PNG Proposed sTER chunk, draft 20060321

File: png-proposed-sTER-chunk-20060321.txt

Status of this Memo

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

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png-mng-misc@lists.sourceforge.net

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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).

PNG `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 subimages within a single PNG image.

The sTER chunk contains:

         Mode: 1 byte
               0: cross-fuse layout
               1: diverging-fuse layout

The sTER chunk with mode==0 or mode=1 indicates that the datastream contains two subimages, encoded within a single PNG image. They are arranged side-by-side, with one subimage intended for presentation to the right eye and the other subimage intended for presentation to the left eye. The left edge of the right subimage 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 subimages must be introduced by the encoder if necessary. The two subimages must have the same dimensions after removal of any padding.

When mode==0, the right-eye image appears at the left and the left-eye image appears at the right, suitable for cross-eyed free viewing. When mode==1, the left-eye image appears at the left and the right-eye image appears at the right, suitable for divergent (wall-eyed) free viewing.

Decoders that are aware of the sTER chunk may display the two images in any suitable manner. Decoders that are not aware of the sTER chunk, and those that recognize the chunk but choose not to treat stereo pairs differently from regular PNG images, will naturally display them side-by-side in a manner suitable for free viewing.

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

In palette images, for compatibility with software not supporting the sTER chunk, the padding pixels must be valid palette indices.

Given two subimages with width subimage_width, encoders can calculate the inter-subimage padding and total width W using the following pseudocode:

    padding        := 7 - ((subimage_width - 1) mod 8)
    W              := 2 * subimage_width + padding

Given an image with width W, decoders can calculate the subimage width and inter-subimage padding using the following pseudocode:

    padding := 15 - ((W - 1) mod 16)
    if (padding > 7) then error
    subimage_width := (W - padding) / 2

Decoders can assume that the samples in the left and right subimages are cosited, such that the subimages and their centers are coincident at the projection plane. Decoders can also assume that the left and right subimages are intended to be presented directly to the right and left eyes of the user/viewer without independent scaling, rotation or displacement. I.e., the subimages will be presented at the same size in the same relative position and orientation to each eye of the viewer.

Encoders should use the pHYs chunk to indicate the pixel's size ratio when it is not 1:1.

It is recommended that encoders use the cross-fusing layout (mode==0), especially when the image centers are separated by more than 65 millimeters when displayed on a typical monitor.

MNG `sTER` proposal

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 any embedded PNG or JNG datastream must be ignored. If not, then the decoder must handle the sTER chunk in the embedded datastream 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, it takes precedence over any sTER chunk found in embedded PNG or JNG datastreams. It has the same meaning as a PNG sTER chunk except that it applies to each subsequent subframe 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. An empty top-level sTER chunk nullifies any prior top-level sTER chunk.
Encoders should use the pHYg chunk or the MAGN chunk to indicate the pixel's size ratio when it is not 1:1.

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, www.adeluc.com, png@adeluc.com

Editor

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

End of PNG sTER Chunk Proposal. Expires 21 Sept 2006.