MNG-0.95e, Draft 68, 19990618

pHYg chunk and global pHYs

This proposal is with respect to MNG 0.95c (Draft 66).

This proposal supersedes MNG-0.95d, Draft 67, 19990615.

MNG-0.95e Draft 68 19990618 is created by making the following changes
to MNG-0.95c Draft 66 19990615:

Section 4.2.5, BASI, is modified to add the words shown in
square brackets:

   Top-level gAMA, sRGB, cHRM, iCCP, bKGD, sBIT, [pHYs,] and sPLT chunks
   are inherited by a BASI datastream in the same manner as by a PNG
   datastream.

Section 4.5.4, pHYs, is modified to delete the words shown in double
square brackets and to add the material shown in square brackets

      4.5.4. [[pHYs]][pHYg] Physical pixel size[, global]

         The MNG [[pHYs]][pHYg] chunk is identical in syntax to the
#ifdef MNG-VLC
         PNG pHYs chunk.  It applies to complete MNG layers and not to
         the individual images within them.
#else
         PNG pHYs chunk.  It applies to complete [full-frame] MNG layers
         and not to the individual images within them.  [Conceptually, a
         MNG viewer that processes the pHYg chunk will first composite
         each image into a full-frame layer, then apply the pHYg scaling
         to the layer, and finally composite the scaled layer into the
         subframe.]
#endif
         [MNG datastreams can include both the pHYs chunk (either at the
         MNG top level or within the PNG and JNG datastreams) and the
         pHYg chunk (only at the MNG top level), to ensure that the images
         are properly displayed either when displayed by a MNG viewer or
         when extracted into a series of individual PNG or JNG datastreams
         and then displayed by a PNG or JNG application.  The pHYs and
         pHYg chunks would normally contain the same values, but this is
         not necessary.]

         The MNG top-level [[pHYs]][pHYg] chunk can be nullified by a
         subsequent empty [[pHYs]][pHYg] chunk appearing in the MNG top level.

Section 4.5.5, Ancillary PNG chunks, is modified to add the
words shown in square brackets:

   The following PNG chunks are also defined at the MNG top level.
   They provide default values to be used in case they are not
   provided in subsequent PNG or JNG datastreams.  Any of these chunks
   can be nullified by the appearance of a subsequent empty chunk
   with the same chunk name.  Such empty chunks are not legal PNG
   or JNG chunks and must only appear in the MNG top level.

       * cHRM, gAMA, iCCP, sRGB, bKGD, sBIT, pHYs

   In the MNG top level, all of these chunks are written as though
   for 16-bit RGBA PNG datastreams.  Decoders are responsible for
   reformatting the chunk data to suit the actual bit depth and color
   type of the datastream that inherits them.

   A MNG editor that writes PNG or JNG datastreams is expected to
   include the top-level cHRM, gAMA, iCCP, and sRGB chunks in the
   generated PNG or JNG datastreams, if the embedded image does
   not contain its own chunks that define the color space.  When
   it writes the sRGB chunk, it should write the gAMA chunk (and
   perhaps the cHRM chunk), in accordance with the PNG
   specification, even though no gAMA or cHRM chunk is present in
   the MNG datastream.  It is also expected to write the [pHYs chunk
   and the] reformatted top-level bKGD chunk in the generated PNG or
   JNG datastreams, and the reformatted sBIT chunk only in generated
   PNG datastreams, when the datastream does not have its own bKGD
   [, pHYs,] or sBIT chunks.

   ...

   MNG editors that write PNG datastreams should ignore the
   sPLT [[and pHYs]] data from the MNG level and simply copy any
   sPLT [[and pHYs]] chunks appearing within the PNG
   datastreams.

Section 6.1.15. oFFs and pHYs, is modified to add the words shown
in square brackets:

   MNG viewers must ignore oFFs and pHYs chunks that appear inside
   a PNG or JNG datastream [or are inherited from the MNG top level].
   MNG editors are expected to treat them as unknown chunks that will
   be handled as described in above (Paragraph 6.1.12).