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Refine WebP Container Spec wrt unknown chunks.

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Change-Id: Ibe950ab7afb2a7349f61c6fb81d587fba9ff27b6
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Urvang Joshi 2012-11-12 11:34:08 -08:00
parent 2ca642e02a
commit 4c4398e2d5
1 changed files with 25 additions and 6 deletions
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doc/webp-container-spec.txt
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@ -13,7 +13,7 @@ end of this file.
WebP Container Specification WebP Container Specification
============================ ============================
_Working Draft, v0.9, 20121105_ _Working Draft, v1.0, 20121109_
* TOC placeholder * TOC placeholder
@ -67,7 +67,7 @@ Terminology & Basics
------------------------ ------------------------
A WebP file contains either a still image (i.e., an encoded matrix of pixels) A WebP file contains either a still image (i.e., an encoded matrix of pixels)
or an [animation](#animation)). Optionally, it can also contain transparency or an [animation](#animation). Optionally, it can also contain transparency
information, color profile and metadata. In case we need to refer only to the information, color profile and metadata. In case we need to refer only to the
matrix of pixels, we will call it the _canvas_ of the image. matrix of pixels, we will call it the _canvas_ of the image.
@ -118,8 +118,8 @@ Chunk Size: 32 bits (_uint32_)
Chunk Payload: _Chunk Size_ bytes Chunk Payload: _Chunk Size_ bytes
: The data payload. If _Chunk Size_ is odd a single padding byte that : The data payload. If _Chunk Size_ is odd, a single padding byte -- that
SHOULD be `0` is added. SHOULD be `0` -- is added.
_ChunkHeader('ABCD')_ _ChunkHeader('ABCD')_
@ -281,6 +281,8 @@ An extended format file consists of:
* An optional 'XMP ' chunk with XMP metadata. * An optional 'XMP ' chunk with XMP metadata.
* An optional list of [unknown chunks](#unknown-chunks).
For a _still image_, the _image data_ consists of a single frame, whereas for For a _still image_, the _image data_ consists of a single frame, whereas for
an _animated image_, it consists of multiple frames. More details about frames an _animated image_, it consists of multiple frames. More details about frames
can be found in the [Animation](#animation) section. can be found in the [Animation](#animation) section.
@ -471,6 +473,8 @@ Frame Data: _Chunk Size_ - `16` bytes
* A [bitstream subchunk](#bitstream-vp8vp8l) for the frame. * A [bitstream subchunk](#bitstream-vp8vp8l) for the frame.
* An optional list of [unknown chunks](#unknown-chunks).
#### Fragments #### Fragments
For images that are represented by fragments, this chunk contains data for For images that are represented by fragments, this chunk contains data for
@ -501,6 +505,7 @@ Fragment Data: _Chunk Size_ - `6` bytes
* An optional [alpha subchunk](#alpha) for the fragment. * An optional [alpha subchunk](#alpha) for the fragment.
* The [bitstream subchunk](#bitstream-vp8vp8l) for the fragment. * The [bitstream subchunk](#bitstream-vp8vp8l) for the fragment.
* An optional list of [unknown chunks](#unknown-chunks).
Note: The width and height of the fragment is obtained from the bitstream Note: The width and height of the fragment is obtained from the bitstream
subchunk. subchunk.
@ -702,8 +707,22 @@ Metadata Working Group's [Guidelines for Handling Metadata][metadata].
#### Unknown Chunks #### Unknown Chunks
A file MAY contain other unknown chunks. Readers SHOULD ignore these chunks. A RIFF chunk (described in [this](#terminology-amp-basics) section) whose _chunk
Writers SHOULD preserve them in their original order. tag_ is different from any of the chunks described in this document, is
considered an _unknown chunk_.
**Rationale**: Allowing unknown chunks gives a provision for future extension
of the format, and also allows storage of any application-specific data.
A file MAY contain unknown chunks:
* At the end of the file as described in [Extended WebP file
header](#extended_header) section.
* At the end of FRGM and ANMF chunks as described in [Fragments](#fragments)
and [Animation](#animation) sections.
Readers SHOULD ignore these chunks. Writers SHOULD preserve them in their
original order.
### Assembling the Canvas from fragments/frames ### Assembling the Canvas from fragments/frames