SMIL 1.0 solved fundamental media synchronization problems and defined a powerful way of choreographing multimedia content. SMIL Boston extends the timing and synchronization support, adding capabilities to the timing model and associated syntax. This section of the document specifies the Timing and Synchronization module.
There are two intended audiences for this module: implementers of SMIL Boston document viewers or authoring tools, and authors of other XML languages who wish to integrate timing and synchronization support. A language with which this module is integrated is referred to as a host language. A document containing SMIL Timing and Synchronization elements and attributes is referred to as a host document.
As this module is used in different profiles (i.e. host languages), the associated syntax requirements may vary. Differences in syntax should be minimized as much as is practical. The semantics of the timing model and of the associated markup must remain consistent across all profiles. Any host language that includes SMIL Boston Timing and Synchronization markup (either via a hybrid DTD or schema, or via namespace qualified extensions) must preserve the semantics of the model defined in this specification.
Some SMIL 1.0 syntax has been changed or deprecated. Only SMIL document players must support the deprecated SMIL 1.0 attribute names as well as the new SMIL Boston names. A SMIL document player is an application that supports playback of SMIL Language documents (i.e. documents with the associated MIME type "application/smil").
This section is informative.
SMIL Timing defines elements and attributes to coordinate and synchronize the presentation of media over time. The term media covers a broad range, including discrete media types such as still images, text, and vector graphics, as well as continuous media types that are intrinsically time-based, such as video, audio and animation.
Three synchronization elements support common timing use-cases:
These elements are referred to as time containers. They group their contained children together into coordinated timelines.
SMIL Timing also provides attributes that can be used to specify an element's timing behavior. Elements have a begin, and a simple duration. The begin can be specified in various ways - for example, an element can begin at a time, based upon when another element begins, or when some event (such as a mouse click) happens. The simple duration defines the basic presentation duration of an element. Elements can be defined to repeat the simple duration, a number of times or for an amount of time. The simple duration and any effects of repeat are combined to define the active duration. When an element's active duration has ended, the element can either be removed from the presentation or frozen (held in its final state), e.g. to fill any gaps in the presentation.
Figure 1 illustrates the basic support of a repeating element within a simple par time container. The corresponding syntax is included with the diagram.
<par begin="0s" dur="33s"> <video begin="1s" dur="10s" repeatCount="2.5" fill="freeze" .../> </par>
Figure 1 - Strip diagram of basic timing support. The starred "Simple*" duration indicates that the simple duration is partial (i.e. it is cut off early).
The attributes that control these aspects of timing can be applied not only to media elements, but to the time containers as well. This allows, for example, an entire sequence to be repeated, and to be coordinated as a unit with other media and time containers. While authors can specify a particular simple duration for a time container, it is often easier to leave the duration unspecified, in which case the simple duration is defined by the contained child elements. When an element does not specify a simple duration, the time model defines an implicit simple duration for the element. For example, the implicit simple duration of a sequence is based upon the sum of the active durations of all the children.
Each time container also imposes certain defaults and
constraints upon the contained children. For example in a
<seq>, elements begin by default right after
the previous element ends, and in all time containers, the active duration
of child elements is constrained not to extend past the end of the
time container's simple duration. Figure 2 illustrates the effects of a repeating
<par> time container as it constrains a
<video> child element.
<par begin="0s" dur="12s" repeatDur="33s" fill="freeze" > <video begin="1s" dur="5s" repeatCount="1.8" fill="freeze" .../> </par>
Figure 2 - Strip diagram of time container constraints upon child elements. The starred "Simple*" durations indicate that the simple duration is partial (i.e. it is cut off early).
The SMIL Timing Model defines how the time container elements and timing attributes are interpreted to construct a time graph. The time graph is a model of the presentation schedule and synchronization relationships. In an ideal environment, the presentation would perform precisely as specified. However, various real-world limitations (such as network delays) can influence the actual playback of media. How the presentation application adapts and manages the presentation in response to media playback problems is termed runtime synchronization behavior. SMIL includes attributes that allow the author to control the runtime synchronization behavior for a presentation.
The SMIL Timing and Synchronization syntax and precise semantics are described in the following section. A set of symbols are used in the semantic descriptions:
B
d
AD
AE
The timing model is defined by building up from the simplest to the most complex concepts: first the basic timing and simple duration controls, followed by the attributes that control repeating and constraining the active duration. Finally, the elements that define time containers are presented.
The time model depends upon several definitions for the host document: A host document is presented over a certain time interval. The start of the interval in which the document is presented is referred to as the document begin. The end of the interval in which the document is presented is referred to as the document end. The difference between the end and the begin is referred to as the document duration. The formal definitions of presentation and document begin and end are left to the host language designer (see also "Required host language definitions").
This section defines the set of timing attributes that are common to all of the SMIL synchronization elements.
@@ Need to define "local time" or find a different term.
The basic timing for an element is described using the begin
and dur attributes. Authors can specify the begin time of an
element in a variety of ways, ranging from simple clock times to the time
that an event (e.g. a mouse-click) happens. The simple duration of an element
is specified as a simple time value. The length of the simple duration is
specified using the dur attribute. The attribute syntax is described
below. The normative syntax rules for each attribute value variant are described
below (in Timing Attribute
Values); a syntax summary is provided here as an aid to the reader.
<seq> time container.
If no begin is specified, the default timing is dependent upon
the time container. Children of a <par> begin by default
when the <par> begins (equivalent to
begin="0"). Children of a <seq> begin by
default when the previous child ends its active duration (equivalent to
begin="0"). Children of an <excl> default
to a begin value of "indefinite".
The begin value can specify a list of times. This can be used to specify multiple "ways" or "rules" to begin an element, e.g. if any one of several events is raised. A list of times can also define multiple begin times, allowing the element to play more than once (this behavior can be controlled, e.g. to only allow the earliest begin to actually be used - see also Restarting elements).
In general, the earliest time in the list determines the begin time of the element. In the case where an element can begin multiple times, the next begin time is the earliest begin time after the current time. There are additional constraints upon the evaluation of the begin time list, detailed in Evaluation of begin and end time lists.
If there is an error in any individual value in the list of begin values,
only the individual value will be ignored (as though it were not specified),
but the rest of the list will not be invalidated. If no legal value is specified
in the list of begin values, the default value for begin will
be used.
When a begin time is specified as a syncbase variant, a marker value or a wallclock value, the defined time must be converted by the implementation to a time that is relative to the parent time container (i.e. to the equivalent of an offset value). This is know as timespace conversion, and is detailed in the section Converting between local and global times.
If there is any error in the argument value syntax for dur ,
the attribute will be ignored (as though it were not specified).
If the element does not have a (valid) dur attribute, the simple
duration for the element is defined to be the implicit duration of the element.
The implicit duration depends upon the type of an element. The primary
distinction is between different types of media elements and time containers.
Note that if a media element has time children (e.g. animate
or area elements), then it is also a
<par> time container. If the media element has no time
children, it is described as a simple media element.
clipBegin and clipEnd attributes on a
media element can override the intrinsic media duration, and will define
the implicit duration. See also the Media Object module.
<seq>, <par> and
<excl> time containers, including media elements that
are also time containers, the implicit simple duration is a function of the
children of the time container. For details see the section
Time container durations.
If the author specifies a simple duration that is longer than the "intrinsic" defined duration for a continuous media element, the ending state of the media (e.g. the last frame of video) will be shown for the remainder of the simple duration. This only applies to visual media - aural media will simply stop playing.
Note that when the simple duration is "indefinite", some simple use cases can yield surprising results. See the related example #4.
Note that when the begin attribute refers to an event, or to
the begin or active end of another element, it may not be possible to calculate
when the begin will happen. For example, if an element is defined to begin
on some event, the begin time will not be known until the event happens.
When such a time becomes known (i.e. when it can be calculated as a presentation
time), the time is said to be resolved (see also the discussion
of Unifying scheduled and interactive
timing).
The following example shows simple offset begin timing. The
<audio> element begins 5 seconds after the
<par> time container begins, and ends 4 seconds later.
<par> <audio src="song1.au" begin="5s" dur="4s" /> </par>
The following example shows syncbase begin timing. The
<img> element begins 2 seconds after the
<audio> element begins.
<par> <audio id="song1" src="song1.au" /> <img src="img1.jpg" begin="song1.begin+2s" /> </par>
Elements can also be specified to begin in response to an event. In this example, the image element begins (appears) when the user clicks on element "show". The image will end (disappear) 3 and a half seconds later.
<text id="show" ... /> <img begin="show.click" dur="3.5s" ... />
In the syntax specifications that follow, allowed white space is indicated as "S", defined as follows (taken from the [XML10] definition for "S"):
S ::= (#x20 | #x9 | #xD | #xA)*
A begin-value-list is a semi-colon separated list of timing specifiers:
begin-value-list ::= begin-value (S";"Sbegin-value-list )? begin-value ::= (offset-value | syncbase-value | syncToPrev-value | event-value | media-marker-value | wallclock-sync-value)
An end-value-list is a semi-colon separated list of timing specifiers:
end-value-list ::= end-value (S";"Send-value-list )? end-value ::= (clock-value | syncbase-value | syncToPrev-value | event-value | media-marker-value | wallclock-sync-value)
Several of the timing specification values have a similar syntax. In addition,
XML ID attributes are allowed to contain the dot '.' separator
character. The backslash character '\' can be used to escape the dot separator
within identifier and event-name references. To parse an individual item
in a value-list, the following approach defines the correct
interpretation.
'+' or '-'), the value should be parsed as an
offset value.
.'
separator characters preceded by a backslash '\' escape character.
.' separator character, then the value
should be parsed as an event-value
with an unspecified (i.e. default) eventbase-element.
.begin" or
".end", then the value should be parsed as a
syncbase-value.
.marker(", then the value
should be parsed as a
media-marker-value.
@@Note that this approach essentially reserves the following tokens:
prev and wallclock for element IDs, and
begin, end and marker for event names.
Clock values have the following syntax:
Clock-val ::= ( Full-clock-val | Partial-clock-val | Timecount-val )
Full-clock-val ::= Hours ":" Minutes ":" Seconds ("." Fraction)?
Partial-clock-val ::= Minutes ":" Seconds ("." Fraction)?
Timecount-val ::= Timecount ("." Fraction)? (Metric)?
Metric ::= "h" | "min" | "s" | "ms"
Hours ::= DIGIT+; any positive number
Minutes ::= 2DIGIT; range from 00 to 59
Seconds ::= 2DIGIT; range from 00 to 59
Fraction ::= DIGIT+
Timecount ::= DIGIT+
2DIGIT ::= DIGIT DIGIT
DIGIT ::= [0-9]
For Timecount values, the default metric suffix is "s" (for seconds). No embedded white space is allowed in clock values, although leading and trailing white space characters will be ignored.
The following are examples of legal clock values:
02:30:03 50:00:10.25 = 50 hours, 10 seconds and 250 milliseconds
02:33 00:10.5 = 10.5 seconds = 10 seconds and 500 milliseconds
3.2h = 3.2 hours = 3 hours and 12
minutes 45min = 45 minutes 30s = 30 seconds 5ms = 5 milliseconds 12.467 = 12 seconds and 467 milliseconds
Fractional values are just (base 10) floating point definitions of seconds.
The number of digits allowed is unlimited (although actual precision may
vary among implementations).
For example:
00.5s = 500 milliseconds 00:00.005 = 5 milliseconds
An offset value has the following syntax:
offset-value ::= ( "+" | "-" )?( Clock-value )
An offset value allows an optional sign on a clock value, and is used to indicate a positive or negative offset. The offset is measured in local time on the parent time container.
The implicit syncbase for an offset value is dependent upon the time container:
<par> or an
<excl>, the offset is relative to the begin of the parent
<par> or <excl>.
<seq>, the offset is relative to the
active end of the previous child. If there is no previous child, the offset
is relative to the begin of the parent <seq>. See also
The seq time container.
Note, only compliant SMIL document players are required to support the SMIL 1.0 syncbase-value syntax. Language designers integrating SMIL Boston Timing and Synchronization into other languages should not support this syntax.
smil-1-syncbase-value ::= "id(" id-ref ")"
( "(" ( "begin" | "end" | clock-value) ")" )?
ID reference values are references to the value of an "id" attribute of another element in the document.
Id-value ::= IDREF
The IDREF is a legal XML identifier.
A syncbase value has the following syntax:
Syncbase-value ::= ( Syncbase-element "." Time-symbol
)
( S ("+"|"-")
S Clock-value )?
Syncbase-element ::= Id-value
Time-symbol ::= "begin" | "end"
A syncbase value starts with a Syncbase-element term defining the value of an "id" attribute of another element referred to as the syncbase element. The syncbase element must be another timed element contained in the host document. In addition, the syncbase element may not be a descendent of the current element. If the syncbase element specification refers to an illegal element, the syncbase-value description is ignored (although the entire time value list is not invalidated - only the particular syncbase value).
The syncbase element is qualified with one of the following time symbols:
The time symbol can be followed by an offset value. The offset value specifies an offset from the time (i.e. the begin or active end) specified by the syncbase and time symbol. The offset is measured in local time on the parent time container. If the clock value is omitted, it defaults to "0".
No embedded white space is allowed between a syncbase element and a time-symbol. White space will be ignored before and after a "+" or "-" for a clock value. Leading and trailing white space characters (i.e. before and after the entire syncbase value) will be ignored.
Examples:
begin="x.end-5s" : Begin
5 seconds before "x" ends
begin=" x.begin " : Begin when
"x" begins
begin="x.begin + 1m" : End 1 minute after
"x" begins
A sync-to-prev value has the following syntax:
SyncToPrev-value ::= ( "prev." Time-symbol )
(
S ("+"|"-")
S Clock-value )?
A sync-to-prev value is much like a syncbase value, except that the reserved token "prev" is used in place of the Syncbase-element term. The Time-symbol and optional Clock-value offset are as defined for syncbase values.
The previous element is the (timed) element that precedes this element within the parent time container (as reflected in the DOM). Note that the parent time container may not be the immediate parent of the current node, in some host documents.
If there is no previous element (i.e. if the current element is the first timed child of the parent time container), then the begin of the parent time container is used as the syncbase (note that the Time-symbol is ignored in this case). The Clock-value offset is nevertheless added to the parent time container begin time, to yield the resulting time value.
@@This requires more complete examples, or we need to include them above somewhere. We need good examples of how this is used.
Examples:
begin="prev.end-5s" :
Begin 5 seconds before the previous element ends
begin=" prev.begin " : Begin
when the previous element begins
begin="prev.begin + 1m" : End 1 minute after
the previous element begins
An event value has the following syntax:
Event-value ::= (
Eventbase-element "." )? Event-symbol
( S ("+"|"-")
S Clock-value )?
Eventbase-element ::= Id-value
An Event value starts with an Eventbase-element term that specifies the
event-base element. The event-base element is the element on which
the event is observed. Given DOM event bubbling, the event-base element may
be either the element that raised the event, or it may be an ancestor element
on which the bubbled event can be observed. Refer to DOM-Level2-Events
[DOM2Events] for details.
The "Id-value" is the value of an attribute declared to be of type ID (per
the XML definition) in the host language, for the event-base element. This
element must be another element contained in the host document.
If the Eventbase-element term is missing, the event-base element defaults
to the element on which the attribute is specified (the current element).
If this element has no associated layout (e.g. a time container in a SMIL
document), then some UI events may not be defined (e.g. mouse events). Note
that certain elements may specify a different default eventbase. E.g. the
SMIL Animation elements (animate, animateMotion,
etc.) specify that the default eventbase is the target element of
the animation. See also [[SMIL Animation]].
The event value must specify an Event-symbol. This term specifies the name of the event that is raised on the Event-base element. The host language designer must specify which types of events can be used. If an integrating language specifies no supported events, the event-base time value is effectively unsupported for that language.
The last term specifies an optional offset-value that is an offset from the time of the event. The offset is measured in local time on the parent time container. If this term is omitted, the offset is 0.
No embedded white space is allowed between an eventbase element and an event-symbol. White space will be ignored before and after a "+" or "-" for a clock value. Leading and trailing white space characters (i.e. before and after the entire eventbase value) will be ignored.
Note that it is not considered an error to specify an event that cannot be raised on the Event-base element (such as click for audio or other non-visual elements). Since the event will never be raised on the specified element, the event-base value is effectively ignored. Similarly, if the host language allows dynamically created events (as supported by DOM-Level2-Events [DOM2Events]), all possible Event-symbol names cannot be specified, and so unrecognized names may not be considered errors. Host language specifications must include a description of legal event names, and/or allow any name to be used.
The semantics of event-based timing are detailed in the section Unifying Scheduling and Interactive Timing.
Examples:
begin=" x.load " : Begin when "load" is observed
on "x"
begin="x.focus+3s" : Begin 3 seconds after an "focus"
event on "x"
Certain types of media can have associated marker values that associate a name with a particular point (i.e. a time) in the media. The media marker value provides a means of defining a begin or end time in terms of these marker values. Note that if the referenced id is not associated with a media element that supports markers, or if the specified marker name is not defined by the media element, the associated time may never be resolved.
Media-Marker-value ::= Id-value ".marker("
S
marker-symbol S ")" )
The marker symbol is a string that must conform to the definition of marker names for the media associated with the Id-value.
Wallclock-sync values have the following syntax. The values allowed are based upon several of the "profiles" described in [DATETIME], which is based upon [ISO8601]. Exactly the components shown here must be present, with exactly this punctuation. Note that the "T" appears literally in the string, to indicate the beginning of the time element, as specified in [ISO8601].
wallclock-val ::= "wallclock(" S (DateTime | WallTime) S ")"
DateTime ::= Date "T" WallTime
Date ::= Years "-" Months "-" Days
WallTime ::= (HHMM-Time | HHMMSS-Time)(TZD)?
HHMM-Time ::= Hours24 ":" Minutes
HHMMSS-Time ::= Hours24 ":" Minutes ":" Seconds ("." Fraction)?
Years ::= 4DIGIT;
Months ::= 2DIGIT; range from 01 to 12
Days ::= 2DIGIT; range from 01 to 31
Hours24 ::= 2DIGIT; range from 00 to 23
4DIGIT ::= DIGIT DIGIT DIGIT DIGIT
TZD ::= "Z" | (("+" | "-") Hours24 ":" Minutes )
Complete date plus hours and minutes: YYYY-MM-DDThh:mmTZD (e.g. 1997-07-16T19:20+01:00) Complete date plus hours, minutes and seconds: YYYY-MM-DDThh:mm:ssTZD (e.g. 1997-07-16T19:20:30+01:00) Complete date plus hours, minutes, seconds and a decimal fraction of a second YYYY-MM-DDThh:mm:ss.sTZD (e.g. 1997-07-16T19:20:30.45+01:00)
Note that the Minutes, Seconds, Fraction, 2DIGIT and DIGIT syntax is as defined for Clock-values. Note that white space is not allowed within the date and time specification.
There are three ways of handling time zone offsets:
No embedded white space is allowed in wallclock values, although leading and trailing white space characters will be ignored.
The presentation engine must be able to convert wallclock-values to a time
within the document. When the document begins, the current wallclock time
must be noted - this is the document wallclock begin. Wallclock
values are then converted to a document time by subtracting the document
wallclock begin, and then converting the time to the element's parent time
space as for any syncbase value, as though the syncbase were the document
body. Note that the resulting begin or end time may be before the begin,
or after end of the parent time container. This is not an error, but the
time container constraints
still apply. In any case, the semantics of the begin and
end attribute govern the interpretation of the wallclock value.
The following examples all specify a begin at midnight on January 1st 2000, UTC
begin="wallclock(2000-01-01Z)" begin="wallclock( 2000-01-01T00:00Z )" begin="wallclock( 2000-01-01T00:00:00Z )" begin="wallclock( 2000-01-01T00:00:00.0Z )" begin="wallclock( 2000-01-01T00:00:00.0Z )" begin="wallclock( 2000-01-01T00:00:00.0-00:00 )"
The following example specifies a begin at 3:30 in the afternoon on July 28th 1990, in the Pacific US time zone:
begin="wallclock( 1990-07-28T15:30-08:00 )"
The following example specifies a begin at 8 in the morning wherever the document is presented:
begin="wallclock( 08:00 )"
New element controls for element time behavior are under discussion. Note that an Accessibility requirement for control of the playback speed is related to (but may end up with different syntax different from) the speed control. In general, these time manipulations are suited to animation and non-linear or discrete media, rather than linear continuous media. Not all continuous media types will support time manipulations, e.g. streaming MPEG 1 video playing backwards. A fallback mechanism is described for these cases.
Three new attributes add support for timing manipulations to SMIL Timing, including control over the speed of an element, and support for acceleration and deceleration. The impact on overall timing and synchronization is described. A definition is provided for reasonable fallback mechanisms for media players that cannot support the time manipulations.
A common general application of timing supports animation. The recent integration of SMIL timing with SVG is a good example of the interest in this area. Animation in the more general sense includes the time-based manipulation of basic transforms, applied to a presentation. Some of the effects supported include motion, scaling, rotation, color manipulation, as well as a host of presentation manipulations with a style framework like CSS.
Animation is often used to model basic mechanics. Many animation use-cases are difficult or nearly impossible to describe without a simple means to control pacing and to apply simple effects that emulate common mechanical phenomena. While it is possible to build these mechanisms into the animation behaviors themselves, this requires that every animation duplicate this support. This makes the framework more difficult to extend and customize. In addition, this model allows any animation behavior to introduce individual syntax and semantics for these mechanisms. This makes the authoring model much harder to learn, and complicates the job of any authoring tool designer as well. Finally, this model precludes the use of these mechanisms on structured animations (e.g. a time container with a series of synchronized animation behaviors).
A much simpler model for providing the necessary support centralizes the needed functionality in the timing framework. This allows all timed elements to support this functionality, and provides a consistent model for authors and tools designers. The most direct means to generalize pacing and related functionality is to transform the pacing of time for a given element. This is an extension of the transform that is implicitly performed to translate from the general document or presentation time space to the adjusted time space for the element (accounting for the begin time of the element, repeat functionality, etc.). Thus, to control the pacing of a motion animation, a transform is applied that adjusts the pacing of local time for the motion element. If time is scaled to advance faster than normal presentation time, the motion will appear to run faster. Similarly, if the pacing of time is dynamically adjusted, acceleration and deceleration effects are easily obtained. This model is detailed in the sections below.
Three basic time manipulations are proposed:
speed
accelerate and
decelerate
autoReverse
This support is often represented to authors as "Play Forwards, then
Backwards". Because so many common use-cases apply repeat to the modified
local time (as in the examples above), this function is modeled as modifying
the simple duration. As such, autoReverse effectively doubles
the simple duration.
When the three features are combined, there is an inherent ordering that can be applied. The accelerate and decelerate features are applied locally on the simple duration, and have no side effects upon the active duration of the element. The autoReverse feature is applied to the simple duration, and doubles it. Thus, autoReverse wraps the effect of accelerate and decelerate. Speed has the broadest effect, scaling the progress of local time for the element. Taken from the perspective of a conversion from the document time-space to the local time-space, speed is applied earliest, autoReverse later and and then accelerate and decelerate are applied latest. See also Details of the time manipulations.
The following motion animation will move the target twice as fast as normal:
<animateMotion dur="10s" repeatCount="2" speed="2.0" path= ... />
The target will move over the path in 5 seconds, and then repeat this motion. The active duration is thus 10 seconds.
The following rotation (a theoretical extension to the animation platform) will produce a simple pendulum swing on the target (assume that it is a pendulum shape with the transform origin at the top):
<animateRotate from="20deg" to="-20deg" dur="1s" repeatCount="indefinite"
accelerate=".5" decelerate=".5" autoReverse="true" ... />
The pendulum swings through an arc in one second, and then back again in
a second. It repeats indefinitely. The acceleration and deceleration are
specified as a proportion of the simple duration (before autoReverse). As
specified, the effect is to accelerate all the way through the downswing,
and then decelerate all through the upswing. This produces a very realistic
looking animation of real-world pendulum motion. The rotate
element itself can be very simple, for example interpolating the rotation
value in a transform matrix.
The speed attribute is supported on all timed elements. The
argument value expresses a multiple of normal play speed that will be applied
to the element and all time descendents. Thus 1.0 is normal speed, and
speed="1" is a no-op, and speed="-1" means play
backwards.
The speed attribute controls the local playback speed of an element, to speed up or slow down the effective rate of play. Note that the speed does not specify an absolute play speed, but rather is relative to the playback speed of the parent time container. Thus if a <par> and one of its children both specify a speed of 50%, the child will play at 25% of normal playback speed .
Legal values are signed floating point values. A value of 0 is not allowed.
The default is "1.0" (no modification of speed).
The details of the speed modification are described in Details of the time manipulations.
These attributes define a simple acceleration and deceleration of element time, within the simple duration. This is useful for animation, motion paths, etc. The values are expressed as a proportion of the simple duration (i.e. between 0 and 1), and are defined such that the simple duration is not affected (although the normal play speed is increased to compensate for the periods of acceleration and deceleration). Note that these attributes apply to the simple duration; if these attributes are combined with repeating behavior, the acceleration and/or deceleration occurs within each repeat iteration.
The sum of accelerate and decelerate must not exceed
1. If it does, the deceleration value will be reduced to make the sum legal
(i.e. the value of accelerate will be clamped to 1, and then
the value of decelerate will be clamped to
1-accelerate ).
The details of the accelerate and decelerate modifications are described in Details of the time manipulations.
In this example, a motion path will accelerate up from a standstill over
the first 2 seconds, run at a faster than normal rate for 4 seconds, and
then decelerate smoothly to a stop during the last 2 seconds. This
makes an animation look more realistic. The animateMotion element
is defined in the Animation section of SMIL Boston.
<img ...>
<animateMotion dur="8s" accelerate=".25" decelerate=".25" .../>
</img>
In this example, the image will "fly in" from off-screen left , and then
decelerate quickly during the last second to "ease in" to place. This assumes
a layout model that supports positioning (a similar effect could be achieved
by animation the position of a region in SMIL layout). The
animate element is defined in the Animation section of SMIL
Boston.
<img ...>
<animate attributeName="left" dur="4s" decelerate=".25"
from="-1000" to="0" additive="sum" />
</img>
This defines "play forwards then backwards" functionality. The use of autoReverse effectively doubles the simple duration. When combined with repeating behavior, each repeat iteration will play once forwards, and once backwards. This is useful for animation, especially for mechanical and pendulum motion.
Argument values are Booleans.
The default value is false (i.e. play normally).
The details of the autoReverse modification are described in Details of the time manipulations.
In this example, a motion path will animate normally for 5 seconds moving
the element 20 pixels to the right, and then run backwards for 5 seconds
(from 20 pixels to the right back to the original position), then forwards
again and then backwards again, leaving the element at its original location.
The active duration of the animation is 20 seconds. The
animateMotion element is defined in the Animation section of
SMIL Boston.
<img ...>
<animateMotion by="20, 0" dur="5s" autoReverse="true" repeatCount="2"/>
</img>
SMIL 1.0 introduced the repeat attribute, which is used to repeat a media
element or an entire time container. SMIL Boston introduces two new controls
for repeat functionality that supercede the SMIL 1.0 repeat
attribute. The new attributes, repeatCount and
repeatDur, provide a semantic that more closely matches typical
use-cases, and the new attributes provide more control over the duration
of the repeating behavior. The SMIL 1.0 repeat attribute is deprecated
in SMIL Boston (it must be supported in SMIL document players for backwards
compatibility).
Repeating an element causes the simple duration to be "played" several times
in sequence. This will effectively copy or loop the contents of
the element media (or an entire timeline in the case of a time container).
The author can specify either how many times to repeat, using
repeatCount, or how long to repeat, using
repeatDur. Each repeat iteration is one instance of
"playing" the simple duration.
If the simple duration is indefinite, the element cannot repeat. In this
case, any repeatCount attribute is ignored, although a
repeatDur attribute value can still constrain the active duration.
See also Computing the Active
Duration.
At most one of repeatCount or repeatDur should
be specified. If both are specified (and the simple duration is not indefinite),
the active duration is defined as the minimum of the specified
repeatDur, and the simple duration multiplied by
repeatCount. For the purposes of this comparison, a defined
value is considered to be "less than" a value of "indefinite".
If the simple duration is indefinite, any repeatCount attribute
will be ignored. Any repeatDur attribute value (other than
"indefinite") will be used to constrain the indefinite simple duration. See
also the examples below describing repeatDur and an indefinite
simple duration.
If the simple duration is 0, any repeatCount attribute will
be ignored. Any repeatDur attribute value will be used to define
the active duration by showing the state of the element for the specified
duration (this may be constrained by an end value - see
Controlling active duration). See also the
examples below describing repeatDur and a simple duration of
0).
@@ If simple duration is 0 and repeatCount is "indefinite" is the active duration 0 or indefinite?
If an element specifying audio media has a simple duration of 0 (e,g, because
of clipBegin and clipEnd values), nothing should
played even if the repeatDur specifies an active duration. The time model
behaves according to the description, but no audio should be played.
These rules are included in the section Computing the Active Duration.
Need to create normative examples that demonstrate the new controls, and the interaction with implicit and explicit simple durations. Examples must also demonstrate the interaction of repeating behavior and time container constraints.
@@ Need to add example of repeatCount < 1 and/or repeatDur < simple duration
In the following example, the 2.5 second simple duration will be repeated twice; the active duration will be 5 seconds.
<audio src="background.au" dur="2.5s" repeatCount="2" />
In the following example, the 3 second (implicit) simple duration will be repeated two full times and then the first half is repeated once more; the active duration will be 7.5 seconds.
<audio src="3second_sound.au" repeatCount="2.5" />
In the following example, the audio will repeat for a total of 7 seconds. It will play fully two times, followed by a fractional part of 2 seconds. This is equivalent to a repeatCount of 2.8.
<audio src="music.mp3" dur="2.5s" repeatDur="7s" />
Note that if the simple duration is zero (0) or indefinite, repeat behavior
is not defined (but repeatDur still contributes to the active
duration). In the following example the simple duration is 0 and indefinite
respectively, and so the repeatCount is effectively ignored.
Nevertheless, this is not considered an error. The active is equal to the
simple duration: for the first element, the active duration is 0, and for
the second element, the active duration is indefinite.
<img src="foo.jpg" repeatCount="2" /> <img src="bar.png" dur="indefinite" repeatCount="2" />
In the following example, the simple duration is 0, and so repeat behavior
is not meaningful. However, the repeatDur determines the active
duration. The effect is that the text is shown for 10 seconds.
<text src="intro.html" repeatDur="10s" />
In the following example, if the audio media is longer than the 5 second
repeatDur, then the active duration will effectively cut short
the simple duration.
<audio src="8second_sound.au" repeatDur="5s" />
The repeatCount and repeatDur attributes can also
be used to repeat an entire timeline (i.e. a time container simple duration),
as in the following example. The sequence has an implicit simple duration
of 13 seconds. It will begin to play after 5 seconds, and then will
repeat the sequence of three images 3 times. The active duration is thus
39 seconds long.
<seq begin="5s" repeatCount="3" > <img src="img1.jpg" dur="5s" /> <img src="img2.jpg" dur="4s" /> <img src="img3.jpg" dur="4s" /> </seq>
The SMIL 1.0 repeat attribute behaves in a manner similar to repeatCount, but it defines the functionality in terms of a sequence that contains the specified number of copies of the element without the repeat attribute. This definition has caused some confusion among authors and implementers. See also the SMIL 1.0 specification [SMIL10].
In particular, there has been confusion concerning the behavior of the SMIL
1.0 end attribute when used in conjunction with the
repeat attribute. SMIL Boston complies with the common practice
of having the end attribute define the element's simple duration
when the deprecated repeat attribute is used. Only SMIL document
players must support this semantic for the end attribute. Only
a single SMIL 1.0 "end" value (i.e. an
offset-value or a
smil-1.0-syncbase-value,
but none of the new SMIL Boston timing) is permitted when used with the
deprecated repeat attribute. If repeat is used
with repeatCount or repeatDur on an element, or
if repeat is used with an illegal end value, the
repeat value is ignored.
<seq> element with the stated number
of copies of the element without the "repeat" attribute as children. All
other attributes of the element, including any begin delay, are included
in the copies.
Note that elements that use the SMIL 1 repeat attribute with
a value of "indefinite" are defined to end immediately after they begin.
I.e. the active duration is effectively defined to be 0. This semantic is
specific to the SMIL 1 repeat attribute, and does not apply
to the new repeatCount and repeatDur attributes.
SMIL Boston provides an additional control over the active duration. The
end attribute allows the author to constrain the active duration
of the animation by specifying an end value using a simple offset, a time
base, an event-base or DOM methods calls. The end attribute
generally constrains the active duration that is otherwise defined
by dur and any repeat behavior, although it will extend an
implicit simple duration (see examples below). The rules for combining
the attributes to compute the active duration are presented in the next section,
Computing the active duration.
The normative syntax rules for each attribute value variant are described in the section Timing Attribute Values; a syntax summary is provided here as an aid to the reader.
endElement() method call.
If end specifies an event-value or syncbase-value that is not
resolved, the value of end is considered to be "indefinite"
until resolved.
The end value can specify a list of times. This can be used to specify multiple "ways" or "rules" to end an element, e.g. if any one of several events is raised. A list of times can also define multiple end times that can correspond to multiple begin times, allowing the element to play more than once (this behavior can be controlled - see also Restarting elements).
In general, the earliest time in the list determines the end value used in Computing the Active Duration. In the case where an element can begin multiple times, the end value used is the earliest end time after the current begin time. There are additional constraints upon the evaluation of the begin and end time lists, detailed in Evaluation of begin and end time lists.
The end value generally constrains all other values, and does not extend
the active duration. However it will extend an implicit simple
duration. In the following example, the dur attribute is
not specified, and so the simple duration is defined to be the implicit media
duration. In this case (and this case only) the value of end
will extend the active duration if it specifies a duration greater than the
implicit (media) duration. For the difference between the implicit
simple duration and the active duration, the ending state of the media (e.g.
the last frame of video) will be shown. This only applies to visual media
- aural media will simply stop playing, or will not play at all if the implicit
simple duration is 0 (e,g, because of clipBegin and
clipEnd values).
In the following example, the video will be shown for 8 seconds, and then the last frame will be shown for 2 seconds.
<video end="10s" src="8-SecondVideo.mpg" .../>
If the end value becomes resolved while the element is still
active, and the resolved time is in the past, the element should end the
active duration immediately. Time dependents defined relative to the end
of this element should be resolved using the computed active end (which may
be in the past), and not the observed active end. These cases arise from
the use of negative offsets in the sync-base and event-base forms, and authors
should be aware of the complexities this can introduce. See also
Handling negative offsets.
In the following example, the active duration will end at the earlier of 10 seconds, or the end of the "foo" element. This is particularly useful if "foo" is defined to begin or end relative to an event.
<audio src="foo.au" dur="2s" repeatDur="10s"
end="foo.end" .../>
In the following example, the active duration will end at 10 seconds, and will cut short the simple duration defined to be 20 seconds. The effect is that only the first half of the element is actually played. For a simple media element, the author could just specify this using the dur attribute. However in other cases, it is sometimes important to specify the simple duration independent of the active duration.
<par> <audio src="music.au" dur="20s" end="10s" ... /> </par>
In the following example, the element begins when the user clicks on the "gobtn" element. The active duration will end 30 seconds after the parent time container begins. Note that if the user has not clicked on the target element before 30 seconds elapse, the element will never begin.
<par> <audio src="music.au" begin="gobtn.click" repeatDur="indefinite" end="30s" ... /> </par>
The defaults for the event syntax make it easy to define simple interactive behavior. The following example stops the image when the user clicks on the element.
<image src="image.jpg" end="click" />
Using end with an event value enables authors to end an element
based on either an interactive event or a maximum active duration. This is
sometimes known as lazy interaction.
In this example, a presentation describes factory processes. Each step is a video, and set to repeat 3 times to make the point clear. Each element can also be ended by clicking on the video, or on some element "next" that indicates to the user that the next step should be shown.
<seq> <video dur="5s" repeatCount="3" end="click; next.click" .../> <video dur="5s" repeatCount="3" end="click; next.click" .../> <video dur="5s" repeatCount="3" end="click; next.click" .../> <video dur="5s" repeatCount="3" end="click; next.click" .../> <video dur="5s" repeatCount="3" end="click; next.click" .../> </seq>
In this case, the active end of each element is defined to be the earlier of 15 (5s dur * 3 repeats) seconds after it begins, or a click on "next". This lets the viewer sit back and watch, or advance the presentation at a faster pace.
This section still needs work - and will change in the next day or two.
The table in Figure 3 defines a set of "forms" for the simple duration. These forms are used in the table in Figure 4 to delineate the possible combinations of attributes that can contribute to the active duration.
dur |
Implicit media duration | Simple Duration | Form |
| number | * | dur value |
explicit finite |
| "indefinite" | * | indefinite | "indefinite" |
| unspecified | 0 | media dur | implicit 0 |
| unspecified | number | media dur | implicit finite |
| unspecified | continuous indefinite | indefinite | implicit indefinite |
| unspecified | unresolved | indefinite | unresolved |
Figure 3 - Describing the simple duration
@@There are two forms of table 4 presented. We are trying to decide which presents the information more effectively. You be the judge which makes more sense, which is clearer, and which we should include. Both use essentially the same terminology, and present the same semantics (i.e. there should be no discrepancy in the semantics described, but rather only differences in how the information is presented).
The table in Figure 4 shows the semantics of all possible combinations of
simple duration, repeatCount and repeatDur, and
end. The following conventions are used in the table:
repeatCount and repeatDur attributes are specified
as either:
end attribute column specifies the end value obtained by
evaluating the the attribute value according to the rules described in
Controlling active duration and
Evaluation of begin and end
time lists. Note that a list of values yields a single end value at any
given point of evaluation.
Note in particular that where a value specifies "unresolved", that the table will be reevaluated (generally using a different row) if and when the associated value becomes resolved. For example if the element specifies:
<audio src="5-second.au" end="foo.click" />
The active duration is initially defined as equal to the (implicit finite) simple duration. If the user clicks on "foo" before 5 seconds, the end value becomes resolved and the active duration table is re-evaluated to be MIN( d, end-B) which causes the element to end at the time of the click.
Some of the rules and results that are implicit in the table, and that should be noted in particular are:
end and dur are specified but neither of
repeatCount or repeatDur are specified, then the
active duration AD is defined as the minimum
of the simple duration and the duration defined by end.end is specified (i.e. none of dur,
repeatCount or repeatDur are specified), then the
active duration AD is defined as the duration
defined by end (in this case end overrides any
implicit simple duration).end and either (or both) of repeatCount
or repeatDur are specified, the active duration
AD is defined by the minimum duration defined
by the respective attributes.AD divided by the simple duration
d (this may yield partial repeat iterations,
just as repeatCount can specify).
Note that while the active duration is computed according to the rules in the table, the parent time container places constraints upon the active duration of all children. These constraints may cut short the active duration of any child, and so override the definition described here. For more information, see the section Time Container constraints on child durations.
The following symbols are used in the table as a shorthand:
@@This is a form of the table that has more rows, but may be clearer in delineating the different cases. It uses the term "unresolved indefinite" for end values, which is semantically equivalent to "unresolved" in the second table.
@@Note that in both table the handling of explicit 0 is a bit messy. This is only needed if we want to say that simple Dur of 0 and repeatCount of "indefinite" yields a 0 active duration, rather than an indefinite AD. The tables would simplify if we went with the latter.
Where the Active duration has a "+" suffix, the value may be reevaluated at some point. The footnotes within the row indicate when the value will be reevaluated. Note that when a value is re-evaluated, a different row in the table may apply.
| Simple duration(d) | repeatCount |
repeatDur |
end |
Active Duration |
| implicit 0 or explicit 0 |
(ignored) | unspecified | unspecified | 0 |
| implicit 0 or explicit 0 |
(ignored) | "indefinite" | unspecified | indefinite |
| explicit finite or implicit finite | unspecified | unspecified | unspecified | d |
| explicit finite or implicit finite | unspecified | unspecified | unresolved indefinite1 or "indefinite"2 | d+ |
| "indefinite" or implicit indefinite | (ignored) | "indefinite" or unspecified | unspecified | indefinite |
| "indefinite", implicit 0, or implicit indefinite | (ignored) | "indefinite" or unspecified | unresolved indefinite1 or "indefinite"2 | indefinite+ |
| unresolved3 | number or unspecified | "indefinite" or unspecified | unresolved indefinite1 or "indefinite"2 | indefinite+ |
| unresolved3 | number | "indefinite" | unspecified | indefinite+ |
| unresolved3 | (ignored) | unspecified | unspecified | indefinite+ |
| explicit finite or implicit finite | number | "indefinite" or unspecified | unspecified | repeatCount*d |
| explicit finite or implicit finite | number | "indefinite" or unspecified | unresolved indefinite1 or "indefinite"2 | repeatCount*d+ |
| (ignored) | "indefinite" or unspecified | number | unspecified | repeatDur |
| (ignored) | "indefinite" or unspecified | number | unresolved indefinite1 or "indefinite"2 | repeatDur+ |
| "indefinite", implicit 0, or implicit indefinite | (ignored) | number | unspecified | repeatDur |
| "indefinite", implicit 0, or implicit indefinite | (ignored) | number | unresolved indefinite1 or "indefinite"2 | repeatDur+ |
| unresolved3 | number | number | unspecified, unresolved indefinite1, or "indefinite"2 | repeatDur+ |
| explicit finite | unspecified | unspecified | number | MIN(d, end-B) |
| explicit finite | unspecified | "indefinite" | number | end-B |
| "indefinite", implicit 0, or implicit indefinite | (ignored) | "indefinite" or unspecified | number | end-B |
| unresolved3 | number | "indefinite" or unspecified | number | end-B+ |
| (ignored) | "indefinite" | "indefinite" or unspecified | number | end-B |
| implicit finite or unresolved | unspecified | "indefinite" or unspecified | number | end-B |
| explicit finite or implicit finite | "indefinite" | "indefinite" or unspecified | unspecified | indefinite |
| (ignored) | unspecified | "indefinite" | unresolved indefinite1 or "indefinite"2 | indefinite+ |
| (ignored) | "indefinite" or unspecified | "indefinite" | unspecified | indefinite |
| (ignored) | "indefinite" | "indefinite" or unspecified | unresolved indefinite1 or "indefinite"2 | indefinite+ |
| explicit finite or implicit finite | number | number | unspecified | MIN(repeatDur,
repeatCount*d) |
| explicit finite or implicit finite | number | number | unresolved indefinite1 or "indefinite"2 | MIN(repeatDur,
repeatCount*d)+ |
| explicit finite or implicit finite | number | number | number | MIN(end-B, repeatDur,
repeatCount*d) |
| (ignored) | "indefinite" or unspecified | number | number | MIN(end-B, repeatDur) |
| explicit finite or implicit finite | number | "indefinite" or unspecified | number | MIN(end-B,
repeatCount*d) |
| "indefinite", implicit 0, or implicit indefinite | (ignored) | number | number | MIN(end-B, repeatDur) |
| unresolved3 | number | number | number | MIN(end-B, repeatDur)+ |
Figure. 4a: Computing the active duration for different combinations
of the simple duration, repeatDur and repeatCount,
and end.
1 reevaluate if/when end becomes resolved by event
2 reevaluate if/when end becomes resolved by DOM
3 reevaluate when simple duration is resolved
@@This is a form of the table that has fewer rows, and does not call out the values that may be re-evaluated.
Note that any row that includes an "unresolved" value may be re-evaluated at some point (i.e. if the value becomes resolved). Note that when a value is re-evaluated, a different row in the table may apply.
| Simple duration d |
repeatCount |
repeatDur |
end |
Active Duration |
| expl. finite | unspecified | unspecified | unspecified, "indefinite" or unresolved | d |
| implicit 0, or implicit finite |
unspecified | unspecified | unspecified | d |
| "indefinite", impl. indefinite or unresolved |
(ignored) | unspecified or "indefinite" | unspecified | indefinite |
| implicit 0 or explicit 0 |
"indefinite" | unspecified | unspecified | 0 |
| "indefinite", implicit finite, impl. indefinite or unresolved |
unspecified or "indefinite" | unspecified or "indefinite" | "indefinite" or unresolved | indefinite |
| expl. finite, implicit 0, or implicit finite |
expl. finite | unspecified or "indefinite" | unspecified, "indefinite" or unresolved | repeatCount*d |
| expl. finite or implicit finite |
unspecified or "indefinite" | expl. finite | unspecified, "indefinite" or unresolved | repeatDur |
| implicit 0, "indefinite", impl. indefinite or unresolved |
(ignored) | expl. finite | unspecified, "indefinite" or unresolved | repeatDur |
| expl. finite | unspecified | unspecified | expl. finite | MIN( d, end-B
) |
| expl. finite | "indefinite" | unspecified or "indefinite" | expl. finite | end-B |
| expl. finite | unspecified or "indefinite" | "indefinite" | expl. finite | end-B |
| implicit finite | unspecified or "indefinite" | unspecified or "indefinite" | expl. finite | end-B |
| implicit 0, "indefinite", impl. indefinite or unresolved |
(ignored) | unspecified or "indefinite" | expl. finite | end-B |
| (anything except a 0 value) | "indefinite" | unspecified or "indefinite" | unspecified, "indefinite" or unresolved | indefinite |
| (anything) | unspecified or "indefinite" | "indefinite" | unspecified, "indefinite" or unresolved | indefinite |
| expl. finite or implicit finite |
expl. finite | expl. finite | unspecified, "indefinite" or unresolved | MIN( repeatCount*d,
repeatDur ) |
| expl. finite or implicit finite |
expl. finite | expl. finite | expl. finite | MIN( repeatCount*d,
repeatDur,
( end-B )) |
| * | unspecified or "indefinite" | expl. finite | expl. finite | MIN( repeatDur,
( end-B )) |
| implicit 0, "indefinite", impl. indefinite or unresolved |
(ignored) | expl. finite | expl. finite | MIN( repeatDur,
( end-B )) |
| expl. finite or implicit finite |
expl. finite | unspecified or "indefinite" | expl. finite | MIN( repeatCount*d,
( end-B )) |
Figure. 4b: Computing the active duration for different combinations
of the simple duration, repeatDur and repeatCount,
and end.
It is possible to combine scheduled and interactive timing, e.g.:
<par dur="30s"> <img id="mutebutton" src="mute.jpg"/> <text src="description.html" /> <audio src="audio.au" end="mutebutton.click"/> </par>
The image and the text appear for the specified duration of the
<par> (30 seconds). The audio will stop early if the image is
clicked before the active end of the audio (which in this case is the duration
of the actual audio media "audio.au").
It is possible to declare both a scheduled duration, as well as an event-based active end. This facilitates what are sometimes called "lazy interaction" use-cases, such as a slideshow that will advance on its own, or in response to user clicks:
<seq> <img src="slide1.jpg" dur="10s" end="click" /> <img src="slide2.jpg" dur="10s" end="click" /> <img src="slide3.jpg" dur="10s" end="click" /> <!-- etc., etc. --> </seq>
In this case, the active end of each element is defined to be the earlier of the specified duration, or a click on the element. This lets the viewer sit back and watch, or advance the slides at a faster pace.
By default when an element's active duration ends, it is no longer presented (or its effect is removed from the presentation, depending upon the type of element). Freezing an element extends it, using the last state presented in the active duration. This can be used to fill gaps in a presentation, or to extend an element as context in the presentation (e.g. with additive animation - see [SMIL-ANIMATION]).
The fill attribute allows an author to specify that an element
should be extended beyond the active duration by freezing the final
state of the element. For discrete media, the media is simply displayed as
it would be during the active duration. For continuous media, the "frame"
that corresponds to the end of the active duration is shown. For algorithmic
media like animation, the value defined for the end of the active duration
should be used. The syntax of the fill attribute is the same as in
SMIL 1.0, with two extensions:
This attribute only has an effect on visual media elements. Non-visual media elements (audio) should ignore this.
Note that <a> and <area> elements are
still sensitive to user activation (e.g. clicks) when frozen. See also the
SMIL 1.0 specification [SMIL10].
The default value of the fill attribute depends on the element
type, and whether the element specifies any of the attributes that define
the simple or active duration.
<par>, <seq> and
<excl>), the default value is "remove".
dur,
end, repeatCount or repeatDur are
specified on the element, then the default value of fill is
"freeze".
An element with fill="freeze" is extended according to the parent
time container:
<par>, the element is frozen to extend to the end
of the simple duration of the <par>. In this case,
fill="freeze" is equivalent to fill="hold".
<seq>, the element is frozen to extend to the begin
of the next element in the <seq>. This will fill any gap
in the presentation (although it may have no effect if the next element begins
immediately).
<excl>, the element is frozen to extend to the begin
of the next element to be activated in the <excl>. This
will fill any gap in the presentation (although it may have no effect if
the next element interrupts the current element). Note that if an element
is paused, the active duration has not ended, and so the fill
attribute does not (yet) apply. See also the section
The excl time container.
The fill attribute can be used to maintain the value of an media
element after the active duration of the element ends:
<par endSync="last"> <video src="intro.mpg" begin= "5s" dur="30s" fill="freeze" /> <audio src="intro.au" begin= "2s" dur="40s"/> </par>
The video element ends 35 seconds after the parent time container began, but the video frame at 30 seconds into the media remains displayed until the audio element ends. The attribute "freezes" the last value of the element for the remainder of the time container's simple duration.
This functionality is also useful to keep prior elements on the screen while
the next item of a <seq> time container prepares to display
as in this example:
<seq> <video id="v1" fill="freeze" src.../> <video id="v2" begin="2s" src.../> </seq>
The first video is displayed and then the last frame is frozen for 2 seconds, until the next element begins. Note that if it takes additional time to download or buffer video "v2" for playback, the first video "v1" will remain frozen until video "v2" actually begins.
@@Need a good example of freeze on a time container, showing both how it extends any frozen children, as well as how it cuts off and freezes any children that were active at the end.
When an element is defined to begin at a simple offset (e.g.
begin="5s" ), there is an unequivocal time when the element
begins. However, if an element is defined to begin relative to an event (e.g.
begin="foo.click" ), the event can happen at any time, and moreover
can happen more than once (e.g. if the user clicks on "foo" several
times). In some cases, it is desirable to restart an element if
a second begin event is received. In other cases, an author may want to preclude
this behavior.
In SMIL Boston, an element can have a list of begin values. In some cases, the intent is to begin at the earliest of the specified times (e.g. when the user clicks on any one of several images). In other cases, the intent is that the element restart when any of the begin times is encountered.
In addition, if an element is defined to begin relative to when another element
begins (using the syncbase- value syntax), the syncbase element can restart.
The restart attribute is used to control the restart behavior
of an element.
restart
= "always | whenNotActive |
never"
The default value for the restart attribute is "always". This may not be a sensible default in all documents. In particular SMIL Boston documents with streaming media may want restart="never" set on all of the elements. In order to not require restart="never" be added to every media element in the document, the WG is considering ways to override the default and set a new default for the document.
Note that there are several ways that an element may be restarted. The behavior
(i.e. to restart or not) in all cases is controlled by the
restart attribute. The different restart cases are:
begin specified as an event-value can be restarted
when the named event fires multiple times.
begin specified as a syncbase value, where the
syncbase element can restart. When an element restarts, other elements defined
to begin relative to the begin or active end of the restarting element may
also restart (subject to the value of restart on these elements).
When an element restarts, the primary semantic is that it behaves as though this were the first time the element had begun, independent of any earlier behavior. Any effect of an element playing earlier is no longer applied, and only the current begin "instance" of the element is reflected in the presentation.
The synchronization relationship between an element and its parent time container is re-established when the element restarts. A new synchronization relationship may be defined. See also Controlling runtime synchronization behavior.
As with any begin time, if an element is scheduled to restart after the end of the parent time container simple duration, the element will not restart.
Note that if the parent time container (or any ascendant time container)
repeats or restarts, any state associated with restart="never"
will be reset, and the element can begin again normally. See also
Resetting element state.
The restart setting for an animation is evaluated when the syncbase element
restarts, when the eventbase event happens, or when the DOM method call (e.g.
beginElement()) happens. For example:
<img id="go_btn" dur="indefinite" .../>
<video id="foo" begin="go_btn.click" ... />
<audio id="bar" begin="foo.begin+2s" dur="10s"
restart="whenNotActive" ..." />
If the user clicks on the "go_btn" image at 5 seconds, element "foo" will begin, and element "bar" will be scheduled to begin at 7 seconds. If the user clicks the image again at 6 seconds, "foo" would restart and "bar" would be rescheduled to start at 8 seconds. If the user clicks again at 9 seconds, "foo" would restart but "bar" will not, as it is set to allow restart only when it is not active.
If an element is currently active when a restart is scheduled, the element should end immediately (at the time of the restart evaluation). It should not continue playing until the rescheduled begin actually happens. For example:
<img id="go_btn" dur="indefinite" .../> <video id="foo" begin="go_btn.click" .../> <audio id="bar" begin="foo.begin+2s" dur="10s" />
If the user clicks the image once at 3 seconds, "foo" begins to play and 2 seconds later "bar" will play as well. If the user clicks again at 6 seconds, "foo" restarts immediately, "bar" is stopped, and "bar" will restart at 8 seconds.
Note that using restart can also allow the author to define a single UI event to both begin and end an element, as follows:
<img id="toggle" dur="indefinite" .../> <audio id="foo" begin="toggle.click" end="toggle.click" repeatDur="indefinite" restart="whenNotActive" .../>
If "foo" were defined with the default restart behavior "always", a second click on the image would simply restart the audio. However, since the second click cannot restart the audio when restart is set to "whenNotActive", the click will just end the active duration and stop the audio. This is sometimes described as "toggle" activation. See also Unifying scheduling and interactive timing.
Note that in SMIL Language documents, a SMIL element cannot be visible before
it begins so having a begin="click" means it won't ever begin. In languages
with timeAction support, this may not be the case. For example,
the following is reasonable:
<span begin="click" end="click" timeAction="class:highlight"> Click here to highlight. Click again to remove highlight. </span>
See also "The SMIL Integration Module" for details of language profiles.
A common use-case requires that the same UI event is used begin an element
and to end the active duration of the element. This is sometimes described
as "toggle" activation, because the UI event toggles the element "on" and
"off". The restart attribute can be used to author this,
as follows:
<img id="foo" begin="bar.click" end="bar.click"
restart="whenNotActive" ... />
If "foo" were defined with the default restart behavior "always", a second
click on the "bar" element would simply restart the element. However, since
the second click cannot restart the element when restart is
set to "whenNotActive", the element ignores the "begin" specification of
the "click" event. The element can then use the "click" event to end the
active duration and stop the element.
This is based upon the event sensitivity semantics described in Unifying Scheduling and Interactive Timing.
SMIL Boston specifies three time containers: <par>, <seq>, and <excl>.
<par> container defines a simple parallel time grouping
in which multiple elements can play back at the same time.
The default syncbase of the child elements of a <par>
is the begin of the <par>. This is the same element
introduced with SMIL 1.0.
The <par> element supports all element timing.
<seq> container defines a sequence of elements in which
elements play one after the other.
This is the same element introduced with SMIL 1.0, but the semantics (and allowed syntax) for child elements of a <seq> are clarified. The default syncbase of a child element is the active end of the previous element. Previous means the element that occurs before this element in the sequence time container. For the first child of a sequence (i.e. where no previous sibling exists), the default syncbase is the begin of the sequence time container.
Child elements may define an offset from the syncbase, but may not define
a different syncbase (i.e. they may not define a begin time relative to another
element, or to an event). Thus, the syncbase of child elements of a sequence
is always the same as the default syncbase. Note however that child elements
may define an end that references other syncbases,
event-bases, etc.
For children of a sequence, only the offset begin values are legal. None of the following begin values may be used:
No constraints are placed upon the end argument values.
The <seq> element itself supports all element timing.
When a hyperlink traversal targets a child of a
<seq>, and the target child is not currently active, part
of the seek action must be to enforce the basic semantic of a
<seq> that only one child may be active a given time.
For details, see Hyperlinks and timing
and specifically Implications
of beginElement() and hyperlinking for seq and excl time containers.
SMIL Boston defines a new time container, <excl>.
<excl> are grouped into categories, and the pause/interruption
behavior of each category can be controlled using the new grouping element
<priorityClass>.
The default syncbase of the child elements of the
<excl> is indefinite (i.e. equivalent to
begin="indefinite").
The <excl> element itself supports all element timing.
With the <excl> time container, common use cases that
were either difficult, or impossible, to author are now easier and possible
to create. The <excl> time container is used to define
a mutually exclusive set of clips, and to describe pausing and resuming behaviors
among these clips. Examples include:
The interactive playlist use case above could be accomplished using a
<par> whose sources have interactive begin times and
end events for all other sources. This would require a prohibitively
long list of values for end to maintain. The
<excl> time container provides a convenient short hand
for this - the element begin times are still interactive, but the
end events do not need to be specified because the
<excl>, by definition, only allows one child element to
play at a time.
The audio descriptions use case is not possible without the pause/resume
behavior provided by <excl> and
<priorityClass>. This use case would be authored with
a video and each audio description as children of the
<excl>. The video element would be scheduled to begin
when the <excl> begins and the audio descriptions, peers
of the video element, would start at scheduled begin times or in response
to stream events raised at specific times.
The dynamic video sub-titles use case requires the "play only one at a time"
behavior of <excl>. In addition, the child elements are
declared in such as way so to preserve the sync relationship to the video:
<par>
<video id="vid" .../>
<excl>
<par begin="englishBtn.click" >
<audio begin="vid1.begin" src="english.au" />
</par>
<par begin="frenchBtn.click" >
<audio begin="vid1.begin" src="french.au" />
</par>
<par begin="swahiliBtn.click" >
<audio begin="vid1.begin" src="swahili.au" />
</par>
</excl>
</par>
The three <par> elements are children of the
<excl>, and so only one can play at a time. The audio
child in each <par> is defined to begin when the video
begins. Each audio can only be active when the parent time container
(<par>) is active, but the begin still specifies the
synchronization relationship. This means that when each
<par> begins, the audio will start playing at some point
in the middle of the audio clip, and in sync with the video.
The <excl> time container is useful in many authoring
scenarios by providing a declarative means of describing complex clip
interactions.
Using priority classes to control the pausing behavior of children of the
<excl> allows the author to group content into categories
of content, and then to describe rules for how each category will interrupt
or be interrupted by other categories. Attributes of the new grouping element
<priorityClass> describe the intended interactions.
The <priorityClass> element is transparent to timing,
and does not participate in or otherwise affect the normal timing behavior
of its children (i.e. it only defines how elements interrupt one another).
Child elements of the <priorityClass> elements are
time-children of the <excl> element (i.e. the parent
<excl> of the <priorityClass> elements).
Each <priorityClass> element describes a group of children,
and the behavior of those children when interrupted by other time-children
of the <excl>. The behavior is described in terms of
peers, and higher and lower priority elements.
Peers are those elements within the same
<priorityClass> element. The priorityClass
elements are assigned priority levels based upon the order in which they
are declared within the excl. The first
<priorityClass> element has highest priority, and the
last has lowest priority.
When one element within the <excl> begins (or would normally
begin) while another is already active, several behaviors may result. The
active element may be paused or stopped, or the interrupting element may
be deferred, or simply blocked from beginning. When elements are paused or
deferred, they are added to a queue of pending elements. When an active element
completes its active duration, the first element (if any) in the queue of
pending elements is made active. The queue is ordered according to rules
described in Pause queue semantics.
The careful choice of defaults makes common use cases very simple. See the examples below.
<excl> time-children, and the
pause/interrupt behavior of the children. If a
<priorityClass> element appears as the child of an
<excl>, then the <excl> can only contain
<priorityClass> elements (i.e. the author cannot mix timed
children and <priorityClass> elements within an
<excl>).
The <excl> element content model is thus (assume that
container content is an updated version of the SMIL 1.0 DTD entity):
<!ENTITY % excl-content "priorityClass* | %container-content;"> <!ELEMENT excl (%excl-content;)*>
The <priorityClass> element supports a simple set of
attributes to describe the behavior of its children:
<!ELEMENT priorityClass %container-content;> <!ATTLIST priorityClass id ID #IMPLIED peers ( stop | pause | defer | never ) 'stop' higher ( stop | pause ) 'pause' lower ( defer | never ) 'defer' >
If no <priorityClass> element is used, all the children
of the <excl> are considered to be peers, with
the default peers
behavior "stop".
Note that the rules define the behavior of the currently active element and the interrupting element. Any elements in the pause queue are not affected (except that their position in the queue may be altered by new queue insertions).
peers = " stop | pause
| defer | never "
<priorityClass> will
interrupt one another.peers.
excl
time container). The paused element is added to the pause queue.
higher = " stop |
pause "
<priorityClass>.<priorityClass> is active, the active child element is
simply stopped.
<priorityClass> is active, the active child element is
paused and will resume when the new (interrupting) element completes its
active duration (subject to the constraints of the excl time
container). The paused element is added to the pause queue.higher attribute.
lower = " defer |
never "
<priorityClass>.<priorityClass> is active, the new
(interrupting) element is deferred until the active element completes its
active duration. This can also be thought of as placing the new element in
the pause queue, paused at its very beginning. The rules for adding the element
to the queue are described below.lower attribute.
<priorityClass> is active, the new (interrupting) element
is prevented from beginning. The begin of the new (interrupting) element
is ignored, and it is not added to the queue.
When an element begin is blocked (ignored) because of the "never" attribute
value, the blocked element does not begin in the time model. The time model
should not propagate begin or end activations to time dependents, nor should
it raise begin or end events.
Note that because of the defaults, the simple cases work without any additional syntax. In the basic case, all the elements default to be peers, and stop one another:
<excl dur="indefinite"> <audio id="song1" .../> <audio id="song2" .../> <audio id="song3" .../> ... <audio id="songN" .../> </excl>
is equivalent to the following with explicit settings:
<excl dur="indefinite">
<priorityClass peers="stop">
<audio id="song1" .../>
<audio id="song2" .../>
<audio id="song3" .../>
...
<audio id="songN" .../>
</priorityClass>
</excl>
If the author wants elements to pause rather than stop, the syntax is:
<excl dur="indefinite">
<priorityClass peers="pause">
<audio id="song1" .../>
<audio id="song2" .../>
<audio id="song3" .../>
...
<audio id="songN" .../>
</priorityClass>
</excl>
The audio description use case for visually impaired users would look very similiar to the previous example:
<excl dur="indefinite">
<priorityClass peers="pause">
<video id="main_video" .../>
<audio id="scene1_description" begin="20s" dur="30s".../>
<audio id="scene2_description" begin="2min" dur="30s" .../>
...
<audio id="sceneN_description" .../>
</priorityClass>
</excl>
This example shows a more complex case of program material and several commercial insertions. The program videos will interrupt one another. The ads will pause the program, but will not interrupt one another.
<excl dur="indefinite">
<priorityClass id="ads" peers="defer">
<video id="advert1" .../>
<video id="advert2" .../>
</priorityClass>
<priorityClass id="program" peers="stop" higher="pause">
<video id="program1" .../>
<video id="program2" .../>
<video id="program3" .../>
<video id="program4" .../>
</priorityClass>
</excl>
Elements that are paused or deferred are placed in a priority-sorted queue of waiting elements. When an active element ends its active duration and the queue is not empty, the first (i.e. highest priority) element in the queue is pulled from the queue and resumed or activated.
The queue semantics are described as a set of invariants and the rules for
insertion and removal of elements. For the purposes of discussion, the child
elements of a <priorityClass> element are considered to
have the priority of that <priorityClass>, and to have
the behavior described by the peers, higher and
lower attributes on the <priorityClass> parent.
excl ends normally
(i.e. not when it is stopped by another, interrupting element),
the element on the front of the queue is pulled off the queue, and resumed
or begun (according to rule 2 or 3).
Note that if an element is active and restarts (subject to the
restart rule), it does not interrupt itself in the sense of
a peer interrupting it. Rather, it simply restarts and the queue is unaffected.
@@This section has more general impact that just in excl, and should perhaps be moved elsewhere. E.g. when an element is paused with the pause() DOM method, this semantic will apply as well.
When an element is paused, a resolved end time for the element may no longer be resolved (although it could be computed in some cases). This change in the end time must be propagated to any sync arc time dependents defined relative to the active end of the paused element. See also the "Propagating Times" section in the Timing draft.
When an element is deferred, sync-arc time-dependents of the element are resolved when the element actually begins, and not when it is placed in the queue. Similarly, the begin event is not raised until the element begins.
<excl>
Although the default begin value for children of an
<excl> is indefinite, scheduled begin times are permitted.
Scheduled begin times on children of the <excl> cause
the element to begin at the specified time, pausing or stopping other siblings
depending on the priorityClass settings (and default values).
To specify that a child of the <excl> should begin playing
by default (i.e., when the <excl> begins), specify
begin="0" on that child element. If children of an
<excl> are scheduled to begin at the same time, the evaluation
proceeds in document order. For each element in turn, the priorityClass semantics
are considered, and elements may be paused, deferred or stopped. For example:
<excl> <img src="image1.jpg" begin="0s" dur="5s"/> <img src="image2.jpg" begin="0s" dur="5s"/> <img src="image3.jpg" begin="0s" dur="5s"/> </excl>
Given the default semantics for excl, the first image will begin and then be immediately stopped by the second image, which will in turn be immediately stopped by the third image. The net result is that only the third image is seen, and it lasts for 5 seconds. Note that the begin and end events for the first two images are raised and propagated to all time dependents. If the behavior is set to "pause" as in this example, the declared order is effectively reversed:
<excl>
<priorityClass peers="pause">
<img src="image1.jpg" begin="0s" dur="5s"/>
<img src="image2.jpg" begin="0s" dur="5s"/>
<img src="image3.jpg" begin="0s" dur="5s"/>
</priorityClass>
</excl>
In this case, the first image will begin and then be immediately paused by
the second image, which will in turn be immediately paused by the third image.
The net result is that the third image is seen for 5 seconds, followed by
the second image for 5 seconds, followed by the 3rd image for 5 seconds.
Note that the begin events for the first two images are raised and propagated
to all time dependents when the excl begins.
In the following slideshow example, images begin at the earlier of their scheduled begin time or when activated by a user input event:
<excl> <img src="image1.jpg" begin="0s".../> <img src="image2.jpg" begin="10s; image1.click".../> <img src="image3.jpg" begin="20s; image2.click".../> </excl>
Note, some surprising results may occur when combining scheduled and interactive
timing within an <excl>. If in the above example, the
user clicks on image1 and then on image2 before ten seconds have elapsed,
image 2 will re-appear at the ten second mark. Image 3 will appear at twenty
seconds. The likely intent of this particular use-case would be better
represented with a seq time container.
Children of the <excl> can be activated by scheduled timing,
hyperlinks, events or DOM methods calls. For all but hyperlink activation,
the <excl> time container must be active for child elements
of the <excl> to be activated. With hyperlink activation,
the document may be seeked to force the parent
<excl> to be active, and a seek may occur to the begin
time target child if it has a resolved begin time. That is, the normal hyperlink
seek semantics apply to a timed child of an <excl>.
With activation via a DOM method call (e.g. the beginElement()
method), the element will be activated at the current time (subject to the
priorityClass semantics), even if the element has a scheduled
begin time. The exclusive semantics of the time container (allowing only
one active element at a time) and all priorityClass semantics
are respected nevertheless.
See also Hyperlinks and timing and specifically Implications of beginElement() and hyperlinking for seq and excl time containers.
The endSync attribute is only valid for
<par> and <excl> time containers, and
media elements with timed children (e.g. animate or
area elements). The endSync attribute controls
the end of the simple duration of these containers, as a function of the
children. This is particularly useful with children that have "unknown" duration,
e.g. an mpeg movie, that must be played through to determine the duration.
@@ Add more info to the effect that: Note that paused children of an <excl> container have not ended their active duration. Moreover (something to explain the "Elements do not have to play to completion, but must have played at least once") (i try ...) elements with multiple activation due to multiple begin and end value has only to play once to be considered as having ended their active duration.
endSync = " first | last | all | id-ref "
<par>, <excl>, or media element
simple duration ends with the earliest active end of all the child elements.
This does not refer to the lexical first child, or to the first child to
start, but rather refers to the first child to end its active duration.<par>, <excl>, or media element
simple duration ends with the last active end of the child elements. This
does not refer to the lexical last child, or to the last child to start,
but rather refers to the last active end of all children that have a resolved
begin time.<par>, <excl>, or media element
simple duration ends when all of the child elements have ended their respective
active durations. Elements with indefinite or unresolved begin times will
keep the simple duration of the time container from ending.<par>, <excl>, or media element
simple duration ends with the specified child. The id must correspond to
one of the immediate children of the <par> time
container. <par ... endSync="movie1" ...>
Semantics of endSync and indeterminate children:
endSync="first" means that the time container must wait for
any element to actually end its active duration. It does not matter whether
the the first element to end was scheduled or interactive.endSync="last" means that the time container must wait for all
elements that have a resolved begin, to end the respective active durations.
Note that elements that had an interactive begin, but that became resolved
before all scheduled elements ended, are added to the set of children that
must end their active durations before the parent can end. This can chain,
so that only one element is running at one point, but before it ends its
active duration another interactive element is resolved. It may even yield
"dead time" (where nothing is playing), if the resolved begin is
after the other elements active end. <excl> that are currently paused
and waiting to resume will keep the simple duration of the time
container from ending.endSync="all" means that every child element of the time container
must end the active duration. In the case of element with multiple begin
times, or restarting elements, note that elements do not have to play to
completion; they just must have played at least once (here "once" refers
to an instance of an active duration, and not to one repeat iteration
of a repeating element). When all elements have completed the active duration
one or more times, the parent can end.<excl> that are currently
paused and waiting to resume (and have not already completed the active duration
at least once) will keep the simple duration of the time container from
ending.endSync=[id-ref] means that the time container must
wait for the referenced element to actually end its active duration. The
id-ref must refer to a child of the time container. If the referenced child
has an indefinite active duration, then the simple duration of the time container
is also indefinite.@@ Do we need a note to call out that in some cases, endSync may define an indefinite simple duration for the time container. This would flow "through the computing the active duration" table accordingly, using "implicit indefinite" as the simple duration.
The following pseudo-code describes the endSync algorithm:
//
// boolean timeContainerHasEnded()
//
// method on time containers called to evaluate whether
// time container has ended, according to the rules of endSync.
// Note: Only supported on par and excl
//
// A variant on this could be called when a child end is updated to
// create a scheduled (predicted) end time for the container.
//
// Note that we never check the end time of children - it don't matter.
//
// Assumes:
// child list is stable during evaluation
// isActive state of children is up to date for current time.
// [In practice, this means that the children must all be
// pre-visited at the current time to see if they are done.
// If the time container is done, and repeats, the children
// may be resampled at the modified time.]
//
// Uses interfaces:
// on TimedNode:
// isActive() tests if node is currently active
// hasStarted() tests if node has (ever) begun
// begin and end begin and end TimeValues of node
//
// on TimeValue (a list of times for begin or end)
// isResolved(t) true if there is a resolved time
// at or after time t
//
boolean timeContainerHasEnded()
{
TimeInstant now = getCurrentTime(); // normalized for time container
boolean assumedResult;
// For first or ID, we assume a false result unless we find a child that has ended
// For last and all, we assume a true result unless we find a dis-qualifying child
if( ( endSyncRule == first ) || ( endSyncRule == ID ) )
assumedResult = false;
else
assumedResult = true;
// Our interpretation of endSync == all:
// we're done when all children have begun, and none is active
//
// loop on each child in collection of timed children,
// and consider it in terms of the endSyncRule
foreach ( child c in timed-children-collection )
{
switch( endSyncRule ) {
case first:
// as soon as we find an ended child, return true.
if( c.hasStarted() & !c.isActive() )
return true;
// else, keep looking (assumedResult is false)
break;
case ID:
// if we find the matching child, just return result
if( endSyncID == c.ID )
return( c.hasStarted() & !c.isActive() );
// else, keep looking (we'll assume the ID is valid)
break;
case last:
// we just test for disqualifying children
// If the child is active, we're definitely not done.
// If the child has not yet begun but has a resolved begin,
// then we're not done. Note that if it has already begun,
// then we do not care if it has more resolved begins.
if( c.isActive()
|| ( !c.hasStarted() && c.begin.isResolved(now) ))
return false;
// else, keep checking (the assumed result is true)
break;
case all:
// we just test for disqualifying children
// all_means_last_done_after_all_begin
// If the child is active, we're definitely not done.
// If the child has not yet begun then we're not done.
// Note that if it has already begun,
// then we do not care if it has more resolved begins.
if( c.isActive() || !c.hasStarted() )
return false;
// else, keep checking (the assumed result is true)
break;
} // close switch
} // close foreach loop
return assumedResult;
} // close timeContainerHasEnded()
The implicit duration of a time container is defined in terms of the children of the container. The children can be thought of as the "media" that is "played" by the time container element. The semantics are specific to each of the defined time container variants.
By default, the simple duration of a <par> is defined
by the endSync=last semantics. The simple duration will end
when all scheduled children have ended their respective active durations.
The simple duration of a <par> container can be controlled
with the dur and endSync attributes. If the
dur attribute is specified, the endSync attribute
is ignored. Using endSync, the end of the simple duration can
be tied to the active end of the first child that finishes, or to the active
end of the last child to finish (the default), or to the active end of a
particular child element.
By default, the simple duration of a <seq> ends with
the active end of the last child of the <seq>.
If any child of a <seq> has an indefinite active duration,
the simple duration of the <seq> is also indefinite.
The implicit simple duration of an <excl> container is
defined the same as for a <par> container, using the
endSync=last semantics. However, since the default timing for
children of <excl> is interactive, it will be common for
<excl> time containers to have indefinite simple duration.
For endSync={last or all}: The time children and the intrinsic
media duration define the simple duration of the media element time container.
If a continuous media duration is longer than the extent of all the time
children, the continuous media duration defines the implicit simple duration
for the media element time container. If the media is discrete, this is defined
as for <par> elements.
For endSync={first}: The time children and the intrinsic media
duration define the simple duration of the media element time container.
The element ends when the first active duration ends, as defined above for
endSync on a <par>, but no sooner than the
end of the intrinsic media duration of continuous media. If the media is
discrete, this is defined as for <par> elements.
For endSync={ID}: This is defined as for
<par> elements.
If the calculated implicit simple duration is longer than the intrinsic duration for a continuous media element, the ending state of the media (e.g. the last frame of video) will be shown for the remainder of the simple duration. This only applies to visual media - aural media will simply stop playing.
This semantic is similar to the case in which the author specifies a simple duration that is longer than the intrinsic duration for a continuous media element. Note that for both cases, although the media element is effectively frozen for the remainder of the simple duration, the time container local time is not frozen during this period, and any children will run normally without being affected by the media intrinsic duration.
Time containers place certain overriding constraints upon the child elements. These constraints can cut short the active duration of any child element.
All time containers share the basic overriding constraint:
While the child may define a sync relationship that places the begin before the parent begin, the child is not active until the parent begins. This is equivalent to the semantic described in Negative begin delays.
If the child defines an active duration (or by the same token a simple duration) that extends beyond the end of the parent simple duration, the active duration of the child will be cut short when the parent simple duration ends. Note that this does not imply that the child duration is automatically shortened, or that the parent simple duration is "inherited" by the child.
For example:
<par dur="10s" repeatDur="25s"> <video dur="6s" repeatCount="2" .../> <text begin="5s" dur="indefinite" .../> <audio begin="prev.end" .../> </par>
The video will play once for 6 seconds, and then a second time but only for
4 seconds - the last 2 seconds will get cut short and will not be seen. The
text shows up for the last 5 seconds of the <par>, and
the indefinite duration is cut short at the end of the simple duration of
the <par>. The audio will not show up at all, since it
is defined to begin at the end of the active duration of the previous element
(the <text> element). Since the text element ends when
the time container ends, the audio would begin after the time container has
ended, and so never is heard. When the <par> repeats the first time,
everything has happens just as it did the first time. However the last repeat
is only a partial repeat (5 seconds), and so on the video will be seen, but
it will not be seen to repeat, and the last second of the video will be cut
off.
Note the time container is itself subject to the same constraints, and so may be cut short by some ascendant time container. When this happens, the children of the time container are also cut off, in the same manner as for the last partial repeat in the example above.
In addition, <excl> time containers allow only one child
to play at once. Subject to the priorityClass semantics, the
active duration of an element may be cut short when another element in the
time container begins.
We need a few good examples to illustrate these concepts.
SMIL 1.0 defined constraints on sync-arc definition (e.g., begin="image1.begin"), allowing references only to qualified siblings. SMIL Boston explicitly removes this constraint. SMIL Boston also adds event-based timing. Both sync-arcs and event-timing are constrained by the parent time container of the associated element as described above.
While a sync-arc is explicitly defined relative to a particular element, if this element is not a sibling element, then the sync is resolved as a sync-relationship to the parent (i.e. to an offset from the parent begin). If the defined sync would place the resolved element begin before the parent time container begin, part of the element will simply be cut off when it first plays. This is not unlike the behavior obtained using clipBegin. However unlike with clipBegin, if the sync-arc defined child element also has repeat specified, only the first iteration will be cut off, and subsequent repeat iterations will play normally. See also Negative begin delays.
Note that in particular, an element defined with a sync-arc begin will not automatically force the parent or any ancestor time container to begin.
For the case that an element with a sync-arc is in a parent (or ancestor) time container that repeats: for each iteration of the parent or ancestor, the element is played as though it were the first time the parent timeline was playing. With each repeat of the parent, the sync-arc will be recalculated to yield a begin time relative to the parent time container. See also the section Resetting element state.
@@ If we allowed events on begin in children of sequence, we would have to refine this language to say that an element is sensitive after the active end of the previous element, and until its own active end.
The parent time container must be active for the child element to receive events.
Sequence children are only sensitive to events during their active duration. In the following example, all children listen to the same end event and it works as expected:
<seq> <img src="img1.jpg" end="foo.click" /> <img src="img2.jpg" end="foo.click" /> <img src="img3.jpg" end="foo.click" /> </seq>
A begin time (ultimately) specifies a synchronization relationship between
the element and the parent time container. Syncbase variants, eventbase,
marker and wallclock timing are implicitly converted to an offset on the
parent time container, just as an offset value specifies this directly. For
children of a <seq>, the result is always a positive offset
from the begin of the <seq> time container. However, for
children of <par> and <excl> time
containers. the computed offset relative to the parent begin time may be
negative.
If the computed begin offset is negative, the time container constraints
specify that the element cannot actually begin until the parent time container
begins. Nevertheless, the element behaves as though it had begun earlier.
A negative begin offset can be thought of as defining a clipBegin value (with
the same magnitude) for the first -- and only the first -- iteration of a
repeated element. If no repeat behavior is specified, a negative begin offset
is equivalent to a clipBegin specification with the same magnitude
as the offset value.
@@This must be updated to reflect the impact of parent time container constraints and the DOM methods (especially including the "paused" state, and possibly Active-to-Active transition for seek()).
State paused: In the paused state, an animation continues to perform the transformation of the specified presentation values that were current at the moment of entering the pause state.
Pause transition: Active to Paused This transition may occur if an animation element has its pause() method called while in the active state, or as a result of <excl> stacking behavior and being interrupted by a sibling of the parent <excl>.
Unpause transition: Paused to Active This transition may occur if an animation element has its unpause() method called while in the pause state, or as a result of <excl> unstacking behavior.
At any moment in time, a timed element is in exactly one of the following states: idle, active, finished or frozen. The state transitions are caused by events called start, restart, freeze and stop. Figure 6 shows the legal transitions between the states of an element:
Figure 6: State diagram of an element
The following sections explain the semantics of the states and transitions of a timed element, and explain how to define the state transitions using timing attributes of the element.
Note that the states and transitions are part of the model, and do not imply a particular implementation. Note also that an element may transition through more than one state in a virtual instant (i.e. with no time spent in a given state).
The presentation effect of timed elements is generally to display media, or to play a timeline (e.g. for time containers). In some cases, the element may be an animation that manipulates the presentation, but does not directly display anything. In some integration scenarios, the presentation effect of the element may be to apply a stylesheet, or to otherwise modify the presentation. In these discussions, the common case of displaying media or playing a timeline is used to describe the states and transitions. The same semantics should be understood to apply to all defined actions or presentation effects, as specified in the language that integrates SMIL Timing and Synchronization.
When the document that contains a timed element is first presented, the element is created in the idle state. This is the common starting state for all timed elements.
When a parent time container repeats or is restarted, all (timed) child elements of the time container will be reset. As part of the element reset, the element is re-initialized to the Idle state. See also Resetting element state.
In the idle state a timed element is inactive and does not affect
the presentation of the document in any way. The element simply waits for
the time or event specified in its begin attribute. Note that
the element may transition immediately to the active state if the element
begins immediately when the document begins.
For an element to become active, the element's parent time container must
be active. Given this, a timed element in the idle state
transitions to the active state when the condition specified in
the begin attribute becomes true. As described in the section
on the begin attribute, this condition may depend upon one of
several factors:
Additionally, an element may be started by a DOM beginElement()
or beginElementAt() method call, or as the result of being
the target of an activated hyperlink.
An element may become active as soon as its parent time container becomes
active, if the condition specified in the begin attribute is
true at that point.
Note that the begin attribute can specify a condition that is
a list of values. The specific semantics of evaluating the list of values
is described in the section Basic - begin and
dur.
In the active state, a timed element displays the associated media
or performs the described timeline associated with the element. The active
state includes the entire active duration of the element. The active duration
of an element is specified by the interaction between the dur,
end, repeatDur, and repeatCount attributes
as detailed in the section Computing
the Active Duration.
If a timed element has the fill attribute set to "freeze",
"hold" or "transition", upon reaching the end of its active duration, the
element will transition to the frozen state.
In the frozen state the element will continue to present the last
defined state of visual media or the timeline state at the end of the active
duration (aural media render nothing during the frozen state). The duration
of the frozen state depends upon the value of the fill attribute
(as described in Freezing elements) and on the
parent time container (as described in
Time Container constraints
on child durations).
The frozen state may have 0 duration, e.g. if the parent time container ends with the element.
If a timed element has the fill attribute set to
remove (the default), upon reaching the end of its active duration,
the element will transition to the finished state. Note that the
active duration of a child element may end when an ascendant time container
ends its simple duration.
In the finished state the timed element does not affect the presentation of the document. The duration of the finished state depends upon the parent time container. The finished state lasts until the end of the current simple duration of the parent time container, or until the element is restarted (whichever comes first).
The ability of an element to make this transition depends upon the value
of the restart attribute. If the restart attribute
value is always or whenNotActive the element will
transition to the active state in response to a DOM
beginElement() or beginElementAt() method
call, or an additional begin event. The restart transition effectively resets
the state of the animation element; the element's simple and active duration
must be recomputed as if it were being started for the first time. See also
Resetting element state.
This transition happens the same way as the Frozen to Active transition
(immediately above). If the element specifies an offset from the syncbase
or eventbase, or if the DOM beginElementAt() method call specifies
a non-0 offset, then the element is returned to the Idle state until it actually
restarts.
An element may receive a DOM beginElement() or
beginElementAt() method call or may receive an additional begin
event while in the active state. In this case, if the value of the
restart attribute is "always" the element will re-transition
to the active state and restart as described above. Any other value
for the restart attribute will prevent this transition from
occurring.
This transition happens the same way as the Active to Active transition
(immediately above). If the element specifies an offset from the syncbase
or eventbase, or if the DOM beginElementAt() method call specifies
a non-0 offset, then the element is returned to the Idle state until it actually
restarts.
An element restart can result from a DOM call or an additional begin event,
subject to the restrictions imposed by the restart attribute.
When in the finished state, an element may re-transition to the active state
if the value of the restart attribute is "always" or
"whenNotActive". If the restart attribute
is set to "never", this transition can not occur.
This transition happens the same way as the Finished to Active transition
(immediately above). If the element specifies an offset from the syncbase
or eventbase, or if the DOM beginElementAt() method call specifies
a non-0 offset, then the element is returned to the Idle state until it actually
restarts.
In this specification, elements are described as having local "time". In particular, many offsets are computed in the local time of a parent time container. However, simple durations can be repeated, and elements can begin and restart in many ways. As such, there is no direct relationship between the local "time" for an element, and the real world concept of time as reflected on a clock.
When the time manipulation attributes are used to adjust the speed and/or pacing within the simple duration, the semantics can be thought of as changing the pace of time in the given interval. An equivalent model is these attributes simply change the pace at which the presentation progresses through the given interval. The two interpretations are equivalent mathematically, and the significant point is that the notion of "time" as defined for the simple duration and "local time" should not be construed as real world clock time. For the purposes of SMIL Timing and Synchronization, "time" can behave quite differently from real world clock time.
The SMIL timing model assumes the most common model for interval timing. This describes intervals of time (i.e. durations) in which the begin time of the interval is included in the interval, but the end time is excluded from the interval. This is also referred to as end-point exclusive timing. This model makes arithmetic for intervals work correctly, and provides sensible models for sequences of intervals.
In the real world, this is equivalent to the way that seconds add up to minutes, and minutes add up to hours. Although a minute is described as 60 seconds, a digital clock never shows more than 59 seconds. Adding one more second to "00:59" does not yield "00:60" but rather "01:00", or 1 minute and 0 seconds. The theoretical end time of 60 seconds that describes a minute interval is excluded from the actual interval.
In the world of media and timelines, the same applies: Let a be a video, a clip of audio, or an animation. Assume "A" begins at 10 and runs until 15 (in any units - it does not matter). If "B" is defined to follow "A", then it begins at 15 (and not at 15 plus some minimum interval). When a runtime actually renders out frames (or samples for audio), and must render the time "15", it should not show both a frame of "A" and a frame of "B", but rather should only show the new element "B". This is the same for audio, or for any interval on a timeline. If the model does not use endpoint-exclusive timing, it will draw overlapping frames, or have overlapping samples of audio, of sequenced animations, etc.
Note that transitions from "A" to "B" also adhere to the interval timing model. They do require that "A" not actually end at 15, and that both elements actually overlap. Nevertheless, the "A" duration is simply extended by the transition duration (e.g. 1 second). This new duration for "A" is also endpoint exclusive - at the end of this new duration, the transition will be complete, and only "B" should be rendered - "A" is no longer needed.
For the time model, several results of this are important: the definition of repeat, and the value sampled during the "frozen" state.
When repeating an element simple duration, the arithmetic follows the end-point exclusive model. Consider the example:
<video dur="4s" repeatCount="4" .../>
At time 0, the simple duration is also at 0, and the first frame of video is presented. This is the inclusive begin of the interval. The simple duration proceeds normally up to 4 seconds. However, the appropriate way to map time on the active duration to time on the simple duration is to use the remainder of division by the simple duration:
simpleTime = REMAINDER( t, d ) where
t is within the active duration
Note: REMAINDER( t, d ) is defined as t -
d*floor(t/d)
Using this, a time of 4 (or 8 or 12) maps to the time of 0 on the simple duration. The endpoint of the simple duration is excluded from (i.e. not actually sampled on) the simple duration.
For most continuous media, this aligns to the internal media model, and so no frames (or audio samples) are ever excluded. However for sampled timeline media (like animation), the distinction is important, and requires a specific semantic for handling the frozen state.
The effect of this semantic upon animation functions is detailed in the [SMIL-ANIMATION] module.
A significant motivation for SMIL Boston is the desire to integrate declarative, determinate scheduling with interactive, indeterminate scheduling. The goal is to provide a common, consistent model and a simple syntax.
Note that "interactive" content does not refer simply to hypermedia with support for linking between documents, but specifically to content within a presentation (i.e. a document) that is activated by some interactive mechanism (often user-input events, but including local hyperlinking as well).
SMIL Boston describes extensions to SMIL 1.0 to support interactive timing of elements. These extensions allow the author to specify that an element should begin or end in response to an event (such as a user-input event like "click"), or to a hyperlink activation, or to a DOM method call.
The syntax to describe this uses
event-value specifications and the
special argument value "indefinite" for the begin and
end attribute values. Event values describe user interface and
other events. If an element should only begin (or end) with a DOM method
call, the begin and end attributes allow the special
value "indefinite" to indicate this. Setting begin="indefinite"
can also be used when a hyperlink will be used to begin the element. The
element will begin when the hyperlink is actuated (usually by the user clicking
on the anchor). It is not possible to control the active end of an element
using hyperlinks.
SMIL Boston represents an evolution from earlier multimedia runtimes. These were typically either pure, static schedulers or pure event-based systems. Scheduler models present a linear timeline that integrates both discrete and continuous media. Scheduler models tend to be good for storytelling, but have limited support for user-interaction. Event-based systems, on the other hand, model multimedia as a graph of event bindings. Event-based systems provide flexible support for user-interaction, but generally have poor scheduling facilities; they are best applied to highly interactive and experiential multimedia.
The SMIL 1.0 model is primarily a scheduling model, but with some flexibility to support continuous media with unknown duration. User interaction is supported in the form of timed hyperlinking semantics, but there was no support for activating individual elements via interaction.
To integrate interactive content into SMIL timing, the SMIL 1.0 scheduler model is extended to support several new concepts: indeterminate timing and event-activation.
With indeterminate timing, an element has an undefined begin or
active end time. The element still exists within the constraints of
the document, but the begin or active end time is determined by some external
activation. Activation may be event-based (such as by a user-input
event), hyperlink based (with a hyperlink targeted at the element), or DOM
based (by a call to the beginElement() or
beginElementAt() methods). From a scheduling perspective,
the time can be thought of as unresolved.
The event-activation support provides a means of associating an event with the begin or active end time for an element. When the event is raised (e.g. when the user clicks on something), the associated time is resolved to a determinate time. The actual begin (or end) time is computed as the time the event is raised plus or minus any specified offset.
The computed time defines the synchronization for the element relative to the parent time container. It is possible for the computed begin or end time to occur in the past, e.g. when a negative offset value is specified, or if there is any appreciable delay between the time the event is raised and when it is handled by the SMIL implementation. See also the section Handling negative offsets.
Note that an event based end will not be activated until the
element has already begun. Any specified end event is ignored
before the element begins.
The constraints imposed on an element by its time container are an important aspect of the event-activation model. In particular, when a time container is itself inactive (e.g. before it begins or after it ends), no events are handled by the children. If the time container is frozen, no events are handled by the children. No event-activation takes place unless the time container of an element is active. For example:
<par begin="10s" dur="5s"> <audio src="song1.au" begin="btn1.click" /> </par>
If the user clicks on the "btn1" element before 10 seconds, or after 15 seconds,
the audio element will not play. In addition, if the audio element
begins but would extend beyond the specified active end of the
<par> container, it is effectively cut off by the active end
of the <par> container.
The SMIL Boston timing model supports synchronization based upon DOM events. These can be user interface generated and other kinds of unpredictable events. The model for handling events is that the notification of the event is delivered to the timing element, and the timing element uses a set of rules to resolve any synchronization dependent upon the event.
The semantics of element sensitivity to events are described by the following set of rules:
@@The above semantic is hard to implement based upon a standard DOM event model. We may have to reconsider and say: Note that if the element and its parent time container are both specified to begin with the same event, the behavior is not defined. DOM Level 2 events does not provide a means to order the registered listeners for an event, and so implementations cannot guarantee that the parent will be activated before the child. Authors should avoid this construct in documents.
begin specifies the event, the element begins and any specification
of the event in end is ignored for this event instance.
begin specifies the event, then the behavior depends upon the
value of restart:
restart="always", then a new begin time is resolved for the
element based on the event time. Any specification of the event in
end is ignored for this event instance.
restart="never" or restart="whenNotActive",
then any begin specification of the event is ignored for this
instance of the event. If end specifies the event, an end value
is resolved based upon the event time, and the active duration is re-evaluated
(according to the rules in Computing
the active duration).
It is important to notice that in no case is a single event occurrence used to resolve both a begin and end time on the same element.
These rules can be used with the restart attribute to describe "toggle" activation use cases, as described in the section: Using restart for toggle activation.
Since the same event instance cannot be used to resolve both the begin and end time on a single element, uses like the following will have behavior that may seem non-intuitive to some people:
<audio src="bounce.wav" begin="foo.click" end="foo.click+3s"
restart="whenNotActive"/>
This example will begin repeating the audio clip when "foo" is clicked, and stop the audio clip 3 seconds after "foo" is clicked a second time. It is incorrect to interpret this example as playing the audio clip for 3 seconds after "foo" is clicked. For that behavior, the following markup should be used:
<audio src="bounce.wav" begin="foo.click" dur="3s"
restart="whenNotActive"/>
Related to event-activation is link-activation. Hyperlinking has defined semantics in SMIL 1.0 to seek a document to a point in time. When combined with indeterminate timing, hyperlinking yields a variant on interactive content. A hyperlink can be targeted at an element that does not have a scheduled begin time. When the link is traversed, the element begins. Note that unlike event activation, the hyperlink activation is not subject to the constraints of the parent time container. The details of when hyperlinks activate an element, and when they seek the document timeline are presented in the section Hyperlinks and timing.
Speed modifies the pace of time for the element and its descendents, and so modifies the interpretation of the normal timing attributes with respect to the normal pace of (real-world) time. The attributes dur and repeatDur always specify a time in unmodified local time for the element. As a result, the observed simple duration and repeat duration for an element with a modified speed is not the same as the specified speed. This is important to making the model be consistent when the speed cascades in the time containment hierarchy, although it can make authoring somewhat more complex.
Note that a speed attribute on an element does not affect the element begin
time. It may affect the element end time, if the end is defined only in terms
of the simple duration or repeat duration. An end value (defined by the
end attribute) is converted to element local time using the
speed value. However, the result is that the active duration is not affected
by the speed value, since the values (syncbase values, eventbase times,
wallclocks times, etc.) are all defined in another timespace and converted
to the local timespace. See also the examples below.
To compute the effect of speed on the simple duration or on the active duration if defined with repeat, the following function is used. This function is also used to convert a time in the parent local timespace to a time in the child local timespace that accounts for the speed attribute.
Tpar is the time in the parent local timespace
Tel is the time in the element local timespace
Tel = (Tpar / speed)
When speed is applied to a time container, it scales the rate of progress through the time container timeline. This effect cascades. When descendents also specify a speed value, the parent speed and the child speed are multiplied to yield the result. For example:
<par speed=2.0> <animation begin="2s" dur="9s" speed=0.75 .../> </par>
The observed rate of play of the animation element is 1.5 times the normal play speed. The element begins 1 second after the par begins (the begin offset is scaled by the parent speed), and ends 6 seconds later (9/1.5).
The following example shows how an event based end combines with time filters:
<par speed=2.0>
<animation begin="2s" dur="9s" speed=0.75
repeatCount="4" end="click" .../>
</par>
This behaves as in the first example, but the animation element will repeat 4 times for a total of 24 seconds (in real time), unless a click happens before that. Whenever the click happens, the element ends. A variant on this demonstrates syncbase timing:
<par speed=2.0>
<img id="foo" dur="30s" .../>
<animation begin="2s" dur="9s" speed=0.75
repeatCount="4" end="click, foo.end" .../>
</par>
The image will display for 15 seconds. The animation element plays at an observed rate of 1.5 times play speed, but it will end after 15 seconds, when the image ends. The animation will have repeated 2.5 times at this point. Note that although the animation has a speed value, this does not impact the semantic of the syncbase timing. When the syncbase, eventbase, wallclock or media marker time is observed to happen, it will be applied anywhere it is used at that real time (although various timespace conversions are applied internally).
Note that in the examples above, the default duration of the
<par> container is defined as endSync="last".
This behavior is not affected by the speed modifications, in the sense that
the observed end of the elements will produce the correct simple duration
on the parent time container.
@@ Need to include notes about how begin offsets are also scaled by the parent timescale, but not the local element scale.
The following example illustrates an important effect of offset time scaling:
<par speed=2.0>
<img id="foo" dur="30s" .../>
<animation begin="2s" dur="9s" speed=0.75
repeatCount="4" end="foo.end+6s" .../>
</par>
The image will display for 15 seconds. The animation element plays at an observed rate of 1.5 times play speed, and it will end after 18 seconds. The offset added to the end of the image is scaled by the parent time container speed. The animation will have repeated 3 times at this point.
The following example illustrates the speed modifications along with the fallback for a video element that can only play normal forward speed.
<par speed=2.0>
<video id="foo" dur="30s" accelerate="0.25" ...>
<area begin="2s" dur="4s" .../>
</video>
<animation begin="2s" dur="9s" speed=0.75
repeatCount="4" end="foo.end+6s" .../>
</par>
The video ignores the acceleration and the speed value, and plays at normal speed for 15 seconds. The simple (and active) duration are still constrained by the speed. The area element reflects what the video is playing, and so the area becomes active after 2 seconds and remains active for 4 seconds. The animation element behaves just as it did in the previous example, and ends after 18 seconds and 3 repeats.
@@ This should perhaps move to the Fallbacks section
Some media will use a fallback speed (e.g. because it cannot play at the requested speed - see the section Fallbacks for time filters on a media element). When this is the case, and the simple duration is defined by the implicit media duration, the results are sometimes less clean, as the following example illustrates. Assume that the video has an intrinsic duration of 30 seconds, and that this cannot be determined until the video plays through to the end (as is sometimes the case).
<par speed=1.5>
<video id="foo" repeatCount="3" accelerate="0.25" ...>
<area begin="2s" dur="4s" .../>
</video>
</par>
The video ignores the acceleration and the speed value, and plays at normal speed for 30 seconds. At this point, the simple duration is resolved, and (accounting for the parent speed) is computed to be 20 seconds. At this point the video *should* be halfway into the second repeat iteration, moving 50% faster than normal playspeed. The video should continue playing for another 30 seconds. The fallback rules would define that each repeat iteration would play the first 20 seconds of the 30 second video. Given the linear behavior of the video element, the first 10 seconds will be shown for the (remainder of the) second repeat iteration, followed by the first 20 seconds of video for the third repeat iteration. Needless to say, this may not produce pleasing results. At the very least, authors should avoid mixing speed modifications with media elements that use the implicit simple duration, and cannot play at the specified speed. As stated elsewhere, the time modifications should be used where appropriate.
@@ Note sure if this next is useful:
Note that the timing of all durations, begin and end times and events, is based upon the computed values using the explicit speed and not the fallback speed. If the media player is nominally playing at the explicit speed, but varies somewhat, then the effective speed, end and durations should be used in the time model (i.e. the behavior of the runtime in computing effective versus desired times, and managing synchronization, should be consistent with the behavior of the media player and timing model when the speed is 1.0, or normal play speed). Try: the rules and mechanisms that a runtime uses to compute effective times versus desired times should be consistently applied if the "attempted" speed is normal playspeed (1.0) or offspeed (e.g. 2.0). If the "attempted" speed is not the desired speed, then a strict interpretation of the desired times should be used for effective times.
@@ Proposal to help authors - is this useful?:
Define a new test attribute canPlaySpeed (?) to support alternate presentations depending upon the capabilities of the media players. Should be evaluated at runtime for the given media and associated player. Evaluates true if the media player will attempt to play the specified speed. Evaluates false otherwise. If applied to a time container, then should consider all the descendents of the time container, and only evaluate to true if all descendents will attempt to play the desired speed. In evaluating this, the cascaded desired speed must be used, and not the explicit speed on each element.
@@ Borrow pictures and description of manipulation from keySplines in SMIL Animation? Need to describe that time is actually manipulated and remapped, but can think of it as progress.
The speed or rate of progress through the simple duration must be increased to account for the acceleration/deceleration and preserve the simple duration. The adjusted speed is described as the run rate. For an element with both acceleration and deceleration, the speed over the simple duration varies from 0 up to the run rate and then back down to 0.
To compute the run rate over the course of the simple duration, the following
formula is used. Let a be the value of accelerate,
and b be the value of decelerate. The run rate
r is then:
r = 1 / ( 1 - a/2 - b/2 )
Thus, for example, if the value of accelerate is 1 (i.e. accelerate throughout the entire simple duration), the run rate is 2.
The speed s(t) at any point in time
t (within the simple duration
d) is defined as a function of the run rate,
as follows:
For: ( 0 <= t < (a*d) ) I.e. in the acceleration interval
s(t) = r * ( t / ( a * d ))
For: ( (a*d) <= t <= (d-(b*d)) ) I.e. in the run-rate interval
s(t) = r
For: ( (d-(b*d)) < t <=d) I.e. in the deceleration intervals(t)= r * ( ( t - (d-(b*d)) ) / ( b*d ))
If in place of t we use
p, the proportional progress through the simple
duration, the equations simplify somewhat:
p=t/d
For: ( 0 <= p < a ) I.e. in the acceleration interval
s(p) = r * ( p / a )
For: ( a <= p <= (1-b) ) I.e. in the run-rate interval
s(p) = r
For: ( (1-b) < p <= 1 ) I.e. in the deceleration interval
s(p) = r * ( ( p - (1-b) ) / b )
In this example, a motion path will accelerate up from a standstill over
the first 2 seconds, run at a faster than normal rate for 4 seconds, and
then decelerate smoothly to a stop during the last 2 seconds. This
makes an animation look more realistic. The animateMotion element
is defined in the Animation section of SMIL Boston.
<img ...>
<animateMotion dur="8s" accelerate=".25" decelerate=".25" .../>
</img>
In this example, the image will "fly in" from offscreen left , and then
decelerate quickly during the last second to "ease in" to place. This assumes
a layout model that supports positioning (a similar effect could be achieved
by animation the position of a region in SMIL layout). The
animate element is defined in the Animation section of SMIL
Boston.
<img ...>
<animate attributeName="left" dur="4s" decelerate=".25"
from="-1000" to="0" additive="sum" />
</img>
A simple example is provided in the syntax description above.
In the following example the motion path will behave as above, but will end
at the earlier of 15 seconds or when the user clicks on the image. If the
element ends at 15 seconds (if the user does not click), the motion path
will leave the element at the end of the defined path, 20 pixels to the right.
Note that repeatDur and end are not affected by
the autoReverse attribute (although repeatCount
is).
<img ...>
<animateMotion by="20, 0" dur="5s" autoReverse="true"
repeatDur="15" end="click" fill="freeze"/>
</img>
Accelerate and decelerate can be combined with autoReverse, and are applied to the unmodified simple duration. For example:
<img ...>
<animateMotion by="20, 0" dur="4s" autoReverse="true"
accelerate=".25" decelerate=".25" />
</img>
This will produce a kind of elastic motion with the path accelerating for 1 second from the original position as it moves to the right, moving slightly faster than normal for 2 seconds, and then decelerating for 1 second as it nears the points 20 pixels to the right. It accelerates back towards the original position and decelerates to the end of the reversed motion path, at the original position.
Speed can also be combined with autoReverse, and modifies the entire effect. This example combines all three time manipulations:
<img ...>
<animateMotion by="20, 0" dur="4s" autoReverse="true"
speed="0.5" accelerate=".25" decelerate=".25" />
</img>
This produces the same effect as in the previous example, except that everything moves half as quickly and takes twice as long. The active duration is 16 seconds.
A theoretical model can be described that assumes that all element local timelines (including any media elements) are completely non-linear and have unconstrained ballistics. This ideal model can be applied to many applications, including pure rendered graphics, text, etc. Nevertheless, many common applications also include media with linear behavior and other constraints on playback. When the timegraph includes media elements that have linear behavior, the model must adapt, and/or provide consistent semantics that accommodate these real world constraints. This sections below include a discussion of these fallback semantics.
Note that while the model does support timegraphs with a mix of linear and non-linear behavior, and defines specific semantics for timegraphs that cannot support the ideal non-linear model, it is not a goal to provide an ideal alternative presentation for all possible timegraphs with such a mix. It is left to authors and authoring tools to apply the time manipulations in appropriate situations. This section describes both the ideal model as well as the semantics associated with linear-media elements.
In the ideal model, the pace or speed of local time can be manipulated arbitrarily. The graph advances (or is sampled, depending upon your perspective) as the presentation time advances. A time container samples each of its children in turn, so that a graph traversal is performed for each render time. Elements that are idle or stopped (i.e. neither active nor frozen) are pruned from the traversal as an optimization. As the traversal moves down the graph (from time containers to children), each local timeline simply transforms the current time from the parent time-space to the local time space, and then samples the local timeline at the transformed current time. Note that the speed and effects of the time filters effectively cascade down the time graph, since each element transforms local time for itself and all descendents.
When linear media are added to this model and the "current time" (sample) traversal encounters a media element, the media element is effectively told to "sample" at a particular position. Given that linear media can not sample arbitrarily, the semantic that is used is to verify the current position of the media (as observed on the player) against the current theoretical timeline position for the timegraph. Within certain limits (e.g. defined by a syncTolerance attribute), divergence from the theoretical timeline position are ignored. So far, this is just a typical implementation of a synchronization manager.
When the speed (and even direction) of the local timeline can vary from normal forward playspeed, an additional parameter is added to the context for the "current time" traversal to indicates the speed of the local timeline. The speed is defined as a proportion of normal forward play speed. Thus a value of "1" is normal forward playspeed, a value of "2" is twice normal forward speed, and a value of "-1" is backwards, at the normal play speed. Note that a value of 0 is not allowed. The speed is often broken down logically into the rate and the direction. The rate is just the absolute value of the speed, and the direction is the "sign" of the speed.
Given the current computed position and speed for the timegraph, a media element with linear behavior can handle the case of non-normal playspeed with the use of fallback semantics. The fallback semantics depend upon how much or how little the media player is capable of. Some may play forwards and backwards but only at the normal rate of play, others may only handle normal forward play speed. The fallback semantics are detailed in the next sections.
@@Describe the properties that the fallback semantics are preserving so that the rationale is clear
When any of the time filters are applied directly to a linear media element, the element can directly examine the attributes and apply one set of fallback semantics. These include:
In any case, the simple duration is still constrained by the computed simple
duration, as modified by the time filters. If the computed simple
duration is shorter than the intrinsic media duration at the fallback rate
(usually if the speed is > 1), the media is cut short just as for a
dur value that overrides the intrinsic media duration. If the
specified simple duration is longer than the intrinsic media duration (i.e.
if the rate is < 1), the media should freeze for the difference, just
as it does for dur in SMIL 1.0. When an element must freeze,
it should ideally respect the direction of the speed, using the last frame
for forward speeds and the first frame for backwards speeds.
Note that the semantics of clipBegin and clipEnd are not affected (i.e. they
are still respected).
The clipBegin and clipEnd semantics are always interpreted in terms of
normal forward play speed. I.e. they are evaluated before any effects
of time filters have been applied to the time model. This is consistent with
the model that they can be evaluated by the media element handler, independent
of the time model.
If a fallback semantic is applied to a media element and the media element has child elements (i.e. is a media time container), the local time as passed through to the children should reflect what the media actually performs. Thus if the accelerate and decelerate are ignored, the local time as passed to the child elements should not be filtered for acceleration and deceleration. This is important for cases like anchors that are timed to associate with points in the media. If an author wishes to apply acceleration to the children of the media (e.g. to accelerate a set of animation children), a wrapping <par> with the time filters applied can be inserted under the media time container. See also the next section for a general discussion of time filters and time containers.
@@ Above semantic may be overly complex. It requires that the time runtime know what the media players can actually do so that the proper timing parameters can be passed down to the children. Is this restriction worth it, since playing with a fallback will lead to a different presentation than intended anyway. The presentation visually will act funny anyway, and the entire thing is constrained by the parent time container, so it may not be as bad as noted.
The effect of these fallbacks is illustrated by some examples. In these examples, the video player is assumed to be capable only of normal forward play speed.
In the following example, the acceleration and deceleration can be safely ignored without side effect. Since they are ignored, the area child should become active 1 second after the video begins (i.e. if the video ignores the acceleration, then the effect is not applied to the children either):
<video accelerate=0.5 decelerate=0.5 src...> <area begin=1s .../> </video>
In the following example, the autoReverse attribute doubles the simple duration, but the video will just play the 3 second simple duration twice, in the forward direction both times. The area child should become active 1 second after the video begins each time (i.e. if the video ignores the reverse play, then the speed change is not applied to the children either):
<video autoReverse="true" dur=3s src...> <area begin=1s .../> </video>
In the following example, the speed attribute decreases the simple duration by 33% (i.e. one third). The video will play the first two seconds at normal playspeed, rather than three seconds at 1.5 times playspeed. The area child should become active 1 second after the video begins each time (i.e. if the video ignores the speed manipulation, then the speed change is not applied to the children either):
<video speed="1.5" dur=3s src...> <area begin=1s .../> </video>
When any of the time filters are applied to a time container element, the implementation should generally respect the time filters and deal with the effects of modified time for each media player. Nevertheless, in some situations it may be desirable to apply a fallback semantic at the time container itself.
@@Need more info on how time containers run backwards. The use of autoReverse is special, as it can generally use the end and begin times from the forward half. Note that time dependencies work backwards, but that event-based timing does not work correctly, and script events may not be fired. This needs more discussion. Need a reasonable means of simplifying the implementation and still allowing backwards play of fully scheduled timegraphs.
@@Examples!
In the theoretical model, each element has a notion of local time that extends from 0 to some duration (the simple duration). To obtain the local time for an element in the simple case with no pacing control, an arithmetic transform is performed for the element and each ascendant time container up to the document root. This transform allows for the begin offset of the element relative to its parent time container, and for any repeat behavior of the element. The pseudo-code for this transform looks like:
@@Edit to make it produce both active and simple time? The only difference is in how the final element is handled. Also need to have a control over unconstrained versus constrained time. Sync arc calculation uses unconstrained time, but real sampling uses end-constrained time that respects freeze.
@@ Edit to use the most recent parent begin if it is active, rather than the constant repeatDuration
// globalToLocalTime()
//
// Recurses up to document root, and then transforms timeIn
// from parent to local time as the recursion unwinds
// Version 1: Ignores fill=freeze, assumes constant pace of time
// assumes repeat duration is constant,
// and assumes all times are resolved.
// Inputs:
// timeIn - the parent simple time
// Outputs:
// timeOut - the resulting local simple time
//
TimeInstant globalToLocalTime( TimeInstant timeIn )
{
TimeInstant timeOut;
Node parent = getParent();
if( !parent.isDocumentTimeRoot() )
{
// We have a parent time container above us.
// First convert timeIn to simple local time for it,
// and then adjust for this node.
timeOut = parent.globalToLocalTime( timeIn );
}
else // Our parent is the top time container
timeOut = timeIn;
// Adjust for begin offset. Assume "begin" has simplified begin time.
timeOut = timeOut - this.begin; // Adjust for begin offset
if( this.repeats() )
{
// Adjust for repeating simple duration.
timeOut = timeOut % this.repeatDuration;
}
// timeOut is now converted to simple local time for this node
return timeOut;
}
@@Next version shows how this is extended to support the time manipulations. For autoReverse, remember the edge case for the endpoint. ?Talk about the imprecision around the reverse point: interval math introduces an epsilon inaccuracy at the end, where epsilon is the sample granularity (e.g. milliseconds in our implementation).
@@How to show the update dependencies of repeatDur, segmentDur? Could just show a method to recompute, that is called on changes. Ignore all optimization issues.
@@@ Need to discuss how to convert a time specified as a syncbase-value (and by extension a wallclock-value or an event in document or system time ) to a time on the parent time container local timeline. Especially given the wallclock stuff, we need to consider the name "local time".
Define the notion of document global time, and note that it is the normalized timeline used to convert between different timespaces. Operations defined are GlobalToLocal, and LocalToGlobal. To convert from one timespace to another, simply convert the first time from local to global, and then from global to local for the second tiespace.
Basic mechanism for global to local conversion is iterate downward from the document body to the element converting the global time to a time on each time container encountered along the way. This is often implemented as a recursive algorithm, where the recursion moves from the local element up to the document body, and the work is done as the recursion "unwinds". For each time container, subtract the begin offset of the time container and then, use the remainder after dividing by the simple duration (which may vary over time - yuck!) or subtract the offset of the current repeat iteration from the begin time (better when working on begin resolution on a reset). Then, apply any filters for time manipulations (speed, acclerate/decelerate, autoReverse).
Basic mechanism for localToGlobal reverses the above algorithm. If the element is active, then the effective begin time of the current iteration of all (repeating) ancestor time containers is used when adding the begin offset. If the element is not active, then for each ancestor time container that is not active, the earliest begin time is used.
Note that the pure conversions do not take into account the clamping of active durations, nor the effects of fill (where time is frozen). Global to local time conversions used to translate between timespaces must ignore these issues, and so may yield a time in the destination local timespace that is well before or well after the simple duration of the element.
An alternate form of the conversion is used when actually sampling the time graph. A time container is only sampled if it is active or frozen, and so no times will be produced that are before a time container begins. If the global to local time conversion for a time container yields a time during which the time container is frozen, the time is clamped to the value of the active end.
In evaluating the list of begin values, one of two questions is asked:
The earliest time is used for example when an element is the target of a hyperlink activation (see also Hyperlinks and timing). The next begin time may be used by a scheduler when an element can begin more than once.
By the same token, a list of end values is evaluated for two cases:
The earliest time is used for example to calculate the simple duration of
a time container defined with endSync. The next end time may
be used by a scheduler when an element can begin more than once.
Begin and end time lists are considered in tandem. Begin times can be constrained by an end specification, as well as by the parent time container. Note that if an end time is not resolved, or is specified as "indefinite", then it is considered to be "after" al begin times. The constraint rules are:
end list are resolved and occur before a begin time, then the
element does not begin.
Thus, for a given point when an element is not active, the next begin and
end time are derived from the begin and end lists
as follows:
If both Steps 1 and 2 fail, then the begin list only specifies resolved times in the past, or after the end of the parent time container. In this case, there is no (valid) begin time after the current time (although for the purposes of a hyperlink that targets the element, even an "invalid" time may be used with additional constraints - see Hyperlinks and timing).
If a begin time is found, then it must be checked against the list of end
times. If no end attribute is specified, the end is "indefinite",
and any begin found above is valid. If any end values are specified, the
list is evaluated as follows:
If a valid begin and end value are found, the end value found with the rules above is the value used in Computing the Active Duration.
When it is specified that the element does not begin, begin and end events will not be raised in the DOM, and time dependents defined relative to the begin or end of this element will not be activated.
In contrast to the rules above, if a beginElement() or a
beginElementAt() call specifies a begin time after the last
end time (with no unresolved end times), the active duration is indefinite,
as though end had been defined as "indefinite". Note that if
beginElement() or beginElementAt() is called when
the parent time container is not active, the method call will have no
effect. If beginElementAt() is called and specifies a
time after the end of the parent simple duration, the method call will have
no effect. See also Supported methods.
Hyperlinking semantics must be specifically defined within the time model in order to ensure predictable behavior. Earlier hyperlinking semantics, such as those defined by SMIL 1.0 are insufficient because they do not handle indeterminate and interactive timing, nor do they handle author-time restart restrictions. Here we extend SMIL 1.0 semantics for use in presentations using elements with indeterminate timing, interactive timing, and author-time restart restrictions.
A hyperlink may be targeted at an element by specifying the value of the
id attribute of an element in the fragment part of the link
locator. Traversing a hyperlink that refers to a timed element will behave
according to the following rules:
beginElement() method call), seek the document
time (forward or back, as needed) to the earliest resolved begin time of
the target element. Note that the begin time may be resolved as a result
of an earlier hyperlink, DOM or event activation. Once the begin time is
resolved (and until the element is reset, e.g. when the parent repeats),
hyperlink traversal always seeks. For a discussion of "reset", see
Resetting element state. Note
also that for an element begin to be resolved, the begin time of all ancestor
elements must also be resolved.
In the above rules, the following additional constraints must also be respected:
Note that the first constraint means that a hyperlink to a child of a time container will never seek to a time earlier than the beginning of the time container. The second constraint implies that a hyperlink to a child that begins after the end of the parent simple duration will seek to the end of the parent, and proceed from there. While this may produce surprising results, it is the most reasonable fallback semantic for what is essentially an error in the presentation.
If a seek of the presentation time is required, it may be necessary to seek either forward or backward, depending upon the resolved begin time of the element and the presentation current time at the moment of hyperlink traversal.
After seeking a document forward, the document should be in the same state as if the user had allowed the presentation to run normally from the current time until reaching the element begin time (but had otherwise not interacted with the document). In particular, seeking the presentation time forward should also begin any other elements that have resolved begin times between the current time and the seeked-to time. The elements that are begun in this manner may still be active, may be frozen, or may already have ended at the seeked-to time. If an element has ended, it logically begins and ends during the seek. The associated DOM events are raised, and all time dependents are updated. Also any elements currently active at the time of hyperlinking should "fast-forward" over the seek interval. These elements may also be active, frozen or already ended at the seeked-to time. The net effect is that seeking forward to a presentation time puts the document into a state identical to that as if the document presentation time advanced undisturbed to reach the seek time.
If the resolved activation time for an element that is the target of a hyperlink traversal occurs in the past, the presentation time must seek backwards. Seeking backwards will rewind any elements active during the seek interval and will turn off any elements that are resolved to begin at a time after the seeked-to time. Note that resolved begin times (e.g. a begin associated with an event) are not cleared or lost by seeking to an earlier time. Note further that seeking to a time before a resolved begin time does not affect the interpretation of a "restart=never" setting for an element; once the begin time is resolved, it cannot be changed or restarted. Subject to the rules above for hyperlinks that target timed elements, hyperlinking to elements with resolved begin times will function normally, advancing the presentation time forward to the previously resolved time. When the document seeks backwards before a resolved begin for an element time, this does not reset the element
These hyperlinking semantics assume that a record is kept of the resolved begin time for all elements, and this record is available to be used for determining the correct presentation time to seek to. Once resolved, begin times are not cleared by hyperlinking. However, they can be overwritten by subsequent resolutions driven by multiple occurrences of an event (i.e. by restarting). For example:
<par begin="0"> <img id="A" begin="10s" .../> <img id="B" begin="A.begin+5s" .../> <img id="C" begin="click" .../> <img id="D" begin="C.begin+5s" .../> ... <a href="#D">Click here!</a> </par>
The begin time of elements "A" and "B" can be immediately resolved to be at 10 and 15 seconds respectively. The begin of elements "C" and "D" are unresolved when the document starts. Therefore activating the hyperlink will have no effect upon the presentation time or upon elements "C" and "D". Now, assume that "C" is clicked at 25 seconds into the presentation. The click on "C" in turn resolves "D" to begin at 30 seconds. From this point on, traversing the hyperlink will cause the presentation time to be seeked to 30 seconds.
If at 60 seconds into the presentation, the user again clicks on "C", "D" will become re-resolved to a presentation time of 65 seconds. Subsequent activation of the hyperlink will result in the seeking the presentation to 65 seconds.
If the time container were defined to repeat, or could restart, then all indeterminate times for children of the time container are cleared (reset to "indefinite") when the parent time container repeats or restarts. See also Resetting element state.
For a child of a sequence time container, if a hyperlink targeted to the child is traversed, this seeks the sequence to the beginning of the child. If the seek is forward in time and the child does not have a resolved begin time, the document time must seek past any scheduled active end on preceding elements, and then activate the referenced child. In such a seek, if the currently active element does not have a resolved active end, it should be ended at the current time. If there are other intervening siblings (between the currently playing element and the targeted element), the document time must seek past all scheduled times, and resolve any unresolved times as seek proceeds (time will resolve to intermediate values of "now" as this process proceeds). As times are resolved, all associated time dependents get notified as the intervening elements are activated and deactivated.
When beginElement() or beginElementAt() is called
for the child of a sequence time container (subject to restart semantics),
any currently active or frozen child is stopped and the new child is begun
at the current time (even if the element has a scheduled begin time). Unlike
hyperlinking, no seek is performed. The sequence will play normally following
the child that is begun with the method call (i.e. as though the child had
begun at its normal time).
@@The above semantic keeps things simple, although it may point up a need for a seekToElement() method in the DOM interface that mimics the hyperlink functionality.
Note that if a hyperlink targets (or if beginElement() or
beginElementAt() is called for) an element A
defined to begin when another element B ends, and the other
element B has (e.g.) an event-base or syncbase end, the
hyperlink or method call will not end element B. It will
only active element A. If the two elements are siblings
within a <seq> or <excl> time container,
the parent time container enforces its semantics and stops (or pauses) the
running element.
@@ What if the target element is in an excl and is in a priorityclass that is defined to be deferred if it tries to interrupt the current (higher priority) element? Do we overrule the priorityClass rules, and just stop or pause the running element?
Note that the presentation agent need not actually prepare any media for elements that are seeked over, but it does need to propagate the sync behavior to all time dependents so that the effect of the seek is correct.
There are several cases in which times may change as the document is presented. In particular, when an element time is defined relative to an event, the time (i.e. the element begin or active end) is resolved when the event occurs. Another case arises with restart behavior - both the begin and active end time of an element can change when it restarts. Since the begin and active end times of one element can be defined relative to the begin or active end of other elements, any changes to times must be propagated throughout the document.
When an element "foo" has a begin or active end time that specifies a syncbase element (e.g. "bar" as below):
<img id="foo" begin="bar.end" .../>
we say that "foo" is a time-dependent of "bar" - that is, the "foo" begin time depends upon the active end of "bar". Any changes to the active end time of "bar" must be propagated to the begin of "foo". The effect of the changes depends upon the state of "foo" when the change happens. The rest of the section describes the specific rules for propagating changes.
Note that it is possible for the syncbase element "bar" to end again, if it is restarted. When "bar" restarts, the a new end time is calculated and all time dependents are notified of the change. For example:
<img id="foo" begin="0" end="bar.end" .../> <img id="bar" begin="btn.click" dur="5s" .../>
Element "foo" will end when "bar" ends, however "bar" can restart on another click. When "bar" restarts, a new end is calculated, and "foo" is notified. However, as "foo" will not restart, the change is ignored. A variant on this illustrates a case when the time change does propagate through:
<img id="foo" begin="0" end="bar.end+10s" .../> <img id="bar" begin="btn.click" dur="5s" .../>
Element "foo" will end 10 seconds after "bar" ends. If "bar" is restarted within 10 seconds of when it first ended, "foo" will still be active, and the changed end time will propagate through. Using example times, if the user clicks on the "btn" element 8 seconds after the parent time container begins, "bar" begins at 8 seconds and will end at 13 seconds. Element "foo" would then end at 23 seconds. If the users clicks "btn" again 3 seconds after "bar" ends, (i.e. at 16 seconds), the end of "bar" now has the value of 21 seconds. This change propagates to "foo", and "pushes out" the end of "foo" until 31 seconds.
The rule is that once an element has ended its active duration, changes that affect its end time are ignored (within the current simple duration of the parent time container).
When an element restarts (or when an ascendant time container repeats or restarts), all child times are recalculated, and may again become indefinite. For example:
<img id="foo" begin="btn.click" end="mouseout" .../> <img id="bar" begin="btn.click" end="foo.end" .../>
Both elements will start when the "btn" element is first clicked. Element "foo" will end when "mouseout" is raised on the img. At this point, the active duration of "bar" will become defined (resolved), and "bar" will end the active duration. If the user clicks on the target element again, both elements will restart, and "bar" will once again have an indefinite active duration.
@@Add additional example with explanation. Note that if user clicks at 4s, image1 is never seen. The resolution of image end happens and sticks when image1 begin is evaluated.
<par> <img id="image3" dur = "24s" end="user.click"/> <img id="image1" begin ="10s" end="image3.end"/> </par>
The use of negative offsets to define begin times merely defines the synchronization relationship of the element. If does not in any way override the time container constraints upon the element, and it cannot override the constraints of presentation time.
If an element has a begin time that resolves to a time before the parent time container begins, the parent time container constraint still applies. For example:
<par> <video begin="-5s" dur="30s" src="movie.mpg" /> </par>
The video element cannot begin before the par begins.
The begin is simply defined to occur "in the past" when the
par begins.
A begin or end time may be specified with a negative offset relative to an event or to a syncbase that is not initially resolved. When the syncbase or eventbase time is (eventually) resolved, the dependent time that is computed with a negative offset may occur in the past. The computed time defines the scheduled synchronization relationship of the element, even if it is not possible to begin or end the element at the computed time.
When a begin time is defined to be in the past, the element begins immediately,
but acts as though it had begun at the specified time (playing from an offset
into the media). The behavior can be thought of as a clipBegin
value applied to the element, that only applies to the first iteration of
repeating elements. The media will actually begin at the time computed according
to the following equation:
Let o be the offset value, d is the simple duration, AD is the active duration. If AD is indefinite, it compares greater than any value of o or ABS(o). REM( x, y ) is defined as x - floor( x/y ). If y is indefinite, REM( x, y ) is just x.
If ABS(o) >= AD the element does not begin.
Else the element media begins at REM( ABS(o), d ).
If the element repeats, the iteration value of the repeat event
has the calculated value based upon the computed begin, and not the observed
number of repeats. Thus for example:
<ref begin="foo.click-8s" dur="3s" repeatCount="10" .../>
The element begins when the user clicks on the element "foo". It begins to
play at 2 seconds into the 3 second simple duration. Any time dependents
are activated relative to the computed begin time, and not the observed begin
time. The begin event is raised when the element begins,
but has a timeStamp value that corresponds to the defined begin
time, 8 seconds earlier. One second later, the element will repeat, and the
associated repeat event will have the iteration value set to
3 (it is zero based). The element will end 22 seconds after the click.
Note: If script authors wish to distinguish between the computed repeat
iterations and observed repeat iterations, they can count actual
repeat events in the associated event handler.
Media elements with an active duration of zero or with the same begin and end time trigger begin and end events, and propagate to time dependents. If an element's end time is before its begin time, no events are triggered (see also Evaluation of begin and end time lists).
Whether or not media is retrieved and/or rendered is implementation dependent.
When a time container repeats or restarts, all descendent children are "reset" with respect to certain state:
restart
semantics is reset. Thus, for example if an element specifies
restart="never", the element can begin again after a reset.
The restart="never" setting is only defined for the extent of
the parent time container simple duration.
When an element restarts, the rules 1 and 2 are also applied to the element itself, although the rule 3 (controlling restart behavior) is not applied.
Note that when any time container ends its simple duration (including when it repeats), all timed children that are still active are ended. See also Time container constraints on child durations.
When an <excl> time container restarts or repeats, in
addition to ending any active or paused children, the pause queue
for the <excl> is cleared.
Proposed new support in SMIL Boston introduces finer grained control over the runtime synchronization behavior of a document. The syncBehavior attribute allows an author to describe for each element whether it must remain in a hard sync relationship to the parent time container, or whether it can be allowed slip with respect to the time container. Thus, if network congestion delays or interrupts the delivery of media for an element, the syncBehavior attribute controls whether the media element can slip while the rest of the document continues to play, or whether the time container must also wait until the media delivery catches up.
The syncBehavior attribute can also be applied to time containers. This controls
the sync relationship of the entire timeline defined by the time
container. In this example, the audio and video elements are defined
with hard or "locked" sync to maintain lip sync, but the "speech"
<par> time container is allowed to slip:
<par> <animation src="..." /> ... <par id="speech" syncBehavior="canSlip" > <video src="speech.mpg" syncBehavior="locked" /> <audio src="speech.au" syncBehavior="locked" /> </par> ... </par>
If either the video or audio must pause due to delivery problems, the entire "speech" par will pause, to keep the entire timeline in sync. However, the rest of the document, including the animation element will continue to play normally. Using the syncBehavior attribute on elements and time containers, the author can effectively describe the "scope" of runtime sync behavior, defining some portions of the document to play in hard sync without requiring that the entire document use hard synchronization.
This functionality also applies when an element first begins, and the media must begin to play. If the media is not yet ready (e.g. if an image file has not yet downloaded), the syncBehavior attribute controls whether the time container must wait until the element media is ready, or whether the element begin can slip until the media is downloaded.
The syncBehavior can affect the effective begin and effective end of an element, but the use of the syncBehavior attribute does not introduce any other semantics with respect to duration.
When the syncBehavior attribute is combined with interactive begin timing for an element, the syncBehavior only applies once the sync relationship of the element is resolved (e.g. when the specified event is raised). If at that point the media is not ready and syncBehavior is specified as "locked", then the parent time container must wait until the media is ready. Once an element with an interactive begin time has begun playing, the syncBehavior semantics described above apply as thought the element were defined with scheduled timing.
The syncBehavior attribute is subordinate to any sync relationships defined by time containers, sync arcs, event arcs, etc. The syncBehavior attribute has no bearing on the formation of the time graph, only the enforcement of it.
@@ Need to address how syncBehavior will interact with restart semantics. In particular, do we re-establish the sync relationship when it restarts (this is my first guess, assuming that restart is allowed). syncBehavior is not supposed to define all the behavior of the element, but rather just the behavior when there is some problem with sync, or when the user pauses or seeks an element. E.g. we do not require that children of seq elements be locked, but we do require that the seq semantics be maintained. If restart is allowed, then that should be orthogonal to the syncBehavior. Note that all other aspects of timing (e.g. repeat, parent constraints and event-based timing override the syncBehavior, so I think we have a precedent.
Note that the semantics of syncBehavior do not describe or require a particular approach to maintaining sync; the approach will be implementation dependent. Possible means of resolving a sync conflict may include:
Additional control is provided over the hard sync model using the syncTolerance attribute. This specifies the amount of slip that can be ignored for an element. Small variance in media playback (e.g. due to hardware inaccuracies) can often be ignored, and allow the overall performance to appear smoother.
@@ We need to move the general definition of the SMIL Default stuff elsewhere, and just specify the possible arg values here. Ideas where it should go?
The value of the syncBehavior and syncTolerance attributes are not inherited, but it is possible to set default behavior for elements or time containers using the SMILdefault syntax below.
The default value for syncTolerance is implementation dependent, but should be no greater than two seconds.
syncBehavior="locked". This allows a locked sync relationship
to ignore a given amount of slew without forcing resynchronization. resolution.
syncBehavior:canSlip
syncBehavior:locked
syncTolerance:
Clock-value
An additional proposed extension allows the author to specify that a particular element should define or control the synchronization for a time container. This is similar to the default behavior of many players that "slave" video and other elements to audio, to accommodate the audio hardware inaccuracies and the sensitivity of listeners to interruptions in the audio playback. The syncMaster attribute allows an author to explicitly define that an element defines the playback "clock" for the time container, and all other elements should be held in sync relative to the syncMaster element.
In practice, linear media often need to be the syncMaster, where non-linear media can more easily be adjusted to maintain hard sync. However, a player cannot always determine which media behaves in a linear fashion and which media behaves in a non-linear fashion. In addition, when there are multiple linear elements active at a given point in time, the player cannot always make the "right" decision to resolve sync conflicts. The syncMaster attribute allows the author to specify the element that has linear media, or that is "most important" and should not be compromised by the syncBehavior of other elements.
The syncMaster attribute interacts with the syncBehavior attribute. An element with syncMaster set to true will define sync for the "scope" of the time container's synchronization behavior. That is, if the syncMaster element's parent time container has syncBehavior="locked", the syncMaster will also define sync for the ancestor time container. The syncMaster will define sync for everything within the closest ancestor time container that is defined with syncBehavior="canSlip".
The syncMaster attribute only applies when an element is active. If more than one element within the syncBehavior scope has the syncMaster attribute set to true, and the elements are both active at any moment in time, the behavior will be implementation dependent.
The default value is false.
The associated property is read-only, and cannot be set by script.
@@ Need to decide whether endSync belongs on media elements (with timed children) or not.
Timing attributes
<!ENTITY % timingAttrs
begin CDATA #IMPLIED
dur CDATA #IMPLIED
end CDATA #IMPLIED
restart (always | never |
whenNotActive) "always"
repeatCount CDATA #IMPLIED
repeatDur CDATA #IMPLIED
fill (remove | freeze | hold) "remove"
>
Runtime sync behavior attributes
<!ENTITY % runtimeSyncBvrAttrs syncBehavior (locked | canSlip) #IMPLIED defaultSyncBehavior (locked | canSlip) "canSlip" syncTolerance CDATA #IMPLIED defaultSyncTolerance CDATA #IMPLIED syncMaster (true | false) "false" >
Time container elements
<!ELEMENT par ???> <!ATTLIST par %timingAttrs %runtimeSyncBvrAttrs id ID #IMPLIED endSync CDATA #IMPLIED >
<!ELEMENT seq ???> <!ATTLIST seq %timingAttrs %runtimeSyncBvrAttrs id ID #IMPLIED >
<!ELEMENT excl ???> <!ATTLIST excl %timingAttrs %runtimeSyncBvrAttrs id ID #IMPLIED endSync CDATA #IMPLIED >
@@When this settles down, fill in subsections in TOC
This section describes what a language designer must actually do to specify the integration of SMIL Timing and Synchronization support into a host language. This includes basic definitions, constraints upon specification, and allowed/supported events.
The host language designer must define some basic concepts in the context of the particular host language. These provide the basis for timing and presentation semantics.
The host language designer must define what "presenting a document" means. A typical example is that the document is displayed on a screen.
The host language designer must define the document begin. Possible definitions are that the document begins when the complete document has been received by a client over a network, or that the document begins when certain document parts have been received.
The host language designer must define the document end. This is typically when the associated application exits or switches context to another document.
@@ Check to see if we really have issues with this, e.g. for specifying floating point values. Can we use this in our definitions of offset value?
The host language must specify the formats supported for numeric attribute
values. This includes integer values and especially floating point values
for attributes such as keyTimes and keySplines.
As a reasonable minimum, host language designers are encouraged to support
the format described in [CSS2]. The specific reference within the CSS
specification for these data types is
4.3.1 Integers and
real numbers.
@@ Need to talk about specifying which elements can be timed, and what it means to time them.
The set of elements that may have timing includes ??? elements defined in host languages.
The host language must specify which event names are legal in event base values. If the host language defines no allowed event names, event-based timing is effectively precluded for the host language.
Host languages may specify that dynamically created events (as per the [DOM2Events] specification) are legal as event names, and not explicitly list the allowed names.
The host language designer may impose stricter constraints upon the error handling semantics. That is, in the case of syntax errors, the host language may specify additional or stricter mechanisms to be used to indicate an error. An example would be to stop all processing of the document, or to halt all animation.
@@Broken link to Handling errors - Do we need a section on this?
Host language designers may not relax the error handling specifications,
or the error handling response (as described in "Handling syntax errors").
For example, host language designers may not define error recovery semantics
for missing or erroneous values in the begin or end
attribute values.
Language designers can choose to integrate SMIL Timing and Synchronization as an independent namespace, or can integrate SMIL Timing and Synchronization names into a new namespace defined as part of the host language. Language designers that wish to put the SMIL Timing and Synchronization functionality in an isolated namespace should use the following namespace:
@@ URI to be confirmed by W3C webmaster
http://www.w3.org/2000/TR/smil20
Any XML-based language that integrates SMIL Timing will inherit the basic interfaces defined in DOM [DOM2] (although not all languages may require a DOM implementation). SMIL Timing specifies the interaction of timing functionality and DOM. SMIL Timing also defines constraints upon the basic DOM interfaces, and specific DOM interfaces to support SMIL Timing.
Much of the related SMIL-DOM functionality is proposed in the [SMIL-DOM] section. We may need to go into further detail on the specific semantics of the interfaces - the sections below are placeholders.
Define rules on element and attribute access (inherit from and point to Core DOM docs for this). Define mutation constraints. This is currently covered in the [SMIL-DOM] section.
SMIL event-timing assumes that the host language supports events, and that the events can be bound in a declarative manner. DOM Level 2 Events [DOM2Events] describes functionality to support this.
The specific events supported are defined by the host language. If no events are defined by a host language, event-timing is effectively omitted.
The [SMIL-DOM] section defines the initial set of time-related events that have been proposed.
SMIL Timing supports two methods for controlling the timing of elements:
beginElement() and endElement(). These methods
are used to begin and end the active duration of an element. Authors can
(but are not required to) declare the timing to respond to the DOM using
the following syntax:
<img begin="indefinite" end="indefinite" .../>
The beginElement(), beginElementAt() and
endElement() methods are all subject to time container constraints
in much the same way that event-based times are. If any of these methods
are called when the parent time container is not active, the methods have
no effect.
Calling beginElement() causes the element to begin in the same
way that an element with event-based begin timing begins. The effective begin
time is the current presentation time at the time of the DOM method call.
Note that beginElement() is subject to the restart
attribute in the same manner that event-based begin timing is. If an element
is specified to disallow restarting at a given point,
beginElement() methods calls must fail. Refer also to the section
Restarting elements.
Calling beginElementAt() causes the element to begin in the
same way that an element begins with event-based begin timing that includes
an offset.
beginElementAt() is positive, then the
element will be restarted (subject to the restart attribute
semantics) at the specified offset into the future from the current time.
If the specified time is past the end of the parent time container simple
duration, the element may be stopped by the restart semantics and yet not
restart (due the the time container constraints).
beginElementAt() is negative, then the
element is restarted in the same manner (and subject to the same constraints)
as for the beginElement() method. However, the element is begun
as though it had begun at the earlier offset. For details see
Handling negative offsets.
Calling endElement() causes an element to end the active duration,
just as end does. Depending upon the value of the
fill attribute, the element effect may no longer be applied,
or it may be frozen at the current effect. Refer also to the section
Freezing elements. If an element is not currently
active (i.e. if it has not yet begun or if it is frozen), the
endElement() method will fail.
Interfaces are currently defined in the [SMIL-DOM] section.
The following concepts are the basic terms used to describe the timing model.
A time graph is used to represent the temporal relations of elements in a document with SMIL timing. Nodes of the time graph represent elements in the document. Parent nodes can "contain" children, and children have a single parent. Siblings are elements that have a common parent. The links or "arcs" of the time graph represent synchronization relationships between the nodes of the graph.
Note that this definition is preliminary.
The time model description uses a set of adjectives to describe particular concepts of timing:
An element is considered to have scheduled timing if the element's start time is given relative to the begin or active end of another element. A scheduled element can be inserted directly into the time graph.
Begin and active end times in SMIL Boston can be specified to be relative
to events that are raised in the document playback environment. This
supports declarative, interactive timing. Interactive in this
sense includes user events such as mouse clicks, events raised by media players
like a mediaComplete event, and events raised by the presentation
engine itself such as a pause event.
More information on the supported events and the underlying mechanism is described in the DOM section of this draft [SMIL-DOM].
In scheduled timing, elements are timed relative to other elements. The syncbase for an element A is the other element B to which element A is relative. More precisely, it is the begin or active end of the other element. The syncbase is not simply a scheduled point in time, but rather a point in the time graph.
Note that this definition is preliminary. The name may also change.
"Sync-arc" is an abbreviation for "synchronization arc". Sync-arcs are used to relate nodes in the time graph, and define the timing relationship between the nodes. A sync-arc relates an element to its syncbase. The sync-arc may be defined implicitly by context, explicitly by id-ref or event name, or logically with special syntax.
Note that this definition is preliminary.
A Clock is a particular timeline reference that can be used for synchronization. A common example that uses real-world local time is referred to as wall-clock timing (e.g. specifying 10:30 local time). Other clocks may also be supported by a given presentation environment.
A hyperlink into or within a timed document may cause a seek of the current presentation time or may activate an element (if it is not in violation of any timing model rules).
During playback, an element may be activated automatically by the progression
of time, via a hyperlink, or in response to an event. When an element is
activated, playback of the element begins.
SMIL includes support for declaring media, using element syntax defined in "The SMIL Media Object Module". The media that is described by these elements is described as either discrete or continuous:
Time containers group elements together in time. They define common, simple synchronization relationships among the grouped child elements. In addition, time containers constrain the time that children may be active. Several containers are defined, each with specific semantics and constraints on its children.
SMIL timing and synchronization support ultimately controls a set of content or media elements. The content includes things like video and audio, images and vector graphics, as well as text or HTML content. SMIL documents use the SMIL media elements to reference this content. XML and HTML documents that integrate SMIL Boston functionality may use SMIL media elements and/or content described by the integrated language (e.g. paragraphs in HTML).
All elements - content/media as well as time containers - support timing markup to describe a begin time and a duration, as well as the ability to play repeatedly. There are several ways to define the begin time. The semantics vary somewhat depending upon an element's time container.
The time model defines two concepts of duration for each element - the simple duration and the active duration. These definitions are closely related to the concept of playing something repeatedly.
The constraints of a parent time container may override the duration of its children. In particular, a child element may not play beyond the simple end of the time container.
The terms for these durations can be modified with the Descriptive Terms for Times, to further distinguish aspects of the time graph.
Time manipulations allow the element's time (within the simple duration) to be filtered or modified. For example the speed of time can be varied to make the element play faster or slower than normal. The filtered time affects all descendents of the element. Several time manipulations are proposed for SMIL Boston. Time manipulation is primarily intended to be used with animation [SMIL-ANIMATION] (W3C members only).
Note that any time manipulation that changes the effective play speed of an element's time may conflict with the basic capabilities of some media players. The use of these manipulations is not recommended with linear media players, or with time containers that contain linear media elements, such as streaming video.
There are a number of unresolved issues with this kind of time manipulation, including issues related to event-based timing and negative play speeds, as well as many media-related issues.
Using simple, scheduled timing, a time graph can be described in which all the times have a known, defined sync relationship to the document timeline. We describe this as determinate timing.
When timing is specified relative to events or external clocks, the sync relationship is not initially defined. We describe this as indeterminate timing.
A time is resolved when the sync relationship is defined, and the time can actually be scheduled on the document time graph.
Indeterminate times that are event-based are resolved when the associated event occurs at runtime - this is described more completely in the section Unifying Scheduling and Interactive Timing. Indeterminate times that are defined relative to external clocks are usually resolved when the document playback begins, and the relationship of the document timeline to the external clock reference is defined.
A determinate time may initially be unresolved, e.g. if it is relative to an unknown time such as the end of a streaming MPEG movie (the duration of an MPEG movie is not known until the entire file is downloaded). When the movie finishes, determinate times defined relative to the end of the movie are resolved.
SMIL 1.0 introduced the notion of synchronization behavior, describing user agent behavior as implementing either "hard synchronization" or "soft synchronization". Using hard sync, the entire presentation would be constrained to the strict description of sync relationships in the time graph. Soft sync allowed for a looser (implementation dependent) performance of the document.
While a document is playing, network congestion and other factors will sometimes interfere with normal playback of media. In a SMIL 1.0 hard sync environment, this will affect the behavior of the entire document. In order to provide greater control to authors, SMIL Boston extends the hard and soft sync model to individual elements. This support allows authors to define which elements and time containers must remain in strict or "hard" sync, and which elements and time containers can have a "soft" or slip sync relationship to the parent time container.
This section includes a set of examples that illustrate both the usage of the SMIL syntax, as well as the semantics of specific constructs. This section is informative.
Note: In the examples below, the additional syntax related to layout and other issues specific to individual document types is omitted for simplicity.
All the children of a <par> begin by default when the <par> begins. For example:
<par> <img id="i1" dur="5s" src="img.jpg" /> <img id="i2" dur="10s" src="img2.jpg" /> <img id="i3" begin="2s" dur="5s" src="img3.jpg" /> </par>
Elements "i1" and "i2" both begin immediately when the par begins, which is the default begin time. The active duration of "i1" ends at 5 seconds into the <par>. The active duration of "i2" ends at 10 seconds into the <par>. The last element "i3" begins at 2 seconds since it has an explicit begin offset, and has a duration of 5 seconds which means its active duration ends 7 seconds after the <par> begins.
An image that illustrated the timeline might be useful here.
Each child of a <seq> begins by default when the previous element ends. For example:
<seq> <img id="i1" begin="0s" dur="5s" src="img1.jpg" /> <img id="i2" dur="10s" src="img2.jpg" /> <img id="i3" begin="1s" dur="5s" src="img3.jpg" /> </seq>
The element "i1" begins immediately, with the start of the <seq>, and ends 5 seconds later. Note: specifying a begin time of 0 seconds is optional since the default begin offset is always 0 seconds. The second element "i2" begins, by default, 0 seconds after the previous element "i1" ends, which is 5 seconds into the <seq>. Element "i2" ends 10 seconds later, at 15 seconds into the <seq>. The last element, "i3", has a begin offset of 1 second specified, so it begins 1 second after the previous element "i2" ends, and has a duration of 5 seconds, so it ends at 21 seconds into the <seq>.
Insert illustration.
<par> <excl> <par id="p1"> ... </par> <par id="p2"> ... </par> </excl> <a href="p1"><img src="Button1.jpg"/></a> <a href="p2"><img src="Button2.jpg"/></a> </par>
This example models jukebox-like behavior. Clicking on the first image activates the media items of parallel container "p1". If the link on the second image is traversed, "p2" is started (thereby deactivating "p1" if it would still be active).
Shouldn't we say, here, exactly where the elements of the selected par in the excl should begin when a click happens, e.g., if we are 10 seconds into the outer par and we click on button 2, does the MPG video in p2 start 10 seconds into its stream (in-sync), or does it start at its time 0?
Note that the specific syntax for beginEvent argument values is still under discussion.
<par> <excl> <par begin="btn1.click"> ... </par> <par begin="btn2.click"> ... </par> </excl> <img id="btn1" src=... /> <img id="btn2" src=... /> </par>
The same jukebox example, using event-based activation.
In these two examples event-based and anchor-based activation look almost identical, maybe we should come up with examples showing the difference and the relative power of each.
<excl> <ref id="a" begin="0s" ... /> <ref id="b" begin="5s" ... /> </excl>
In the example above, the beginning of "b" deactivates "a" (assuming that a is still active after 5 seconds). Note that this could also be modeled using a sequence with an explicit duration on the children. While the determinate syntax is allowed, this is not expected to be a common use-case scenario.
Issue - should we preclude the use of determinate timing on children of
excl? Other proposals would declare one child (possibly the first)
to begin playing by default. Proposals include an attribute on the
<excl> container that indicate one child to begin playing by default.
For simple media elements (i.e. media elements that are not time containers) that reference discrete media, the implicit duration is defined to be indefinite. This can lead to surprising results, as in this example:
<seq> <img src="img1.jpg" /> <video src="vid2.mpg" /> <video src="vid3.mpg" /> </seq>
The default syncbase of a sequence is defined to be the effective active end of the previous element in the sequence, unless the active duration is indefinite in which case the default syncbase is the begin of the previous element. In the example, the implicit duration of the image is used to defined the simple and active durations. As a result, the default begin of the second element causes it to begin at the same time as the image. Thus, the image will not show at all! Authors will generally specify an explicit duration for any discrete media elements.
There is an important difference between the semantics of end and dur.
The dur attribute, in conjunction with the begin time, specifies the simple
duration for an element.
This is the duration that is repeated when the element also has a repeat specified. The attribute end on the other hand overrides the active duration of the element. If the element does not have repeat specified, the active duration is the same as the simple duration. However, if the element has repeat specified, then the end will override the repeat, but will not affect the simple duration. For example:
<seq repeat="10" end="stopBtn.click"> <img src="img1.jpg" dur="2s" /> <img src="img2.jpg" dur="2s" /> <img src="img3.jpg" dur="2s" /> </seq>
The sequence will play for 6 seconds on each repeat iteration. It will play through 10 times, unless the user clicks on a "stopBtn" element before 60 seconds have elapsed.
When an implementation supports the SMIL-DOM, it will be possible to make an element begin or end the active duration using script or some other browser extension. When an author wishes to describe an element as interactive in this manner, the following syntax can be used:
<audio src="song1.au" begin="indefinite" />
The element will not begin until the SMIL-DOM beginElement()
method is called.
This is a placeholder for a set of authoring guidelines intended to help authors avoid potential mistakes and confusion, and to suggest best practices as intended by the authors.
SMIL 1.0 defines the model for timing, including markup to define element timing, and elements to define parallel and sequence time containers. This version introduces some syntax variations and additional functionality, including:
The complete syntax is described here, including syntax that is unchanged from SMIL 1.0.