PNG Proposed sTER chunk, draft 20060315

File: png-proposed-sTER-chunk-20060315.txt

Status of this Memo

   This document is an informal draft of the PNG development group.

   Comments on this document can be sent to the PNG specification
   maintainers at png-mng-misc@lists.sourceforge.net

   Distribution of this memo is unlimited.

   At present, the latest version of this document is available on the
   World Wide Web from
   ftp://ftp.simplesystems.org/pub/libpng/png-group/documents/.

Notices

   Copyright (C) 2006 Glenn Randers-Pehrson

   Permission is granted to copy and distribute this document for any
   purpose and without charge, provided that the copyright notice and
   this notice are preserved, and that any substantive changes or
   deletions from the original are clearly marked.

Abstract

   This document describes a special-purpose chunk type that has been
   proposed for use in various PNG (Portable Network Graphics) and
   related multi-image applications. It has not yet been approved for
   registration by the PNG developers, and therefore is experimental and
   its format is subject to change. The proposed chunk is (with an
   unregistered name that can be used in test implementations shown in
   parentheses): sTER (stER).

sTER proposal

   It is proposed to add the following section to the document
   "Extensions to the PNG 1.2 Specification, Version 1.2.0"

  3.6. sTER Indicator of Stereo Image

   When present, the sTER chunk indicates that the datastream contains a
   stereo pair of images within a single PNG image.

   The sTER chunk contains:

         Mode: 1 byte (reserved; must be zero)

   The sTER chunk with mode==0 indicates that the datastream contains two
   images, encoded within a single PNG image. They are arranged
   side-by-side, with the left image intended for presentation to the
   right eye and the right image intended for presentation to the left
   eye. The left edge of the right image must be on a column that is
   evenly divisible by eight, so that if interlacing is employed the two
   images will have coordinated interlacing. Padding columns between the
   two images must be introduced by the encoder if necessary. The two
   images must have the same dimensions after removal of any padding.

   Decoders that are aware of the sTER chunk can display the two images
   in any suitable manner. Decoders that are not aware of the sTER chunk
   will naturally display them side-by-side in a manner suitable for
   cross-eyed viewing.

   If present, the sTER chunk must appear before the first IDAT chunk.

  Handling of the sTER chunk in MNG datastreams

   It is proposed to add the following paragraphs to the document "MNG
   (Multiple-image Network Graphics) Format, Version 1.0":

   At the end of the list in Paragraph 4.2.3, IHDR, PNG chunks, IEND, add
   another bullet:

     * If the PNG sTER chunk appears in the top-level MNG datastream with
       mode==0, any sTER chunk appearing in the PNG datastream must be
       ignored. If not, then the decoder must handle the sTER chunk in
       the same manner as it handles them in standalone PNG or JNG
       datastreams.

   At the end of Paragraph 4.6, Ancillary PNG chunks, add another bullet:

     * sTER This PNG chunk is also defined at the MNG top level. When
       active, with mode==0, it takes precedence over any sTER chunk
       found in embedded PNG or MNG datastreams. It has the same meaning
       as a PNG sTER chunk except that it applies to subsequent complete
       frames rather than to a single PNG image, and the image dimensions
       and padding are determined from the subframe boundaries instead of
       from the PNG IHDR data. If the subframe boundaries change within a
       frame, then the dimensions and padding of the stereo pair are
       determined from the minimum dimensions that enclose all of the
       subframes in the frame. An empty top-level sTER chunk nullifies
       any prior top-level sTER chunk.

   In paragraph 5.2, Ancillary JNG chunks, after the word "sCAL", add
   ", sTER".

   Add paragraph 10.8, Interlacing and the sTER chunk

   It is recommended that when the sTER chunk is active and the two
   images are conveyed in separate PNG or JNG datastreams, interlacing
   not be used.

Contributors

   Names of contributors not already listed in the PNG specification are
   presented in alphabetical order:

     * Adeluc, png@adeluc.com

  Editor

     * Glenn Randers-Pehrson, glennrp@users.sourceforge.net

   End of PNG sTER Chunk Proposal. Expires 15 Sept 2006.