10 Text

Contents

• 10.1 Introduction
• 10.2 Characters and their corresponding glyphs
• 10.3 Fonts, font tables and baselines
• 10.4 The 'text' element
• 10.5 The 'tspan' element
• 10.6 The 'tref' element
• 10.7 Text layout
  • 10.7.1 Text layout introduction
  • 10.7.2 Setting the inline progression direction
  • 10.7.3 Glyph orientation within a text run
  • 10.7.4 Relationship with bidirectionality
• 10.8 Text rendering order
• 10.9 Alignment properties
  • 10.9.1 Text alignment properties
  • 10.9.2 Baseline alignment properties
• 10.10 Font selection properties
• 10.11 Spacing properties
• 10.12 Text decoration
• 10.13 Text on a path
  • 10.13.1 Introduction to text on a path
  • 10.13.2 The 'textPath' element
  • 10.13.3 Text on a path layout rules
• 10.14 Alternate glyphs
• 10.15 White space handling
• 10.16 Text selection
• 10.17 DOM interfaces

10.1 Introduction

Text that is to be rendered as part of an SVG document fragment is specified using the 'text' element. The characters to be drawn are expressed as XML character data [XML10] inside the 'text' element.

SVG's 'text' elements are rendered like other graphics elements. Thus, coordinate system transformations, painting, clipping and masking features apply to 'text' elements in the same way as they apply to shapes such as paths and rectangles.

Each 'text' element causes a single string of text to be rendered. SVG performs no automatic line breaking or word wrapping. To achieve the effect of multiple lines of text, use one of the following methods:

• The author or authoring package pre-computes the line breaks and uses multiple 'text' elements (one for each line of text).
• The author or authoring package needs pre-computes the line breaks and uses a single 'text' element with one or more 'tspan' child elements with appropriate values for attributes x, y, dx and dy to set new start positions for those characters which start new lines. (This approach allows user text selection across multiple lines of text -- see Text selection and clipboard operations.)
• Express the text to be rendered in another XML namespace such as XHTML [XHTML] embedded inline within a 'foreignObject' element. (Note: the exact semantics of this approach are not completely defined at this time.)

The text strings within 'text' elements can be rendered in a straight line or rendered along the outline of a 'path' element. SVG supports the following international text processing features for both straight line text and text on a path:

• horizontal and vertical orientation of text
• left-to-right or bidirectional text (i.e., languages which intermix right-to-left and left-to-right text, such as Arabic and Hebrew)
• when SVG fonts are used, automatic selection of the correct glyph corresponding to the current form for Arabic and Han text

Because SVG text is packaged as XML character data [XML10]:

• Text data in SVG content is readily accessible to the visually impaired (see Accessibility Support)
• In many viewing scenarios, the user will be able to search for and select text strings and copy selected text strings to the system clipboard (see Text selection)
• XML-compatible Web search engines will find text strings in SVG content with no additional effort over what they need to do to find text strings in other XML documents

Multi-language SVG content is possible by substituting different text strings based on the user's preferred language.

For accessibility reasons, it is recommended that text which is included in a document have appropriate semantic markup to indicate its function. See SVG accessibility guidelines for more information.
 

10.2 Characters and their corresponding glyphs

In XML [XML10], textual content is defined in terms of a sequence of XML characters, where each character is defined by a particular Unicode code point [UNICODE]. Fonts, on the other hand, consists of a collection of glyphs and other associated information, such as font tables. A glyph is a presentable form of one or more characters (or a part of a character in some cases). Each glyph consists of some sort of identifier (in some cases a string, in other cases a number) along with drawing instructions for rendering that particular glyph.

In many cases, there is a one-to-one mapping of Unicode characters (i.e., Unicode code points) to glyphs in a font. For example, it is common for a font designed for Latin languages (where the term Latin is used for European languages such as English with alphabets similar to and/or derivative to the Latin language) to contain a single glyph for each of the standard ASCII characters (i.e., A-to-Z, a-to-z, 0-to-9, plus the various punctuation characters found in ASCII). Thus, in most situations, the string "XML", which consists of three Unicode characters, would be rendered by the three glyphs corresponding to "X", "M" and "L", respectively.

In various other cases, however, there is not a strict one-to-one mapping of Unicode characters to glyphs. Some of the circumstances when the mapping is not one-to-one:

• Ligatures - For best looking typesetting, it is often desirable that particular sequences of characters are rendered as a single glyph. An example is the word "office". Many fonts will define an "ffi" ligature. When the word "office" is rendered, sometimes the user agent will render the glyph for the "ffi" ligature instead of rendering distinct glyphs (i.e., "f", "f" and "i") for each of the three characters. Thus, for ligatures, multiple Unicode characters map to a single glyph. (Note that for proper rendering of some languages, ligatures are required for certain character combinations.)
• Composite characters - In various situations, commonly used adornments such as diacritical marks will be stored once in a font as a particular glyph and then composed with one or more other glyphs to result in the desired character. For example, it is possible that a font engine might render the é character by first rendering the glyph for e and then rendering the glyph for ´ (the accent mark) such that the accent mark will appear over the e. In this situation, a single Unicode character maps to multiple glyphs.
• Glyph substitution - Some typography systems examine the nature of the textual content and utilize different glyphs in different circumstances. For example, in Arabic, the same Unicode character might render as any of four different glyphs, depending on such factors as whether the character appears at the start, the end or the middle of a sequence of cursively joined characters. Different glyphs might be used for punctuation character depending on inline progression direction (e.g., horizontal vs. vertical). In these situations, a single Unicode character might map to one of several alternative glyphs.
• In some languages, particular sequences of characters will be converted into multiple glyphs such that parts of a particular character are in one glyph and the remainder of that character is in another glyph.
• Alternative glyph specification - SVG contains a facility for the author to explicitly specify that a particular sequence of Unicode characters is to be rendered using a particular glyph. (See Alternate glyphs.) When this facility is used, multiple Unicode characters map to a single glyph.

In many situations, the algorithms for mapping from characters to glyphs are system-dependent, resulting in the possibility that the rendering of text might be (usually slightly) different when viewed in different user environments. If the author of SVG content requires precise selection of fonts and glyphs, then the recommendation is that the necessary fonts (potentially subsetted to include only the glyphs needed for the given document) be available either as SVG fonts embedded within the SVG content or as WebFonts posted at the same Web location as the SVG content.

Throughout this chapter, the term character shall be equivalent to the definition of a character in XML [XML10].

10.3 Fonts, font tables and baselines

A font consists of a collection of glyphs together with the information (the font tables) necessary to use those glyphs to present characters on some medium. The combination of the collection of glyphs and the font tables is called the font data. The font tables include the information necessary to map characters to glyphs, to determine the size of glyph areas and to position the glyph area. Each font table consists of one or more font characteristics, such as the font-weight and font-style.

The geometric font characteristics are expressed in a coordinate system based on the EM box. (The EM is a relative measure of the height of the glyphs in the font; see CSS2 em square.) The box 1 EM high and 1 EM wide is called the design space. This space is given a geometric coordinates by sub-dividing the EM into a number of units-per-em.

Note: Units-per-em is a font characteristic. A typical value for units-per-EM is 1000 or 2048.

The coordinate space of the EM box is called the design space coordinate system. For scalable fonts, the curves and lines that are used to draw a glyph are represented using this coordinate system.

Note: Most often, the (0,0) point in this coordinate system is positioned on the left edge of the EM box, but not at the bottom left corner. The Y coordinate of the bottom of a roman capital letter is usually zero. And the descenders on lower case roman letters have negative coordinate values.

SVG assumes that the font tables will provide at least three font characteristics: an ascent, a descent and a set of baseline-tables. The ascent is the distance to the top of the EM box from the (0,0) point of the font; the descent is the distance to the bottom of the EM box from the (0.0) point of the font. The baseline-table is explained below.

Note: Within an OpenType font, for horizontal writing-modes, the ascent and descent are given by the sTypoAscender and sTypoAscender entries in the OS/2 table. For vertical writing-modes, the descent (the distance, in this case from the (0,0) point to the left edge of the glyph) is normally zero because the (0,0) point is on the left edge. The ascent for vertical writing-modes is either 1 em or is specified by the Ideographic top baseline value in the OpenType Base table for vertical writing-modes.

In horizontal writing-modes, the glyphs of a given script are positioned so that a particular point on each glyph, the alignment-point, is aligned with the alignment-points of the other glyphs in that script. The glyphs of different scripts, for example, western, northern indic and far-eastern scripts, are typically aligned at different points on the glyph. For example, western glyphs are aligned on the bottoms of the capital letters, northern indic glyphs are aligned at the top of a horizontal stroke near the top of the glyphs and far-eastern glyphs are aligned either at the bottom or center of the glyph. Within a script and within a line of text having a single font-size, the sequence of alignment-points defines, in the inline- progression-direction, a geometric line called a baseline. Western and most other alphabetic and syllabic glyphs are aligned to an "alphabetic" baseline, the northern indic glyphs are aligned to a "hanging" baseline and the far-eastern glyphs are aligned to an "ideographic" baseline.

A baseline-table specifies the position of one or more baselines in the design space coordinate system. The function of the baseline table is to facilitate the alignment of different scripts with respect to each other when they are mixed on the same text line. Because the desired relative alignments may depend on which script is dominant in a line (or block), there may be a different baseline table for each script. In addition, different alignment positions are needed for horizontal and vertical writing modes. Therefore, the font may have a set of baseline tables: typically, one or more for horizontal writing-modes and zero or more for vertical writing-modes.

Note: Some fonts may not have values for the baseline tables. Heuristics are suggested for approximating the baseline tables when a given font does not supply baseline tables.

SVG further assumes that for each glyph in the font data for a font, there is are two width values, two alignment-baselines and two alignment-points, one each for horizontal writing-modes and the other for vertical writing-modes. (Even though it is specified as a width, for vertical writing-modes the width is used in the vertical direction.) The script to which a glyph belongs determines an alignment-baseline to which the glyph is to be aligned. The inline-progression-direction position of the alignment-point is on the start-edge of the glyph.

Properties related to baselines are described below under Baseline alignment properties.

In addition to the font characteristics required above, a font may also supply substitution and positioning tables that can be used by a formatter to re-order, combine and position a sequence of glyphs to make one or more composite glyphs. The combination may be as simple as a ligature, or as complex as an indic syllable which combines, usually with some re-ordering, multiple consonants and vowel glyphs.

10.4 The 'text' element

The 'text' element defines a graphics element consisting of text. The XML [XML10] character data within the 'text' element, along with relevant attributes and properties and character-to-glyph mapping tables within the font itself, define the glyphs to be rendered. (See Characters and their corresponding glyphs.) The attributes and properties on the 'text' element indicate such things as the writing direction, font specification and painting attributes which describe how exactly to render the characters. Subsequent sections of this chapter describe the relevant text-specific attributes and properties, particular text layout and bidirectionality.

Since 'text' elements are rendered using the same rendering methods as other graphics elements, all of the same coordinate system transformations, painting, clipping and masking features that apply to shapes such as paths and rectangles also apply to 'text' elements.

It is possible to apply a gradient, pattern, clipping path, mask or filter to text. When one of these facilities is applied to text and keyword objectBoundingBox is used (see Object bounding box units) to specify a graphical effect relative to the "object bounding box", then the object bounding box units are computed relative to the entire 'text' element in all cases, even when different effects are applied to different 'tspan' elements within the same 'text' element.

The 'text' element renders its first glyph (after bidirectionality reordering) at the initial current text position, which is established by the x and y attributes on the 'text' element (with possible adjustments due to the value of the 'text-anchor' property, the presence of a 'textPath' element containing the first character, and/or an x, y, dx or dy attributes on a 'tspan', 'tref' or 'altGlyph' element which contains the first character). After the glyph(s) corresponding to the given character is(are) rendered, the current text position is updated for the next character. In the simplest case, the new current text position is the previous current text position plus the glyphs' advance value (horizontal or vertical). See text layout for a description of glyph placement and glyph advance.

<!ENTITY % textExt "" >
<!ELEMENT text (#PCDATA|desc|title|metadata|
                tspan|tref|textPath|altGlyph|a|animate|set|
                animateMotion|animateColor|animateTransform
                %geExt;%textExt;)* >
<!ATTLIST text
  %stdAttrs;
  %testAttrs;
  %langSpaceAttrs;
  externalResourcesRequired %Boolean; #IMPLIED
  class %ClassList; #IMPLIED
  style %StyleSheet; #IMPLIED
  %PresentationAttributes-FillStroke;
  %PresentationAttributes-FontSpecification;
  %PresentationAttributes-Graphics;
  %PresentationAttributes-TextContentElements;
  %PresentationAttributes-TextElements;
  transform %TransformList; #IMPLIED
  %graphicsElementEvents;
  x %Coordinate; #IMPLIED
  y %Coordinate; #IMPLIED
  textLength %Length; #IMPLIED
  lengthAdjust (spacing|spacingAndGlyphs) #IMPLIED >

Example text01 below expresses all values in physical units such as centimeters and points. The 'text' element contains the text string "Hello, out there" which will be rendered onto the canvas using the Verdana font family with font size of 12 points with the glyphs filled with the color blue.

<?xml version="1.0" standalone="no"?>
<!DOCTYPE svg PUBLIC "-//W3C//DTD SVG 20001102//EN" 
  "http://www.w3.org/TR/2000/CR-SVG-20001102/DTD/svg-20001102.dtd">
<svg width="10cm" height="3cm">
  <desc>Example text01 - 'Hello, out there' in blue</desc>

  <text x="2.5cm" y="1.5cm" 
        style="font-family:Verdana; font-size:16pt; fill:blue">
    Hello, out there
  </text>
</svg>

Example text01

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Example text02 below expresses the x and y attributes and the 'font-size' property in the user coordinate system set up by the viewBox attribute on the 'svg' element. The 'text' element contains the text string "Text in user space."

<?xml version="1.0" standalone="no"?>
<!DOCTYPE svg PUBLIC "-//W3C//DTD SVG 20001102//EN" 
  "http://www.w3.org/TR/2000/CR-SVG-20001102/DTD/svg-20001102.dtd">
<svg width="10cm" height="3cm" viewBox="0 0 1000 300">
  <desc>Example text02 - Text in user space</desc>

  <text x="250" y="150" 
        style="font-family:Verdana; font-size:42.333; fill:blue">
    Text in user space
  </text>
</svg>

Example text02

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10.5 The 'tspan' element

Within a 'text' element, text and font properties and the current text position can be adjusted with absolute or relative coordinate values by including a 'tspan' element.

<!ENTITY % tspanExt "" >
<!ELEMENT tspan (#PCDATA|desc|title|metadata|tspan|tref|altGlyph|a|animate|set|animateColor
                %tspanExt;)* >
<!ATTLIST tspan
  %stdAttrs;
  %testAttrs;
  %langSpaceAttrs;
  externalResourcesRequired %Boolean; #IMPLIED
  class %ClassList; #IMPLIED
  style %StyleSheet; #IMPLIED
  %PresentationAttributes-FillStroke;
  %PresentationAttributes-FontSpecification;
  %PresentationAttributes-Graphics;
  %PresentationAttributes-TextContentElements;
  %graphicsElementEvents;
  x %Coordinates; #IMPLIED
  y %Coordinates; #IMPLIED
  dx %Lengths; #IMPLIED
  dy %Lengths; #IMPLIED
  rotate CDATA #IMPLIED
  textLength %Length; #IMPLIED
  lengthAdjust (spacing|spacingAndGlyphs) #IMPLIED >

The x, y, dx, dy and rotate on the 'tspan' element are useful in high-end typography scenarios where individual glyphs require exact placement. These attributes are useful for minor positioning adjustments between characters or for major positioning adjustments, such as moving the current text position to a new location to achieve the visual effect of a new line of text. Multi-line 'text' elements are possible by defining different 'tspan' elements for each line of text, with attributes x, y, dx and/or dy defining the position of each 'tspan'. (An advantage of such an approach is that users will be able to perform multi-line text selection.)

In situations where micro-level positioning adjustment are necessary for advanced typographic control, the SVG content designer needs to ensure that the necessary font will be available for all viewers of the document (e.g., package up the necessary font data in the form of an SVG font or an alternative WebFont format which is stored at the same Web site as the SVG content) and that the viewing software will process the font in the expected way (the capabilities, characteristics and font layout mechanisms vary greatly from system to system). If the SVG content contains x, y, dx or dy attribute values which are meant to correspond to a particular font processed by a particular set of viewing software and either of these requirements is not met, then the text might display with poor quality.

The following additional rules apply to attributes x, y, dx, dy and rotate when they contain a list of numbers:

• When a single XML character maps to a single glyph - In this case, the i-th value for the x, y, dx, dy and rotate attributes is applied to the glyph that corresponds to the i-th character.
• When a single XML character maps to multiple glyphs (e.g., when an accent glyph is placed on top of a base glyph) - In this case, the i-th value for the x, y, dx and dy values are applied (i.e., the current text position is adjusted) before rendering the first glyph. The rotation transformation corresponding to the i-th rotate value is applied to the glyphs and to the inter-glyph advance values corresponding to this character on a group basis (i.e., the rotation value creates a temporary new rotated coordinate system, and the glyphs corresponding to the character are rendered into this rotated coordinate system).
• When multiple XML characters map to a single glyph (e.g., when a ligature is used) - Suppose that the i-th and (i+1)-th XML characters map to a single glyph. In this case, the i-th value for the x, y, dx, dy and rotate attributes all apply when rendering the glyph. The (i+1)-th values, however, for x, y and rotate are ignored (exception: the final rotate value in the list would still apply to subsequent characters), whereas the dx and dy are applied to the subsequent XML character (i.e., the (i+2)-th character), if one exists, by translating the current text position by the given amounts before rendering the first glyph associated with that character.
• When there is a many-to-many mapping of characters to glyphs (e.g., when three characters map to two glyphs, such as when the first glyph expresses the first character and half of the second character, and the second glyph expresses the other half of the second character plus the third character) - Suppose that the i-th, (i+1)-th and (i+2)-th XML characters map to two glyphs. In this case, the i-th value for the x, y, dx and dy values are applied (i.e., the current text position is adjusted) before rendering the first glyph. The rotation transformation corresponding to the i-th rotate value is applied to both the two glyphs and the glyph advance values for the first glyph on a group basis (i.e., the rotation value creates a temporary new rotated coordinate system, and the two glyphs are rendered into the temporary rotated coordinate system). The (i+1)-th and (i+2)-th values, however, for the x, y and rotate attributes are not applied (exception: the final rotate value in the list would still apply to subsequent characters), whereas the (i+1)-th and (i+2)-th values for the dx and dy attributes are applied to the subsequent XML character (i.e., the (i+3)-th character), if one exists, by translating the current text position by the given amounts before rendering the first glyph associated with that character.
• Relationship to bidirectionality - As described below in the discussion on bidirectionality, text is laid out in a two-step process, where any bidirectional text is first re-ordered into a left-to-right string, and then text layout occurs with the re-ordered text string. Whenever the character data within a 'tspan' element is re-ordered, the corresponding elements within the x, y, dx, dy and rotate are also re-ordered to maintain the correspondence. For example, suppose that you have the following 'tspan' element:

  <tspan dx="11 12 13 14 15 0 21 22 23 0 31 32 33 34 35 36">Latin and Hebrew</span>
  

  and that the word "Hebrew" will be drawn right-to-left. First, the character data and the corresponding values in the dx list will be reordered, such that the text string will be "Latin and werbeH" and the list of values for the dx attribute will be "11 12 13 14 15 0 21 22 23 0 36 35 34 33 32 31". After this re-ordering, the glyphs corresponding to the characters will be positioned using standard left-to-right layout rules.
• Nested 'tspan' elements - The x, y, dx, dy and rotate attributes on a given 'tspan' element apply only to the character data that is directly within that 'tspan' element and do not apply to the character data within child (i.e., nested) 'tspan' elements. If the nested 'tspan' elements require positioning adjustments or rotation values, the nested 'tspan' elements need to specify x, y, dx, dy and rotate values for their own character data.

The following examples show basic use of the 'tspan' element.

Example tspan01 uses a 'tspan' element to indicate that the word "not" is to use a bold font and have red fill.

<?xml version="1.0" standalone="no"?>
<!DOCTYPE svg PUBLIC "-//W3C//DTD SVG 20001102//EN" 
  "http://www.w3.org/TR/2000/CR-SVG-20001102/DTD/svg-20001102.dtd">
<svg width="10cm" height="3cm">
  <desc>Example tspan01 - using tspan to change visual attributes</desc>

  <g style="font-family:Verdana; font-size:12pt">
    <text x="2cm" y="1.5cm" style="fill:blue">
      You are
        <tspan style="font-weight:bold; fill:red">not</tspan>
      a banana.
    </text>
  </g>
</svg>

Example tspan01

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Example tspan02 uses the dx and dy attributes on the 'tspan' element to adjust the current text position horizontally and vertically for particular text strings within a 'text' element.

<?xml version="1.0" standalone="no"?>
<!DOCTYPE svg PUBLIC "-//W3C//DTD SVG 20001102//EN" 
  "http://www.w3.org/TR/2000/CR-SVG-20001102/DTD/svg-20001102.dtd">
<svg width="10cm" height="3cm">
  <desc>Example tspan02 - using tspan's dx and dy attributes 
        for incremental positioning adjustments</desc>

  <g style="font-family:Verdana; font-size:12pt">
    <text x="2cm" y="1.5cm" style="fill:blue">
      But you
        <tspan dx="2em" dy="-.5cm" style="font-weight:bold; fill:red">
          are
        </tspan>
        <tspan dy="1cm">
           a peach!
        </tspan>
    </text>
  </g>
</svg>

Example tspan02

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Example tspan03 uses the x and y attributes on the 'tspan' element to establish a new absolute current text position for each glyph to be rendered. The example shows two lines of text within a single 'text' element. Because both lines of text are within the same 'text' element, the user will be able to select through both lines of text and copy the text to the system clipboard in user agents that support text selection and clipboard operations,

<?xml version="1.0" standalone="no"?>
<!DOCTYPE svg PUBLIC "-//W3C//DTD SVG 20001102//EN" 
  "http://www.w3.org/TR/2000/CR-SVG-20001102/DTD/svg-20001102.dtd">
<svg width="10cm" height="3cm">
  <desc>Example tspan03 - using tspan's x and y attributes 
        for multiline text and precise glyph positioning</desc>

  <g style="font-family:Verdana; font-size:12pt">
    <text style="fill:rgb(255,164,0)">
      <tspan x="3.0cm 3.5cm 4.0cm 4.5cm 5.0cm 5.5cm 6.0cm 6.5cm" y="1cm">
        Cute and
      </tspan>
      <tspan x="3.75cm 4.25cm 4.75cm 5.25cm 5.75cm" y="2cm">
         fuzzy
      </tspan>
    </text>
  </g>
</svg>

Example tspan03

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10.6 The 'tref' element

The textual content for a 'text' can be either character data directly embedded within the 'text' element or the character data content of a referenced element, where the referencing is specified with a 'tref' element.

<!ENTITY % trefExt "" >
<!ELEMENT tref (desc|title|metadata|animate|set|animateColor
                %trefExt;)* >
<!ATTLIST tref
  %stdAttrs;
  %xlinkRefAttrs;
  xlink:href %URI; #REQUIRED
  %testAttrs;
  %langSpaceAttrs;
  externalResourcesRequired %Boolean; #IMPLIED
  class %ClassList; #IMPLIED
  style %StyleSheet; #IMPLIED
  %PresentationAttributes-FillStroke;
  %PresentationAttributes-FontSpecification;
  %PresentationAttributes-Graphics;
  %PresentationAttributes-TextContentElements;
  %graphicsElementEvents;
  x %Coordinates; #IMPLIED
  y %Coordinates; #IMPLIED
  dx %Lengths; #IMPLIED
  dy %Lengths; #IMPLIED
  rotate CDATA #IMPLIED
  textLength %Length; #IMPLIED
  lengthAdjust (spacing|spacingAndGlyphs) #IMPLIED >

All character data within the referenced element, including character data enclosed within additional markup, will be rendered.

The x, y, dx, dy and rotate attributes have the same meanings as for the 'tspan' element. The attributes are applied as if the 'tref' element was replaced by a 'tspan' with the referenced character data (stripped of all supplemental markup) embedded within the hypothetical 'tspan' element.

Example tref01 shows how to use character data from a different element as the character data for a given 'tspan' element. The first 'text' element (with id="ReferencedText") will not draw because it is part of a 'defs' element. The second 'text' element draws the string "Inline character data". The third 'text' element draws the string "Reference character data" because it includes a 'tref' element which is a reference to element "ReferencedText", and that element's character data is "Referenced character data".

<?xml version="1.0" standalone="no"?>
<!DOCTYPE svg PUBLIC "-//W3C//DTD SVG 20001102//EN" 
  "http://www.w3.org/TR/2000/CR-SVG-20001102/DTD/svg-20001102.dtd">
<svg width="10cm" height="3cm">
  <defs>
    <text id="ReferencedText">
      Referenced character data
    </text>
  </defs>
  <desc>Example tref01 - inline vs reference text content</desc>

  <text x="1cm" y="1cm" style="font-size:12pt; fill:blue">
    Inline character data
  </text>
  <text x="1cm" y="2cm" style="font-size:12pt; fill:red">
    <tref xlink:href="#ReferencedText"/>
  </text>
</svg>

Example tref01

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10.7 Text layout

10.7.1 Text layout introduction

This section describes the text layout features supported by SVG, which includes support for various international writing directions, such as left-to-right (e.g., Latin scripts) and bidirectional (e.g., Hebrew or Arabic) and vertical (e.g., Asian scripts). The descriptions in this section assume straight line text (i.e., text that is either strictly horizontal or vertical with respect to the current user coordinate system). Subsequent sections describe the supplemental layout rules for text on a path.

SVG does not provide for automatic line breaks or word wrapping, which makes internationalized text layout for SVG relatively simpler than it is for languages which support formatting of multi-line text blocks.

For each 'text' element, the SVG user agent determines the current reference orientation. For standard horizontal or vertical text (i.e., no text-on-a-path), the reference orientation is the vector pointing towards negative infinity in Y within the current user coordinate system. (Note: in the initial coordinate system, the reference orientation is up.) For text on a path, the reference orientation is reset with each character.

Based on the reference orientation and the value for property 'writing-mode', the SVG user agent determines the current inline progression direction. For left-to-right text, the inline progression direction points 90 degrees clockwise from the reference orientation vector. For right-to-left text, the inline progression points 90 degrees counter-clockwise from the reference orientation vector. For top-to-bottom text, the inline progression direction points 180 degrees from the reference orientation vector.

The shift direction is the direction towards which the baseline table moves due to positive values for property 'baseline-shift'. The shift direction is such that a positive value shifts the baseline table towards the topmost entry in the parent's baseline table.

In processing a given 'text' element, the SVG user agent keeps track of the current text position. The initial current text position is established by the x and y attributes on the 'text' element.

The current text position is adjusted after each glyph to establish a new current text position at which the next glyph shall be rendered. The adjustment to the current text position is based on the current inline progression direction, glyph-specific advance values corresponding to the glyph orientation of the glyph just rendered, kerning tables in the font and the current values of various attributes and properties, such as the spacing properties and any x, y, dx and dy attributes on 'tspan', 'tref' or 'altGlyph' elements. If a glyph does not provide explicit advance values corresponding to the current glyph orientation, then an appropriate approximation should be used. For vertical text, a suggested approximation is the sum of the ascent and descent values for the glyph. Another suggested approximation for an advance value for both horizontal and vertical text is the size of an em (see units-per-em).

For each glyph to be rendered, the SVG user agent determines an appropriate alignment-point on the glyph which will be placed exactly at the current text position. The alignment-point is determined based on glyph cell metrics in the glyph itself, the current inline progression direction and the glyph orientation relative to the inline progression direction. For most uses of Latin text (i.e., 'writing-mode:lr', 'text-anchor:start', and 'alignment-baseline:baseline') the alignment-point in the glyph will be the intersection of left edge of the glyph cell (or some other glyph-specific x-axis coordinate indicating a left-side origin point) with the Latin baseline of the glyph. For many cases with top-to-bottom vertical text layout, the reference point will be either a glyph-specific origin point based on the set of vertical baselines for the font or the intersection of the center of the glyph with its top line (see [CSS2] for a definition of top line). If a glyph does not provide explicit origin points corresponding to the current glyph orientation, then an appropriate approximation should be used, such as the intersection of the left edge of the glyph with the appropriate horizontal baseline for the glyph or intersection of the top edge of the glyph with the appropriate vertical baseline. If baseline tables are not available, user agents should establish baseline tables that reflect common practice.

Adjustments to the current text position are either absolute position adjustments or relative position adjustments. An absolute position adjustment occurs in the following circumstances:

• At the start of a 'text' element
• At the start of each 'textPath' element
• For each character within a 'tspan', 'tref' and 'altGlyph' element which has an x or y attribute value assigned to it explicitly

All other position adjustments to the current text position are relative position adjustments.

Each absolute position adjustment defines a new text chunk. Absolute position adjustments impact text layout in the following ways:

• Ligatures only occur when a set of characters which might map to a ligature are all in the same text chunk.
• Each text chunk represents a separate block of text for alignment due to 'text-anchor' property values.
• Reordering of characters due to bidirectionality only occurs within a text chunk. Reordering does not happen across text chunks.

The following additional rules apply to ligature formation:

• As in [CSS2], when the resultant space between two characters is not the same as the default space, user agents should not use ligatures; thus, if there are non-default values for properties 'kerning' or 'letter-spacing', the user agent should not use ligatures.
• Ligature formation should not be enabled for the glyphs corresponding to characters within different DOM text nodes; thus, characters separated by markup should not use ligatures.
• As mentioned above, ligature formation should not be enabled for the glyphs corresponding to characters within different text chunks.

10.7.2 Setting the inline progression direction

The 'writing-mode' property specifies whether the initial inline progression direction for a 'text' element shall be left-to-right, right-to-left, or top-to-bottom. The 'writing-mode' property applies only to 'text' elements; the property is ignored for 'tspan', 'tref', 'altGlyph' and 'textPath' sub-elements. (Note that the inline progression direction can change within a 'text' element due to the Unicode bidirectional algorithm and properties 'direction' and 'unicode-bidi'. For more on bidirectional text, see Relationship with bidirectionality.)

lr-tb | lr

Sets the initial inline progression direction to left-to-right, as is common in most Latin-based documents. For most characters, the current text position is advanced from left to right after each glyph is rendered. (When the character data includes characters which are subject to the Unicode bidirectional algorithm, the text advance rules are more complex. See Relationship with bidirectionality).

rl-tb | rl

Sets the initial inline progression direction to right-to-left, as is common in Arabic or Hebrew scripts. (See Relationship with bidirectionality.)

tb-rl | tb

Sets the initial inline progression direction to top-to-bottom, as is common in Asian scripts. Though hardly as frequent as horizontal, this type of vertical layout also occurs in Latin based documents, particularly in table column or row labels. In most cases, the vertical baselines running through the middle of each glyph are aligned.

10.7.3 Glyph orientation within a text run

In some cases, it is required to alter the orientation of a sequence of characters relative to the inline progression direction. The requirement is particularly applicable to vertical layouts of East Asian documents, where sometimes narrow-cell Latin text is to be displayed horizontally and other times vertically.

Two properties control the glyph orientation relative to the reference orientation for each of the two possible inline progression directions. 'glyph-orientation-vertical' controls glyph orientation when the inline progression direction is vertical. 'glyph-orientation-horizontal' controls glyph orientation when the inline progression direction is horizontal.

<angle>

The value of the angle is an <integer> restricted to the range of -360 to +360 in 90-degree increments.
A value of 0 indicates that all glyphs are unrotated relative to the reference orientation, resulting in glyphs which are stacked vertically on top of each other. A value of 90 indicates a rotation of 90 degrees clockwise relative to the reference orientation. Negative angle values are computed modulo 360; thus, a value of -90 is equivalent to a value of 270.

auto

The glyph orientation relative to the inline progression direction is determined automatically based on the Unicode character number of the first rendered glyph.

Full-width ideographic and full-width Latin glyphs (excluding ideographic punctuation) are oriented as if an <angle> of "0" had been specified (i.e., glyphs are unrotated relative to the reference orientation, resulting in glyphs which are stacked vertically on top of each other).

Ideographic punctuation and other ideographic glyphs having alternate horizontal and vertical forms shall use the vertical form of the glyph.

Text which is not full-width will be set as if an <angle> of "90" had been specified; thus, narrow-cell Latin text will be rotated 90 degree clockwise versus full-width ideographic and full-width Latin text.

Note that a value of auto will generally produce the expected results in common uses of mixing Japanese with European characters; however, the exact algorithms are based on complex interactions between many factors, including font design, and thus different algorithms might be employed in different processing environments. For precise control, specify explicit <angle> values.

Glyphs corresponding to Arabic or Hebrew characters that are not full-width have the same orientation as narrow-cell Latin.

The glyph orientation affects the amount that the current text position advances as each glyph is rendered. When the inline progression direction is vertical and the 'glyph-orientation-vertical' results in an orientation angle that is a multiple of 180 degrees, then the current text position is incremented according to the vertical metrics of the glyph. Otherwise, if the 'glyph-orientation-vertical' results in an orientation angle that is not a multiple of 180 degrees, then the current text position is incremented according to the horizontal metrics of the glyph.

The text layout diagrams in this section use the following symbols:

- wide-cell glyph (e.g. Han) which is the n-th glyph in the text run
- narrow-cell glyph (e.g. Latin) which is the n-th glyph in the text run

The orientation which the above symbols assume in the diagrams corresponds to the orientation that the Unicode characters they represent are intended to assume when rendered in the user agent. Spacing between the glyphs in the diagrams is usually symbolic, unless intentionally changed to make a point.

The diagrams below illustrate different uses of 'glyph-orientation-vertical'. The diagram on the left shows the result of the mixing of full-width ideographic glyphs with narrow-cell Latin glyphs when 'glyph-orientation-vertical' for the Latin characters is either auto or 90. The diagram on the right show the result of mixing full-width ideographic glyphs with narrow-cell Latin glyphs when Latin glyphs are specified to have a 'glyph-orientation-vertical' of 0.

<angle>

The value of the angle is an <integer> restricted to the range of -360 to +360 in 90-degree increments.
A value of 0 indicates that all glyphs are unrotated with respect to the reference orientation, resulting in glyphs which are positioned side by side. A value of 90 indicates an orientation of 90 degrees clockwise from the reference orientation. Angle values are computed modulo 360; thus, a value of -90 is equivalent to a value of 270.

The glyph orientation affects the amount that the current text position advances as each glyph is rendered. When the reference orientation direction is horizontal and the 'glyph-orientation-horizontal' results in an orientation angle that is a multiple of 180 degrees, then the current text position is incremented according to the horizontal metrics of the glyph. Otherwise, if the 'glyph-orientation-vertical' results in an orientation angle that is not a multiple of 180 degrees, then the current text position is incremented according to the vertical metrics of the glyph.

10.7.4 Relationship with bidirectionality

The characters in certain scripts are written from right to left. In some documents, in particular those written with the Arabic or Hebrew script, and in some mixed-language contexts, text in a single line may appear with mixed directionality. This phenomenon is called bidirectionality, or "bidi" for short.

The Unicode standard ([UNICODE], section 3.11) defines a complex algorithm for determining the proper directionality of text. The algorithm consists of an implicit part based on character properties, as well as explicit controls for embeddings and overrides. The SVG user agent applies this bidirectional algorithm when determining the layout of characters within a 'text' element. The 'direction' and 'unicode-bidi' properties allow authors to override the inherent directionality of the content characters and thus explicitly control how the elements and attributes of a document language map to this algorithm. These two properties are applicable to all characters who glyphs are perpendicular to the inline progression direction.

In most cases, the bidirectional algorithm from [UNICODE] produces the desired result automatically, and overriding this algorithm properly is usually quite complex. Therefore, in most cases, authors are discouraged from assigning values to these properties.

A more complete discussion of bidirectionality can be found in the "Cascading Style Sheets (CSS) level 2" specification [CSS2].

The processing model for bidirectional text is as follows. The user agent processes the characters which are provided in logical order (i.e., the order the characters appear in the original document, either via direct inclusion of via indirect reference due a 'tref' element). The user agent determines the set of independent blocks within each of which it should apply the Unicode bidirectional algorithm. Each text chunk represents an independent block of text. Additionally, any change in glyph orientation due to processing of properties 'glyph-orientation-horizontal' or 'glyph-orientation-vertical' will subdivide the independent blocks of text further. After processing the Unicode bidirectional algorithm and properties 'direction' and 'unicode-bidi' on each of the independent text blocks, the user agent will have a potentially re-ordered list of characters which are now in left-to-right rendering order. Simultaneous with re-ordering of the characters, the dx, dy and rotate attributes on the 'tspan' and 'tref' elements are also re-ordered to maintain the original correspondence between characters and attribute values. While kerning or ligature processing might be font-specific, the preferred model is that kerning and ligature processing occurs between combinations of characters or glyphs after the characters have been re-ordered.

This property specifies the base writing direction of text and the direction of embeddings and overrides (see 'unicode=bidi') for the Unicode bidirectional algorithm. For the 'direction' property to have any effect, the 'unicode=bidi' property's value must be 'embed' or 'override'.

Except for any additional information provided in this specification, the normative definition of the property is in [CSS2].

The 'direction' property applies only to glyphs oriented perpendicular to the inline progression direction, which includes the usual case of horizontally-oriented Latin or Arabic text and the case of narrow-cell Latin or Arabic characters rotated 90 degrees clockwise relative to a top-to-bottom inline progression direction.

Except for any additional information provided in this specification, the normative definition of the property is in [CSS2].

10.8 Text rendering order

The glyphs associated with the characters within a 'text' element are rendered in the logical order of the characters in the original document, independent of any re-ordering necessary to implement bidirectionality. Thus, for text that goes right-to-left visually, the glyphs associated with the rightmost character are rendered before the glyphs associated with the other characters.

Additionally, each distinct glyph is rendered in its entirety (i.e., it is filled and stroked as specified by the 'fill' and 'stroke' properties) before the next glyph gets rendered.

10.9 Alignment properties

10.9.1 Text alignment properties

The 'text-anchor' property is used to align (start-, middle- or end-alignment) a string of text relative to a given point.

The 'text-anchor' property is applied to each individual text chunk within a given 'text' element. Each text chunk has an initial current text position, which represents the point in the user coordinate system resulting from (depending on context) application of the x and y attributes on the 'text' element, any x or y attribute values on a 'tspan', 'tref' or 'altGlyph' element assigned explicitly to the first rendered character in a text chunk, or determination of the initial current text position for a 'textPath' element.

Values have the following meanings:

start

The rendered characters are aligned such that the start of the text string is at the initial current text position. For Latin or Arabic, which is usually rendered horizontally, this is comparable to left alignment. For Asian text with a vertical primary text direction, this is comparable to top alignment.

middle

The rendered characters are aligned such that the middle of the text string is at the current text position. (For text on a path, conceptually the text string is first laid out in a straight line. The midpoint between the start of the text string and the end of the text string is determined. Then, the text string is mapped onto the path with this midpoint placed at the current text position.)

end

The rendered characters are aligned such that the end of the text string is at the initial current text position. For Latin text in its usual orientation, this is comparable to right alignment.

10.9.2 Baseline alignment properties

An overview of baseline alignment and baseline tables can be found above in Fonts, font tables and baselines.

One of the characteristics of international text is that there are different baselines (different alignment points) for glyphs in different scripts. For example, in horizontal writing, ideographic scripts, such as Han Ideographs, Katakana, Hiragana, and Hangul, alignment occurs with a baseline near the bottoms of the glyphs; alphabetic based scripts, such as Latin, Cyrillic, Hebrew, Arabic, align a point that is the bottom of most glyphs, but some glyphs descend below the baseline; and Indic based scripts are aligned at a point that is near the top of the glyphs.

When different scripts are mixed on a line of text, an adjustment must be made to ensure that the glyphs in the different scripts are aligned correctly with one another. Open Type [OPENTYPE] fonts have a Baseline table (BASE) [OPENTYPE-BASETABLE] that specifies the offsets of the alternative baselines from the current baseline.

SVG uses a similar baseline table model that assumes one script (at one font-size) is the "dominant run" during processing of a 'text' element; that is, all other baselines are defined in relation to this dominant run. The baseline of the script with the dominant run is called the dominant baseline. So, for example, if the dominant baseline is the alphabetic baseline, there will be offsets in the baseline table for the alternate baselines, such as the ideographic baseline and the Indic baseline. There will also be an offset for the math baseline which is used for some math fonts. Note that there are separate baseline tables for horizontal and vertical writing-modes. The offsets in these tables may be different for horizontal and vertical writing.

The baseline table established at the start of processing of a 'text' element is called the dominant baseline table.

Because the value of the 'font-family' property is a list of fonts, to insure a consistent choice of baseline table we define the nominal font in a font list as the first font in the list for which a glyph is available. This is the first font that could contain a glyph for each character encountered. (For this definition, glyph data is assumed to be present if a font substitution is made or if the font is synthesized.) This definition insures a content independent determination of the font and baseline table that is to be used.

The value of the 'font-size' property on the 'text' element establishes the dominant baseline table font size.

The model assumes that each glyph has a 'alignment-baseline' value which specifies the baseline with which the glyph is to be aligned. (The 'alignment-baseline' is called the "Baseline Tag" in the OpenType baseline table description.) The initial value of the 'alignment-baseline' property uses the baseline identifier associated with the given glyph. Alternate values for 'alignment-baseline' can be useful for glyphs such as a "*" which are ambiguous with respect to script membership.

The model assumes that the font from which the glyph is drawn also has a baseline table, the font baseline table. This baseline table has offsets in units-per-em from the (0,0) point to each of the baselines the font knows about. In particular, it has the offset from the glyph's (0,0) point to the baseline identified by the 'alignment-baseline'.

The offset values in the baseline table are in "design units" which means fractional units of the EM. CSS calls these "units-per-em" [CSS2-UNITSPEREM]. Thus, the current 'font-size' is used to determine the actual offset from the dominant baseline to the alternate baselines.

The glyph is aligned so that its baseline identified by its 'alignment-baseline' is aligned with the baseline with the same name from the dominant baseline table.

The offset from the dominant baseline of the parent to the baseline identified by the 'alignment-baseline' is computed using the dominant baseline table and dominant baseline table font size. The font baseline table and font size applicable to the glyph are used to compute the offset from the identified baseline to the (0,0) point of the glyph. This second offset is subtracted from the first offset to get the position of the (0,0) point in the shift direction. Both offsets are computed by multiplying the baseline value from the baseline table times the appropriate font size value.

If the 'alignment-baseline' identifies the dominant baseline, then the first offset is zero and the glyph is aligned with the dominant baseline; otherwise, the glyph is aligned with the chosen alternate baseline.

The baseline identifiers below are used in this specification. Some of these are determined by baseline-tables contained in the nominal font. Others are computed from other font data as described below.

alphabetic

This identifies the baseline used by most alphabetic and syllabic scripts. These include, but are not limited to the western, southern indic, southeast asian (non-ideographic) scripts.

ideographic

This identifies the baseline used by ideographic scripts. For historical reasons, this baseline is at the bottom of the ideographic EM box and not in the center of the ideographic EM box. See the 'central' baseline. The ideographic scripts include Chinese, Japanese, Korean and Vietnamese Chu Nom.

hanging

This identifies the baseline used by northern indic scripts. These scripts include Devanagari, Gurmurhki and Bengali.

mathematical

This identifies the baseline used by mathematical symbols.

central

This identifies a computed baseline that is at the center of the EM box. This baseline lies halfway between the text-before-edge and text-after-edge baselines. For ideographic fonts, this baseline is often used to align the glyphs; it is an alternative to the ideographic baseline.

middle

This identifies a computed baseline that is offset from the alphabetic baseline in the shift direction by 1/2 the value of the x-height font characteristic.

text-before-edge

This identifies the before-edge of the EM box. The position of this baseline may be specified in the baseline-table or it may be calculated. The position of this baseline is normally around or at the top of the ascenders, but it may not encompass all accents that can appear above a glyph. For ideographic fonts, the position of this baseline is normally 1 EM in the shift direction from the "ideographic" baseline. However, some ideographic fonts have a reduced width in the inline-progression-direction to allow tighter setting. When such a font, designed only for vertical writing-modes, is used in a horizontal writing-mode, the text-before-edge" baseline may me less than 1 EM from the text-after-edge.

text-after-edge

This identifies the after-edge of the EM box. The position of this baseline may be specified in the baseline-table or it may be calculated. For fonts with descenders, this is normally around or at the bottom of the descenders. For these fonts the value of the "descent' font characteristic is used. For ideographic fonts, the position of this baseline is normally at the "ideographic" baseline.

There are, in addition, two computed baseline that are only defined for line areas. Since SVG does not support the notion of computations based on line areas, the two computed baselines are mapped as follows:

before-edge

For SVG, this is equivalent to text-before-edge.

after-edge

For SVG, this is equivalent to text-after-edge.

There are also four baselines that are defined only for horizontal writing-modes.

top

This baseline is the same as the "before-edge" baseline in a horizontal writing-mode and is undefined in a vertical writing mode.

text-top

This baseline is the same as the "text-before-edge" baseline in a horizontal writing-mode and is undefined in a vertical writing mode.

bottom

This baseline is the same as the "after-edge" baseline in a horizontal writing-mode and is undefined in a vertical writing mode.

text-bottom

This baseline is the same as the "text-after-edge" baseline in a horizontal writing-mode and is undefined in a vertical writing mode.

The baseline-alignment properties follow.

The "dominant-baseline" property is used to determine or re-determine a scaled-baseline-table. A scaled-baseline-table is a compound value with three components: a dominant-baseline, a baseline-table and a baseline-table font-size. When the initial value, "auto", would give an incorrect result, this property can be used to explicitly set the desire scaled-baseline-table.

Values for the property have the following meaning:

auto

If this property occurs on a 'text' element, then the computed value depends on the value of the 'writing-mode' property. If the "writing-mode" is horizontal, then the value the dominant-baseline component is "alphabetic", else if the "writing-mode" is vertical, then the value the dominant-baseline component is "center". The baseline-table font-size component is set to the value of the "font-size" property on the formatting object on which the "dominant-baseline" property occurs.

If this property occurs on a 'tspan', 'tref', 'altGlyph' or 'textPath' element, then the dominant-baseline and the baseline-table components remain the same as those of the parent formatting object. If the computed "baseline-shift" value actually shifts the baseline, then the baseline-table font-size component is set to the value of the "font-size" property on the formatting object on which the "dominant-baseline" property occurs, otherwise the baseline-table font-size remains the same as that of the parent formatting object. If there is no parent formatting object, the scaled-baseline-table value is constructed as above for 'text' elements.

use-script

The dominant-baseline and the baseline-table components are set by determining the predominant script of the character data content. The "writing-mode", whether horizontal or vertical, is used to select the appropriate set of baseline-tables and the dominant baseline is used to select the baseline-table that corresponds to that baseline. The baseline-table font-size component is set to the value of the "font-size" property on the formatting object on which the "dominant-baseline" property occurs.

no-change

The dominant-baseline, the baseline-table, and the baseline-table font-size remain the same as that of the parent formatting object.

reset-size

The dominant-baseline and the baseline-table remain the same, but the baseline-table font-size is changed to the value of the "font-size" property on this formatting object. This re-scales the baseline-table for the current "font-size".

ideographic

The dominant-baseline is set to the "ideographic" baseline using the baseline-table and baseline-table font-size of the parent area, the baseline-table is changed to correspond to the "ideographic" baseline, and the baseline-table font-size is changed to the value of the "font-size" property on this formatting object.

alphabetic

The dominant-baseline is set to the "alphabetic" baseline using the baseline-table and baseline-table font-size of the parent area, the baseline-table is changed to correspond to the "alphabetic" baseline, and the baseline-table font-size is changed to the value of the "font-size" property on this formatting object.

hanging

The dominant-baseline is set to the "hanging" baseline using the baseline-table and baseline-table font-size of the parent area, the baseline-table is changed to correspond to the "hanging" baseline, and the baseline-table font-size is changed to the value of the "font-size" property on this formatting object.

mathematical

The dominant-baseline is set to the "mathematical" baseline using the baseline-table and baseline-table font-size of the parent area, the baseline-table is changed to correspond to the "mathematical" baseline, and the baseline-table font-size is changed to the value of the "font-size" property on this formatting object.

If there is no baseline table in the nominal font or if the baseline table lacks an entry for the desired baseline, then the User Agent may use heuristics to determine the position of the desired baseline.

This property specifies how an object is aligned with respect to its parent. That is, to which of the parent's baselines the alignment-point of this object is aligned. The alignment-point defaults to the baseline with the same name as the value. That is, for the position of "ideographic" alignment-point is the position of the "ideographic" baseline in the object being aligned.

Values have the following meanings:

auto

For SVG, this value represents the dominant-baseline of the script to which the character belongs.

baseline

The alignment-point of the object being aligned is aligned with the dominant-baseline of the parent area.

before-edge

The alignment-point of the object being aligned is aligned with the "before-edge" baseline of the parent area.

text-before-edge

The alignment-point of the object being aligned is aligned with the "text-before-edge" baseline of the parent area.

central

The alignment-point of the object being aligned is aligned with the "central" baseline of the parent area.

middle

The alignment-point of the object being aligned is aligned with the "middle" baseline of the parent area.

after-edge

The alignment-point of the object being aligned is aligned with the "after-edge" baseline of the parent area.

text-after-edge

The alignment-point of the object being aligned is aligned with the "text-after-edge" baseline of the parent area.

ideographic

The alignment-point of the object being aligned is aligned with the "ideographic" baseline of the parent area.

alphabetic

The alignment-point of the object being aligned is aligned with the "alphabetic" baseline of the parent area.

hanging

The alignment-point of the object being aligned is aligned with the "hanging" baseline of the parent area.

mathematical

The alignment-point of the object being aligned is aligned with the "mathematical" baseline of the parent area.

top

The alignment-point of the object being aligned is aligned with the "top" baseline of the parent area if the writing-mode is horizontal. Otherwise, the dominant-baseline is used.

bottom

The alignment-point of the object being aligned is aligned with the "bottom" baseline of the parent area if the writing-mode is horizontal. Otherwise, the dominant-baseline is used.

text-top

The alignment-point of the object being aligned is aligned with the "text-top" baseline of the parent area if the writing-mode is horizontal. Otherwise, the dominant-baseline is used.

text-bottom

The alignment-point of the object being aligned is aligned with the "text-bottom" baseline of the parent area if the writing-mode is horizontal. Otherwise, the dominant-baseline is used.

The 'baseline-shift' property allows repositioning of the dominant-baseline relative to the dominant-baseline of the parent area. The shifted object might be a sub- or superscript. Within the shifted object, the whole baseline-table is offset; not just a single baseline. The amount of the shift is determined from information from the parent area, the sub- or superscript offset from the nominal font of the parent area, percent of the "line-height" of the parent area or an absolute value.

In SVG, the 'baseline-shift' property represents a supplemental adjustment to the baseline tables. The 'baseline-shift' property shifts the baseline tables for each glyph to temporary new positions, for example to lift the glyph into superscript or subscript position, but it does not effect the current text position. When the current text position is adjusted after rendering a glyph to take into account glyph advance values, the adjustment happens as if there were no baseline shift.

'baseline-shift' properties can nest. Each nested 'baseline-shift' is added to previous baseline shift values.

Values for the property have the following meaning:

baseline

There is no baseline shift; the dominant-baseline remains in its original position.

sub

The dominant-baseline is shifted to the default position for subscripts. The offset to this position is determined by the font data for the nominal font as adjusted by the dominant baseline-table font-size. If there is no applicable font data the User Agent may use heuristics to determine the offset.

super

The dominant-baseline is shifted to the default position for superscripts. The offset to this position is determined by the font data for the nominal font as adjusted by the dominant baseline-table font-size. If there is no applicable font data the User Agent may use heuristics to determine the offset.

<percentage>

The computed value of the property is this percentage multiplied by the computed "line-height" of the 'text' element. The dominant-baseline is shifted in the shift direction (positive value) or opposite to the shift direction (negative value) of the parent area by the computed value. A value of "0%" is equivalent to "baseline".

<length>

The dominant-baseline is shifted in the shift direction (positive value) or opposite to the shift direction (negative value) of the parent area by the <length< value. A value of "0cm" is equivalent to "baseline".

10.10 Font selection properties

SVG uses the following font specification properties. Except for any additional information provided in this specification, the normative definition of the property is in [CSS2]. Any SVG-specific notes about these properties are contained in the descriptions below.

This property indicates which font family is to be used to render the text, specified as a prioritized list of font family names and/or generic family names. Except for any additional information provided in this specification, the normative definition of the property is in [CSS2].

This property specifies whether the text is to be rendered using a normal, italic or oblique face. Except for any additional information provided in this specification, the normative definition of the property is in [CSS2].

This property indicates whether the text is to be rendered using the normal glyphs for lowercase characters or using small-caps glyphs for lowercase characters. Except for any additional information provided in this specification, the normative definition of the property is in [CSS2].

This property refers to the boldness or lightness of the glyphs used to render the text, relative to other fonts in the same font family. Except for any additional information provided in this specification, the normative definition of the property is in [CSS2].

This property indicates the desired amount of condensing or expansion in the glyphs used to render the text. Except for any additional information provided in this specification, the normative definition of the property is in [CSS2].

This property refers to the size of the font from baseline to baseline when multiple lines of text are set solid in a multiline layout environment. For SVG, if a <length> is provided without a unit identifier (e.g., an unqualified number such as 128), the SVG user agent processes the <length> as a height value in the current user coordinate system.

If a <length> is provided with one of the unit identifiers (e.g., 12pt or 10%), then the SVG user agent converts the <length> into a corresponding value in the current user coordinate system by applying the rules described in Units.

Except for any additional information provided in this specification, the normative definition of the property is in [CSS2].

This property allows authors to specify an aspect value for an element that will preserve the x-height of the first choice font in a substitute font. Except for any additional information provided in this specification, the normative definition of the property is in [CSS2].

Shorthand property for setting 'font-style', 'font-variant', 'font-weight', 'font-size', 'line-height' and 'font-family'. The 'line-height' property has no visual effect in SVG. Conforming SVG Viewers are not required to support the various system font options (caption, icon, menu, message-box, small-caption and status-bar) and can use a system font or one of the generic fonts instead.

Except for any additional information provided in this specification, the normative definition of the property is in [CSS2].

10.11 Spacing properties

Three properties affect the space between characters and words:

• 'kerning' indicates whether the user agent should adjust inter-glyph spacing based on kerning tables that are included in the relevant font (i.e., enable auto-kerning) or instead disable auto-kerning and instead set inter-character spacing to a specific length (typically, zero).
• 'letter-spacing' indicates an amount of space that is to be added between text characters supplemental to any spacing due to the 'kerning' property.
• 'word-spacing' indicates the spacing behavior between words.

The value of auto indicates that the user agent should adjust inter-glyph spacing based on kerning tables that are included in the font that will be used (i.e., enable auto-kerning).

If a <length> is provided, then auto-kerning is disable. Instead, inter-character spacing is set to the given <length>. The most common scenario, other than auto, is to set 'kerning' to a value of 0 so that auto-kerning is disabled.

If a <length> is provided without a unit identifier (e.g., an unqualified number such as 128), the SVG user agent processes the <length> as a width value in the current user coordinate system.

If a <length> is provided with one of the unit identifiers (e.g., .25em or 1%), then the SVG user agent converts the <length> into a corresponding value in the current user coordinate system by applying the rules described in Units.

When a <length> is provided, its value is added to the inter-character spacing value specified by the 'letter-spacing' property.

This property specifies spacing behavior between text characters supplemental to any spacing due to the 'kerning' property.

For SVG, if a <length> is provided without a unit identifier (e.g., an unqualified number such as 128), the SVG user agent processes the <length> as a width value in the current user coordinate system.

If a <length> is provided with one of the unit identifiers (e.g., .25em or 1%), then the SVG user agent converts the <length> into a corresponding value in the current user coordinate system by applying the rules described in Units.

Except for any additional information provided in this specification, the normative definition of the property is in [CSS2].

This property specifies spacing behavior between words. For SVG, if a <length> is provided without a unit identifier (e.g., an unqualified number such as 128), the SVG user agent processes the <length> as a width value in the current user coordinate system.

If a <length> is provided with one of the unit identifiers (e.g., .25em or 1%), then the SVG user agent converts the <length> into a corresponding value in the current user coordinate system by applying the rules described in Units.

Except for any additional information provided in this specification, the normative definition of the property is in [CSS2].

10.12 Text decoration

This property describes decorations that are added to the text of an element. Conforming SVG Viewers are not required to support the blink value.

Except for any additional information provided in this specification, the normative definition of the property is in [CSS2].

The CSS2 specification [CSS2] defines the behavior of the 'text-decoration' property using the terminology "block-level elements" and "inline elements". For the purposes of the 'text-decoration' property and SVG, a 'text' element represents a block-level element and any of the potential children of a 'text' element (e.g., a 'tspan') represent inline elements.

Also, the CSS2 definition of 'text-decoration' specifies that the "color of the decorations" remain the same on descendant elements. Since SVG offers a painting model consisting of the ability to apply various types of paint (see Painting: Filling, Stroking and Marker Symbols) to both the interior (i.e., the "fill") and the outline (i.e., the "stroke") of text, for SVG the 'text-decoration' property is defined such that, for an element which has a specified value for the 'text-decoration' property, all decorations on its content and that of its descendants are rendered using the same fill and stroke properties as are present on the given element. If the 'text-decoration' property is specified on a descendant, then that overrides the ancestor.

Example textdecoration01, which uses an internal CSS style sheet, provides examples for 'text-decoration'. The first line of text has no value for 'text-decoration', so the initial value of 'text-decoration:none' is used. The second line shows 'text-decoration:line-through'. The third line shows 'text-decoration:underline'. The fourth line illustrates the rule whereby decorations are rendered using the same fill and stroke properties as are present on the element for which the 'text-decoration' is specified. Since 'text-decoration' is specified on the 'text' element, all text within the 'text' element has its underline rendered with the same fill and stroke properties as exist on the 'text' element (i.e., blue fill, red stroke), even though the various words have different fill and stroke property values. However, the word "different" explicitly specifies a value for 'text-decoration'; thus, its underline is rendered using the fill and stroke properties as the 'tspan' element that surrounds the word "different" (i.e., yellow fill, darkgreen stroke):

<?xml version="1.0" standalone="no"?>
<!DOCTYPE svg PUBLIC "-//W3C//DTD SVG 20001102//EN" 
  "http://www.w3.org/TR/2000/CR-SVG-20001102/DTD/svg-20001102.dtd">
<svg width="12cm" height="4cm" viewBox="0 0 1200 400">
  <desc>Example textdecoration01 - behavior of 'text-decoration' property</desc>
  <defs>
    <style type="text/css"><![CDATA[
      text { font-size:60; fill:blue; stroke:red; stroke-width: 1 }
    ]]></style>
  </defs>
  <rect x="1" y="1" width="1198" height="398" style="fill:none; stroke:blue"/>
  <text x="100" y="75">Normal text</text>
  <text x="100" y="165" style="text-decoration:line-through">Text with line-through</text>
  <text x="100" y="255" style="text-decoration:underline">Underlined text</text>
  <text x="100" y="345" style="text-decoration:underline">
    <tspan>One </tspan>
    <tspan style="fill:yellow; stroke:purple">word </tspan>
    <tspan style="fill:yellow; stroke:black">has </tspan>
    <tspan style="fill:yellow; stroke:darkgreen; text-decoration:underline">different</tspan>
    <tspan style="fill:yellow; stroke:blue"> underlining</tspan>
  </text>
</svg>

Example textdecoration01

View this example as SVG (SVG-enabled and CSS-enabled browsers only)
 

10.13 Text on a path

10.13.1 Introduction to text on a path

In addition to text drawn in a straight line, SVG also includes the ability to place text along the shape of a 'path' element. To specify that a block of text is to be rendered along the shape of a 'path', include the given text within a 'textPath' element which includes an xlink:href attribute with a URI reference to a 'path' element.

10.13.2 The 'textPath' element

<!ENTITY % textPathExt "" >
<!ELEMENT textPath (#PCDATA|desc|title|metadata|tspan|tref|altGlyph|a|animate|set|animateColor
                %textPathExt;)* >
<!ATTLIST textPath
  %stdAttrs;
  %xlinkRefAttrs;
  xlink:href %URI; #REQUIRED
  %langSpaceAttrs;
  %testAttrs;
  externalResourcesRequired %Boolean; #IMPLIED
  class %ClassList; #IMPLIED
  style %StyleSheet; #IMPLIED
  %PresentationAttributes-FillStroke;
  %PresentationAttributes-FontSpecification;
  %PresentationAttributes-Graphics;
  %PresentationAttributes-TextContentElements;
  %graphicsElementEvents;
  startOffset CDATA #IMPLIED
  textLength %Length; #IMPLIED
  lengthAdjust (spacing|spacingAndGlyphs) #IMPLIED
  method (align|stretch) #IMPLIED
  spacing (auto|exact) #IMPLIED >

The path data coordinates within the referenced 'path' element are assumed to be in the same coordinate system as the current 'text' element, not in the coordinate system where the 'path' element is defined. The transform attribute on the referenced 'path' element represents a supplemental transformation relative to the current user coordinate system for the current 'text' element, including any adjustments to the current user coordinate system due to a possible transform attribute on the current 'text' element. For example, the following fragment of SVG content:

<svg>
  <g transform="translate(25,25)">
    <defs>
      <path id="path1" transform="scale(2)" d="..." style="fill:none; stroke:red"/>
    </defs>
  </g>
  <text transform="rotate(45)">
    <textPath xlink:href="#path1">Text along path1</textPath>
  </text>
</svg>

should have the same effect as the following:

<svg>
  <g transform="rotate(45)">
    <defs>
      <path id="path1" transform="scale(2)" d="..." style="fill:none; stroke:red"/>
    </defs>
    <text>
      <textPath xlink:href="#path1">Text along path1</textPath>
    </text>
  </g>
</svg>

Note that the transform="translate(25,25)" has no effect on the 'textPath' element, whereas the transform="rotate(45)" applies to both the 'text' and the use of the 'path' element as the referenced shape for text on a path.

Example toap01 provides a simple example of text on a path:

<?xml version="1.0" standalone="no"?>
<!DOCTYPE svg PUBLIC "-//W3C//DTD SVG 20001102//EN" 
  "http://www.w3.org/TR/2000/CR-SVG-20001102/DTD/svg-20001102.dtd">
<svg width="10cm" height="3cm" viewBox="0 0 1000 300">
  <defs>
    <path id="MyPath"
          d="M 100 200 
             C 200 100 300   0 400 100
             C 500 200 600 300 700 200
             C 800 100 900 100 900 100" />
  </defs>
  <desc>Example toap01 - simple text on a path</desc>

  <use xlink:href="#MyPath" style="fill:none; stroke:red" />
  <text style="font-family:Verdana; font-size:42.3333; fill:blue">
    <textPath xlink:href="#MyPath">
      We go up, then we go down, then up again
    </textPath>
  </text>
</svg>

Example toap01

View this example as SVG (SVG-enabled browsers only)
 

Example toap02 shows how 'tspan' elements can be included within 'textPath' elements to adjust styling attributes and adjust the current text position before rendering a particular glyph. The first occurrence of the word "up" is filled with the color red. Attribute dy is used to lift the word "up" from the baseline.

<?xml version="1.0" standalone="no"?>
<!DOCTYPE svg PUBLIC "-//W3C//DTD SVG 20001102//EN" 
  "http://www.w3.org/TR/2000/CR-SVG-20001102/DTD/svg-20001102.dtd">
<svg width="10cm" height="3cm" viewBox="0 0 1000 300">
  <defs>
    <path id="MyPath"
          d="M 100 200 
             C 200 100 300   0 400 100
             C 500 200 600 300 700 200
             C 800 100 900 100 900 100" />
  </defs>
  <desc>Example toap02 - tspan within textPath</desc>

  <use xlink:href="#MyPath" style="fill:none; stroke:red" />
  <text style="font-family:Verdana; font-size:42.3333; fill:blue">
    <textPath xlink:href="#MyPath">
      We go 
      <tspan dy="-30" style="fill:red">
        up
      </tspan>
      <tspan dy="30">
        ,
      </tspan>
      then we go down, then up again
    </textPath>
  </text>
</svg>

Example toap02

View this example as SVG (SVG-enabled browsers only)
 

Example toap03 demonstrates the use of the startOffset attribute on the 'textPath' element to specify the start position of the text string as a particular position along the path. Notice that glyphs that fall off the end of the path are not rendered (see text on a path layout rules).

<?xml version="1.0" standalone="no"?>
<!DOCTYPE svg PUBLIC "-//W3C//DTD SVG 20001102//EN" 
  "http://www.w3.org/TR/2000/CR-SVG-20001102/DTD/svg-20001102.dtd">
<svg width="10cm" height="3cm" viewBox="0 0 1000 300">
  <defs>
    <path id="MyPath"
          d="M 100 200 
             C 200 100 300   0 400 100
             C 500 200 600 300 700 200
             C 800 100 900 100 900 100" />
  </defs>
  <desc>Example toap03 - text on a path with startOffset attribute</desc>

  <use xlink:href="#MyPath" style="fill:none; stroke:red" />
  <text style="font-family:Verdana; font-size:42.3333; fill:blue">
    <textPath xlink:href="#MyPath" startOffset="80%">
      We go up, then we go down, then up again
    </textPath>
  </text>
</svg>

Example toap03

View this example as SVG (SVG-enabled browsers only)
 

10.13.3 Text on a path layout rules

Conceptually, for text on a path the target path is stretched out into either a horizontal or vertical straight line segment. For horizontal text layout flows, the path is stretched out into a hypothetical horizontal line segment such that the start of the path is mapped to the left of the line segment. For vertical text layout flows, the path is stretched out into a hypothetical vertical line segment such that the start of the path is mapped to the top of the line segment. The standard text layout rules are applied to the hypothetical straight line segment and the result is mapped back onto the target path. Vertical and bidirectional text layout rules also apply to text on a path.

The reference orientation is determined individually for each glyph that is rendered along the path. For horizontal text layout flows, the reference orientation for a given glyph is the vector that starts at the intersection point on the path to which the glyph is attached and which points in the direction 90 degrees counter-clockwise from the angle of the curve at the intersection point. For vertical text layout flows, the reference orientation for a given glyph is the vector that starts at the intersection point on the path to which the glyph is attached and which points in the direction 180 degrees from the angle of the curve at the intersection point.

Example toap04 will be used to illustrate the particular layout rules for text on a path that supplement the basic text layout rules for straight line horizontal or vertical text.

<?xml version="1.0" standalone="no"?>
<!DOCTYPE svg PUBLIC "-//W3C//DTD SVG 20001102//EN" 
  "http://www.w3.org/TR/2000/CR-SVG-20001102/DTD/svg-20001102.dtd">
<svg width="10cm" height="3cm" viewBox="0 0 1000 300">
  <defs>
    <path id="MyPath"
          d="M 100 100 
             C 150 100 250 200 300 200
             C 350 200 450 100 500 100
             C 550 100 650 200 700 200
             C 750 200 850 100 900 100" />
  </defs>
  <desc>Example toap04 = text on a path layout rules</desc>

  <use xlink:href="#MyPath" style="fill:none; stroke:red" />
  <text style="font-family:Verdana; font-size:63.5; fill:blue">
    <textPath xlink:href="#MyPath">
      Choose shame or get war 
    </textPath>
  </text>
</svg>

Example toap04

View this example as SVG (SVG-enabled browsers only)
 

The following picture does an initial zoom in on the first glyph in the 'text' element.

The small dot above shows the point at which the glyph is attached to the path. The box around the glyph shows the glyph is rotated such that its horizontal axis is parallel to the tangent of the curve at the point at which the glyph is attached to the path. The box also shows the glyph's charwidth (i.e., the amount which the current text position advances horizontally when the glyph is drawn using horizontal text layout).

The next picture zooms in further to demonstrate the detailed layout rules.

For left-to-right horizontal text layout along a path (i.e., when the glyph orientation is perpendicular to the inline progression direction), the layout rules are as follows:

• Determine the startpoint-on-the-path for the first glyph using attribute startOffset and property 'text-anchor'. For 'text-anchor:start', startpoint-on-the-path is the point on the path which represents the point on the path which is startOffset distance along the path from the start of the path, calculated using the user agent's distance along the path algorithm. For 'text-anchor:middle', startpoint-on-the-path is the point on the path which represents the point on the path which is [ startOffset minus half of the total advance values for all of the glyphs in the 'textPath' element ]. distance along the path from the start of the path, calculated using the user agent's distance along the path algorithm. For 'text-anchor:end', startpoint-on-the-path is the point on the path which represents the point on the path which is [ startOffset minus the total advance values for all of the glyphs in the 'textPath' element ]. Before rendering the first glyph, the horizontal component of the startpoint-on-the-path is adjusted to take into account various horizontal alignment text properties and attributes, such as a dx attribute value on a 'tspan' element. (In the picture above, the startpoint-on-the-path is the leftmost dot on the path.)
• Determine the glyph's charwidth (i.e., the amount which the current text position advances horizontally when the glyph is drawn using horizontal text layout). (In the picture above, the charwidth is the distance between the two dots at the side of the box.)
• Determine the point on the curve which is charwidth distance along the path from the startpoint-on-the-path for this glyph, calculated using the user agent's distance along the path algorithm. This point is the endpoint-on-the-path for the glyph. (In the picture above, the endpoint-on-the-path for the glyph is the rightmost dot on the path.)
• Determine the midpoint-on-the-path, which is the point on the path which is "halfway" (user agents can choose either a distance calculation or a parametric calculation) between the startpoint-on-the-path and the endpoint-on-the-path. (In the picture above, the midpoint-on-the-path is shown as a white dot.)
• Determine the glyph-midline, which is the vertical line in the glyph's coordinate system that goes through the glyph's x-axis midpoint. (In the picture above, the glyph-midline is shown as a dashed line.)
• Position the glyph such that the glyph-midline passes through the midpoint-on-the-path and is perpendicular to the line through the startpoint-on-the-path and the endpoint-on-the-path.
• Align the glyph vertically relative to the midpoint-on-the-path based on property 'alignment-baseline' and any specified values for attribute dy on a 'tspan' element. In the example above, the 'alignment-baseline' property is unspecified, so the initial value of 'alignment-baseline:baseline' will be used. There are no 'tspan' elements; thus, the baseline of the glyph is aligned to the midpoint-on-the-path.
• For each subsequent glyph, set a new startpoint-on-the-path as the previous endpoint-on-the-path, but with appropriate adjustments taking into account horizontal kerning tables in the font and current values of various attributes and properties, including spacing properties and 'tspan' elements with values provided for attributes dx and dy. All adjustments are calculated as distance adjustments along the path, calculated using the user agent's distance along the path algorithm.
• Glyphs whose midpoint-on-the-path are off either end of the path are not rendered.
• Continue rendering glyphs until there are no more glyphs.

Comparable rules are used for top-to-bottom vertical text layout along a path (i.e., when the glyph orientation is parallel with the inline progression direction), the layout rules are as follows:

• Determine the startpoint-on-the-path using the same method as for horizontal text layout along a path, except that before rendering the first glyph, the horizontal component of the startpoint-on-the-path is adjusted to take into account varoius vertical alignment text properties and attributes, such as a dy attribute value on a 'tspan' element.
• Determine the glyph's charheight (i.e., the amount which the current text position advances vertically when the glyph is drawn using vertical text layout).
• Determine the point on the curve which is charheight distance along the path from the startpoint-on-the-path for this glyph, calculated using the user agent's distance along the path algorithm. This point is the endpoint-on-the-path for the glyph.
• Determine the midpoint-on-the-path, which is the point on the path which is "halfway" (user agents can choose either a distance calculation or a parametric calculation) between the startpoint-on-the-path and the endpoint-on-the-path.
• Determine the glyph-midline, which is the horizontal line in the glyph's coordinate system that goes through the glyph's y-axis midpoint.
• Position the glyph such that the glyph-midline passes through the midpoint-on-the-path and is perpendicular to the line through the startpoint-on-the-path and the endpoint-on-the-path.
• Align the glyph horizontally (where horizontal is relative to the glyph's coordinate system) relative to the midpoint-on-the-path based on property 'alignment-baseline' and any specified values for attribute dx on a 'tspan' element.
• For each subsequent glyph, set a new startpoint-on-the-path as the previous endpoint-on-the-path, but with appropriate adjustments taking into account vertical kerning tables in the font and current values of various attributes and properties, including spacing properties and 'tspan' elements with values provided for attributes dx and dy. All adjustments are calculated as distance adjustments along the path, calculated using the user agent's distance along the path algorithm.
• Glyphs whose midpoint-on-the-path are off either end of the path are not rendered.
• Continue rendering glyphs until there are no more glyphs.

In the calculations above, if either the startpoint-on-the-path or the endpoint-on-the-path is off the end of the path, then extend the path beyond its end points with a straight line that is parallel to the tangent at the path at its end point so that the midpoint-on-the-path can still be calculated.

When the inline progression direction is horizontal, then any x attributes on 'tspan', 'tref' or 'altGlyph' elements represent new absolute offsets along the path, thus providing explicit new values for startpoint-on-the-path. Any y attributes on 'tspan', 'tref' or 'altGlyph' elements are ignored. When the inline progression direction is horizontal, then any y attributes on 'tspan', 'tref' or 'altGlyph' elements represent new absolute offsets along the path, thus providing explicit new values for startpoint-on-the-path. Any x attributes on 'tspan', 'tref' or 'altGlyph' elements are ignored.

10.14 Alternate glyphs

There are situations such as ligatures, special-purpose fonts (e.g., a font for music symbols) or alternate glyphs for Asian text strings where it is required that a different set of glyphs is used than the glyph(s) which normally corresponds to the given character data.

The 'altGlyph' element provides control over the glyphs used to render particular character data.

<!ENTITY % altGlyphExt "" >
<!ELEMENT altGlyph (#PCDATA %altGlyphExt;)* >
<!ATTLIST altGlyph
  %stdAttrs;
  %xlinkRefAttrs;
  xlink:href %URI; #IMPLIED 
  glyphRef CDATA #IMPLIED
  format CDATA #IMPLIED
  %testAttrs;
  %langSpaceAttrs;
  externalResourcesRequired %Boolean; #IMPLIED
  class %ClassList; #IMPLIED
  style %StyleSheet; #IMPLIED
  %PresentationAttributes-FillStroke;
  %PresentationAttributes-FontSpecification;
  %PresentationAttributes-Graphics;
  %PresentationAttributes-TextContentElements;
  %graphicsElementEvents;
  x %Coordinates; #IMPLIED
  y %Coordinates; #IMPLIED
  dx %Lengths; #IMPLIED
  dy %Lengths; #IMPLIED
  rotate CDATA #IMPLIED >

If the references to alternate glyphs do not result in successful identification of alternate glyphs to use, then the character(s) that are inside of the 'altGlyph' element are rendered as if there were not an 'altGlyph' element surrounding those characters.

An 'altGlyph' element either references a 'glyph' element or an 'altGlyphDef' element via its xlink:href attribute or identifies a glyph by means of font selection properties, a glyph identifier and a font format. If the xlink:href attribute is specified, it takes precedence, and the other glyph identification attributes and properties are ignored.

The 'altGlyphDef' element defines a set of possible glyph substitutions.

<!ENTITY % altGlyphDefExt "" >
<!ELEMENT altGlyphDef ((glyphRef+|altGlyphItem+) %altGlyphDefExt;) >
<!ATTLIST altGlyphDef
  %stdAttrs; >

An 'altGlyphDef' can contain either of the following:

• In the simplest case, an 'altGlyphDef' contains one or more 'glyphRef' elements. Each 'glyphRef' element references a single glyph within a particular font. If all of the referenced glyphs are available, then these glyphs are rendered instead of the character(s) inside of the referencing 'altGlyph' element. If any of the referenced glyphs are unavailable, then the character(s) that are inside of the 'altGlyph' element are rendered as if there were not an 'altGlyph' element surrounding those characters.
• In the more complex case, an 'altGlyphDef' contains one or more 'altGlyphItem' elements. Each 'altGlyphItem' represents a candidate set of substitute glyphs. Each 'altGlyphItem' contains one or more 'glyphRef' elements. Each 'glyphRef' element references a single glyph within a particular font. The first 'altGlyphItem' in which all referenced glyphs are available is chosen. The glyphs referenced from this 'altGlyphItem' are rendered instead of the character(s) that are inside of the referencing 'altGlyph' element. If none of the 'altGlyphItem' elements result in a successful match (i.e., none of the 'altGlyphItem' elements has all of its referenced glyphs available), then the character(s) that are inside of the 'altGlyph' element are rendered as if there were not an 'altGlyph' element surrounding those characters.

The 'altGlyphItem' element defines a candidate set of possible glyph substitutions. The first 'altGlyphItem' element whose referenced glyphs are all available is chosen. Its glyphs are rendered instead of the character(s) that are inside of the referencing 'altGlyph' element

<!ENTITY % altGlyphItemExt "" >
<!ELEMENT altGlyphItem (glyphRef+ %altGlyphItemExt;) >
<!ATTLIST altGlyphItem
  %stdAttrs; >

The 'glyphRef' element defines a possible glyph to use.

<!ELEMENT glyphRef EMPTY >
<!ATTLIST glyphRef
  %stdAttrs;
  %xlinkRefAttrs;
  xlink:href %URI; #IMPLIED
  class %ClassList; #IMPLIED
  style %StyleSheet; #IMPLIED
  %PresentationAttributes-FontSpecification;
  glyphRef CDATA #IMPLIED
  format CDATA #IMPLIED
  x %Number; #IMPLIED
  y %Number; #IMPLIED
  dx %Number; #IMPLIED
  dy %Number; #IMPLIED >

A 'glyphRef' either references a 'glyph' element in an SVG document fragment via its xlink:href attribute or identifies a glyph by means of font selection properties, a glyph identifier and a font format. If insufficient attributes and properties have been specified to identify a glyph, then the 'glyphRef' is processed in the same manner as when a glyph reference is fully specified, but the given glyph is not available. If the xlink:href attribute is specified, it takes precedence, and the other glyph identification attributes and properties are ignored.

10.15 White space handling

SVG supports the standard XML attribute xml:space to specify the handling of white space characters within a given 'text' element's character data. The SVG user agent has special processing rules associated with this attribute as described below. These are behaviors that occur subsequent to XML parsing [XML10] and any construction of a Document Object Model [DOM2].

xml:space is an inheritable attribute which can have one of two values:

• default (the initial/default value for xml:space) - When xml:space="default", the SVG user agent will do the following using a copy of the original character data content. First, it will remove all newline characters. Then it will convert all tab characters into space characters. Then, it will strip off all leading and trailing space characters. Then, all contiguous space characters will be consolidated.
• preserve - When xml:space="preserve", the SVG user agent will do the following using a copy of the original character data content. It will convert all newline and tab characters into space characters. Then, it will draw all space characters, including leading, trailing and multiple contiguous space characters. Thus, when drawn with xml:space="preserve", the string "a   b" (three spaces between "a" and "b") will produce a larger separation between "a" and "b" than "a b" (one space between "a" and "b").

The following examples illustrate that line indentation can be important when using xml:space="default". The fragments below show two pairs of equivalent 'text' elements. Each pair consists of two equivalent 'text' elements, with the first 'text' element using xml:space='default' and the second using xml:space='preserve'. For these examples, there is no extra white space at the end of any of the lines (i.e., the line break occurs immediately after the last visible character).

[01]  <text xml:space='default'>
[02]    WS example
[03]    indented lines
[04]  </text>
[05]  <text xml:space='preserve'>WS example indented lines</text>
[06]
[07]  <text xml:space='default'>
[08]WS example
[09]non-indented lines
[10]  </text>
[11]  <text xml:space='preserve'>WS examplenon-indented lines</text>

The first pair of 'text' elements above show the effect of indented character data. The attribute xml:space='default' in the first 'text' element instructs the user agent to:

• convert all tabs (if any) to space characters,
• strip out all line breaks (i.e., strip out the line breaks at the end of lines [01], [02] and [03]),
• strip out all leading space characters (i.e., strip out space characters before "WS example" on line [02]),
• strip out all trailing space characters (i.e., strip out space characters before "</text>" on line [04]),
• consolidate all intermediate space characters (i.e., the space characters before "indented lines" on line [03]) into a single space character.

The second pair of 'text' elements above show the effect of indented character data. The attribute xml:space='default' in the third 'text' element instructs the user agent to:

• convert all tabs (if any) to space characters,
• strip out all line breaks (i.e., strip out the line breaks at the end of lines [07], [08] and [09]),
• strip out all leading space characters (there are no leading space characters in this example),
• strip out all trailing space characters (i.e., strip out space characters before "</text>" on line [10]),
• consolidate all intermediate space characters into a single space character (in this example, there are no intermediate space characters).

Note that XML parsers are required to convert the standard representations for a newline indicator (e.g., the literal two-character sequence "#xD#xA" or the standalone literals #xD or #xA) into the single character #xA before passing character data to the application. Thus, each newline in SVG will be represented by the single character #xA, no matter what representation for newlines might have been used in the original resource. (See XML end-of-line handling.)

Any features in the SVG language or the SVG DOM that are based on character position number, such as the x, y, dx, dy and rotate attributes on the 'tspan', 'tref' 'altGlyph' elements, are based on character position after applying the white space handling rules described here. In particular, if xml:space="default", it is often the case that white space characters are removed as part of processing. Character position numbers index into the text string after the white space characters have been removed per the rules in this section.

The xml:space attribute is:

    Animatable: no.

10.16 Text selection and clipboard operations

Conforming SVG viewers on systems which have the capacity for text selection (e.g., systems which are equipped with a pointer device such as a mouse) and which have system clipboards for copy/paste operations are required to support:

• user selection of text strings in SVG content
• the ability to copy selected text strings to the system clipboard

A text selection operation starts when all of the following occur:

• the user positions the pointing device over a glyph that has been rendered as part of a 'text' element, initiates a select operation (e.g., pressing the standard system mouse button for select operations) and then moves the pointing device while continuing the select operation (e.g., continuing to press the standard system mouse button for select operations)
• no other visible graphics element has been painted above the glyph at the point at which the pointing device was clicked
• no links or events have been assigned to the 'text' , 'tspan' or 'textPath' , element(s) (or their ancestors) associated with the given glyph.

As the text selection operation proceeds (e.g., the user continues to press the given mouse button), all associated events with other graphics elements are ignored (i.e., the text selection operation is modal) and the SVG user agent shall dynamically indicate which characters are selected by an appropriate highlighting technique, such as redrawing the selected glyphs with inverse colors. As the pointer is moved during the text selection process, the end glyph for the text selection operation is the glyph within the same 'text' element whose glyph cell is closest to the pointer. All characters within the 'text' element whose position within the 'text' element is between the start of selection and end of selection shall be highlighted, regardless of position on the canvas and regardless of any graphics elements that might be above the end of selection point.

Once the text selection operation ends (e.g., the user releases the given mouse button), the selected text will stay highlighted until an event occurs which cancels text selection, such as a pointer device activation event (e.g., pressing a mouse button).

Detailed rules for determining which characters to highlight during a text selection operation are provided in Text selection implementation notes.

For systems which have system clipboards, the SVG user agent is required to provide a user interface for initiating a copy of the currently selected text to the system clipboard. It is sufficient for the SVG user agent to post the selected text string in the system's appropriate clipboard format for plain text, but it is preferable if the SVG user agent also posts a rich text alternative which captures the various font properties associated with the given text string.

For bidirectional text, the user agent must support text selection in logical order, which will result in discontinuous highlighting of glyphs due to the bidirectional reordering of characters. User agents can provide an alternative ability to select bidirectional text in visual rendering order (i.e., after bidirectional text layout algorithms have been applied), with the result that selected character data might be discontinous logically. In this case, if the user requests that bidirectional text be copied to the clipboard, then the user agent is required to make appropriate adjustments to copy only the visually selected characters to the clipboard.

When feasible, it is recommended that generators of SVG attempt to order their text strings to facilitate properly ordered text selection within SVG viewing applications such as Web browsers.

10.17 DOM interfaces

The following interfaces are defined below: SVGTextContentElement, SVGTextElement, SVGTextPositioningElement, SVGTSpanElement, SVGTRefElement, SVGTextPathElement, SVGAltGlyphElement, SVGAltGlyphDefElement, SVGAltGlyphItemElement, SVGGlyphRefElement.

Interface SVGTextContentElement

The SVGTextContentElement interface is inherited by various text-related interfaces, such as SVGTextElement, SVGTSpanElement, SVGTRefElement, SVGAltGlyphElement and SVGTextPathElement.

IDL Definition

interface SVGTextContentElement : 
                SVGElement,
                SVGTests,
                SVGLangSpace,
                SVGExternalResourcesRequired,
                SVGStylable,
                events::EventTarget { 

  // lengthAdjust Types
  const unsigned short LENGTHADJUST_UNKNOWN   = 0;
  const unsigned short LENGTHADJUST_SPACING     = 1;
  const unsigned short LENGTHADJUST_SPACINGANDGLYPHS     = 2;

  readonly attribute SVGAnimatedLength      textLength;
  readonly attribute SVGAnimatedEnumeration lengthAdjust;

  long     getNumberOfChars (  );
  float    getComputedTextLength (  );
  float    getSubStringLength ( in unsigned long charnum, in unsigned long nchars )
                  raises( DOMException );
  SVGPoint getStartPositionOfChar ( in unsigned long charnum )
                  raises( DOMException );
  SVGPoint getEndPositionOfChar ( in unsigned long charnum )
                  raises( DOMException );
  SVGRect  getExtentOfChar ( in unsigned long charnum )
                  raises( DOMException );
  float    getRotationOfChar ( in unsigned long charnum )
                  raises( DOMException );
  long     getCharNumAtPosition ( in SVGPoint point );
  void     selectSubString ( in unsigned long charnum, in unsigned long nchars )
                  raises( DOMException );
};

Definition group lengthAdjust Types

Defined constants

LENGTHADJUST_UNKNOWN		The enumeration was set to a value that is not one of predefined types. It is invalid to attempt to define a new value of this type or to attempt to switch an existing value to this type.
LENGTHADJUST_SPACING		Corresponds to value spacing.
LENGTHADJUST_SPACINGANDGLYPHS		Corresponds to value spacingAndGlyphs.

Attributes

readonly SVGAnimatedLength textLength

Corresponds to attribute textLength on the given element.

readonly SVGAnimatedEnumeration lengthAdjust

Corresponds to attribute lengthAdjust on the given element. The value must be one of the length adjust constants specified above.

Methods

getNumberOfChars

Returns the total number of characters to be rendered within the current element. Includes characters which are included via a 'tref' reference.

No Parameters

Return value

long

Total number of characters.

No Exceptions

getComputedTextLength

The total sum of all of the advance values from rendering all of the characters within this element, including the advance value on the glyphs (horizontal or vertical), the effect of properties 'kerning', 'letter-spacing' and 'word-spacing' and adjustments due to attributes dx and dy on 'tspan' elements. For non-rendering environments, the user agent shall make reasonable assumptions about glyph metrics.

No Parameters

Return value

float

The text advance distance.

No Exceptions

getSubStringLength

The total sum of all of the advance values from rendering the specified substring of the characters, including the advance value on the glyphs (horizontal or vertical), the effect of properties 'kerning', 'letter-spacing' and 'word-spacing' and adjustments due to attributes dx and dy on 'tspan' elements. For non-rendering environments, the user agent shall make reasonable assumptions about glyph metrics.

Parameters

in unsigned long charnum		The index of the first character in the substring, where the first character has an index of 0.
in unsigned long nchars		The number of characters in the substring.

Return value

float

The text advance distance.

Exceptions

DOMException

INDEX_SIZE_ERR: Raised if the charnum is negative or if charnum+nchars is greater than or equal to the number of characters at this node.

getStartPositionOfChar

Returns the current text position before rendering the character in the user coordinate system for rendering the glyph(s) that correspond to the specified character. The current text position has already taken into account the effects of any inter-character adjustments due to properties 'kerning', 'letter-spacing' and 'word-spacing' and adjustments due to attributes x, y, dx and dy. If multiple consecutive characters are rendered inseparably (e.g., as a single glyph or a sequence of glyphs), then each of the inseparable characters will return the start position for the first glyph.

Parameters

in unsigned long charnum

The index of the character, where the first character has an index of 0.

Return value

SVGPoint

The character's start position.

Exceptions

DOMException

INDEX_SIZE_ERR: Raised if the charnum is negative or if charnum is greater than or equal to the number of characters at this node.

getEndPositionOfChar

Returns the current text position after rendering the character in the user coordinate system for rendering the glyph(s) that correspond to the specified character. This current text position does not take into account the effects of any inter-character adjustments to prepare for the next character, such as properties 'kerning', 'letter-spacing' and 'word-spacing' and adjustments due to attributes x, y, dx and dy. If multiple consecutive characters are rendered inseparably (e.g., as a single glyph or a sequence of glyphs), then each of the inseparable characters will return the end position for the last glyph.

Parameters

in unsigned long charnum

The index of the character, where the first character has an index of 0.

Return value

SVGPoint

The character's end position.

Exceptions

DOMException

INDEX_SIZE_ERR: Raised if the charnum is negative or if charnum is greater than or equal to the number of characters at this node.

getExtentOfChar

Returns a tightest rectangle which defines the minimum and maximum X and Y values in the user coordinate system for rendering the glyph(s) that correspond to the specified character. The calculations assume that all glyphs occupy the full standard glyph cell for the font. If multiple consecutive characters are rendered inseparably (e.g., as a single glyph or a sequence of glyphs), then each of the inseparable characters will return the same extent.

Parameters

in unsigned long charnum

The index of the character, where the first character has an index of 0.

Return value

SVGRect

The rectangle which encloses all of the rendered glyph(s).

Exceptions

DOMException

INDEX_SIZE_ERR: Raised if the charnum is negative or if charnum is greater than or equal to the number of characters at this node.

getRotationOfChar

Returns the rotation value relative to the current user coordinate system used to render the glyph(s) corresponding to the specified character. If multiple glyph(s) are used to render the given character and the glyphs each have different rotations (e.g., due to text-on-a-path), the user agent shall return an average value (e.g., the rotation angle at the midpoint along the path for all glyphs used to render this character). The rotation value represents the rotation that is supplemental to any rotation due to properties 'glyph-orientation-horizontal' and 'glyph-orientation-vertical'; thus, any glyph rotations due to these properties are not included into the returned rotation value. If multiple consecutive characters are rendered inseparably (e.g., as a single glyph or a sequence of glyphs), then each of the inseparable characters will return the same rotation value.

Parameters

in unsigned long charnum

The index of the character, where the first character has an index of 0.

Return value

float

The rotation angle.

Exceptions

DOMException

INDEX_SIZE_ERR: Raised if the charnum is negative or if charnum is greater than or equal to the number of characters at this node.

getCharNumAtPosition

Returns the index of the character whose corresponding glyph cell bounding box contains the specified point. The calculations assume that all glyphs occupy the full standard glyph cell for the font. If no such character exists, a value of -1 is returned. If multiple such characters exist, the character within the element whose glyphs were rendered last (i.e., take into account any reordering such as for bidirectional text) is used. If multiple consecutive characters are rendered inseparably (e.g., as a single glyph or a sequence of glyphs), then the user agent shall allocate an equal percentage of the text advance amount to each of the contributing characters in determining which of the characters is chosen.

Parameters

in SVGPoint point

A point in user space.

Return value

long

The index of the character which is at the given point, where the first character has an index of 0.

No Exceptions

selectSubString

Causes the specified substring to be selected just as if the user selected the substring interactively.

Parameters

in unsigned long charnum		The index of the start character which is at the given point, where the first character has an index of 0.
in unsigned long nchars		The number of characters in the substring. If nchars specifies more characters than are available, then the substring will consist of all characters starting with charnum until the end of the list of characters.

No Return Value

Exceptions

DOMException

INDEX_SIZE_ERR: Raised if the charnum is negative or if charnum is greater than or equal to the number of characters at this node.

Interface SVGTextElement

The SVGTextElement interface corresponds to the 'text' element.

IDL Definition

interface SVGTextElement : 
                SVGTextContentElement,
                SVGTransformable { 

  readonly attribute SVGAnimatedLength x;
  readonly attribute SVGAnimatedLength y;
};

Attributes

readonly SVGAnimatedLength x

Corresponds to attribute x on the given 'text' element.

readonly SVGAnimatedLength y

Corresponds to attribute y on the given 'text' element.

Interface SVGTextPositioningElement

The SVGTextPositioningElement interface is inherited by text-related interfaces: SVGTSpanElement, SVGTRefElement and SVGAltGlyphElement.

IDL Definition

interface SVGTextPositioningElement : SVGTextContentElement { 
  readonly attribute SVGAnimatedLengthList x;
  readonly attribute SVGAnimatedLengthList y;
  readonly attribute SVGAnimatedLengthList dx;
  readonly attribute SVGAnimatedLengthList dy;
  readonly attribute SVGAnimatedNumberList rotate;
};

Attributes

readonly SVGAnimatedLengthList x

Corresponds to attribute x on the given element.

readonly SVGAnimatedLengthList y

Corresponds to attribute y on the given element.

readonly SVGAnimatedLengthList dx

Corresponds to attribute dx on the given element.

readonly SVGAnimatedLengthList dy

Corresponds to attribute dy on the given element.

readonly SVGAnimatedNumberList rotate

Corresponds to attribute rotate on the given element.

Interface SVGTSpanElement

The SVGTSpanElement interface corresponds to the 'tspan' element.

IDL Definition

interface SVGTSpanElement : SVGTextPositioningElement {};

Interface SVGTRefElement

The SVGTRefElement interface corresponds to the 'tref' element.

IDL Definition

interface SVGTRefElement : 
                SVGTextPositioningElement,
                SVGURIReference {};

Interface SVGTextPathElement

The SVGTextPathElement interface corresponds to the 'textPath' element.

IDL Definition

interface SVGTextPathElement : 
                SVGTextContentElement,
                SVGURIReference { 

  // textPath Method Types
  const unsigned short TEXTPATH_METHODTYPE_UNKNOWN   = 0;
  const unsigned short TEXTPATH_METHODTYPE_ALIGN     = 1;
  const unsigned short TEXTPATH_METHODTYPE_STRETCH     = 2;
  // textPath Spacing Types
  const unsigned short TEXTPATH_SPACINGTYPE_UNKNOWN   = 0;
  const unsigned short TEXTPATH_SPACINGTYPE_AUTO     = 1;
  const unsigned short TEXTPATH_SPACINGTYPE_EXACT     = 2;

  readonly attribute SVGAnimatedLength              startOffset;
  readonly attribute SVGAnimatedEnumeration method;
  readonly attribute SVGAnimatedEnumeration spacing;
};

Definition group textPath Method Types

Defined constants

TEXTPATH_METHODTYPE_UNKNOWN		The enumeration was set to a value that is not one of predefined types. It is invalid to attempt to define a new value of this type or to attempt to switch an existing value to this type.
TEXTPATH_METHODTYPE_ALIGN		Corresponds to value align.
TEXTPATH_METHODTYPE_STRETCH		Corresponds to value stretch.

Definition group textPath Spacing Types

Defined constants

TEXTPATH_SPACINGTYPE_UNKNOWN		The enumeration was set to a value that is not one of predefined types. It is invalid to attempt to define a new value of this type or to attempt to switch an existing value to this type.
TEXTPATH_SPACINGTYPE_AUTO		Corresponds to value auto.
TEXTPATH_SPACINGTYPE_EXACT		Corresponds to value exact.

Attributes

readonly SVGAnimatedLength startOffset

Corresponds to attribute startOffset on the given 'textPath' element.

readonly SVGAnimatedEnumeration method

Corresponds to attribute method on the given 'textPath' element. The value must be one of the method type constants specified above.

readonly SVGAnimatedEnumeration spacing

Corresponds to attribute spacing on the given 'textPath' element. The value must be one of the spacing type constants specified above.

Interface SVGAltGlyphElement

The SVGAltGlyphElement interface corresponds to the 'altGlyph' element.

IDL Definition

interface SVGAltGlyphElement : 
                SVGTextPositioningElement,
                SVGURIReference { 

           attribute DOMString glyphRef;
                       // raises DOMException on setting
           attribute DOMString format;
                       // raises DOMException on setting
};

Attributes

DOMString glyphRef

Corresponds to attribute glyphRef on the given 'glyphSub' element.

Exceptions on setting

DOMException

NO_MODIFICATION_ALLOWED_ERR: Raised on an attempt to change the value of a readonly attribute.

DOMString format

Corresponds to attribute format on the given 'glyphSub' element.

Exceptions on setting

DOMException

NO_MODIFICATION_ALLOWED_ERR: Raised on an attempt to change the value of a readonly attribute.

Interface SVGAltGlyphDefElement

The SVGAltGlyphDefElement interface corresponds to the 'altGlyphDef' element.

IDL Definition

interface SVGAltGlyphDefElement : SVGElement {};

Interface SVGAltGlyphItemElement

The SVGAltGlyphItemElement interface corresponds to the 'altGlyphItem' element.

IDL Definition

interface SVGAltGlyphItemElement : SVGElement {};

Interface SVGGlyphRefElement

The SVGGlyphRefElement interface corresponds to the 'glyphSub' element.

IDL Definition

interface SVGGlyphRefElement : 
                SVGElement,
                SVGURIReference,
                SVGStylable { 

           attribute DOMString glyphRef;
                       // raises DOMException on setting
           attribute DOMString format;
                       // raises DOMException on setting
           attribute float    x;
                       // raises DOMException on setting
           attribute float    y;
                       // raises DOMException on setting
           attribute float    dx;
                       // raises DOMException on setting
           attribute float    dy;
                       // raises DOMException on setting
};

Attributes

DOMString glyphRef

Corresponds to attribute glyphRef on the given 'glyphSub' element.

Exceptions on setting

DOMException

NO_MODIFICATION_ALLOWED_ERR: Raised on an attempt to change the value of a readonly attribute.

DOMString format

Corresponds to attribute format on the given 'glyphSub' element.

Exceptions on setting

DOMException

NO_MODIFICATION_ALLOWED_ERR: Raised on an attempt to change the value of a readonly attribute.

float x

Corresponds to attribute x on the given element.

Exceptions on setting

DOMException

NO_MODIFICATION_ALLOWED_ERR: Raised on an attempt to change the value of a readonly attribute.

float y

Corresponds to attribute y on the given element.

Exceptions on setting

DOMException

NO_MODIFICATION_ALLOWED_ERR: Raised on an attempt to change the value of a readonly attribute.

float dx

Corresponds to attribute dx on the given element.

Exceptions on setting

DOMException

NO_MODIFICATION_ALLOWED_ERR: Raised on an attempt to change the value of a readonly attribute.

float dy

Corresponds to attribute dy on the given element.

Exceptions on setting

DOMException

NO_MODIFICATION_ALLOWED_ERR: Raised on an attempt to change the value of a readonly attribute.