PNG Proposed sTER chunk, draft 20060322
File: png-proposed-sTER-chunk-20060322.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).
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 22 Sept 2006.