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.

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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.