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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
============================
_Working Draft, v0.9, 20121105_
_Working Draft, v1.0, 20121109_
* TOC placeholder
@ -67,7 +67,7 @@ Terminology & Basics
------------------------
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
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
: The data payload. If _Chunk Size_ is odd a single padding byte that
SHOULD be `0` is added.
: The data payload. If _Chunk Size_ is odd, a single padding byte -- that
SHOULD be `0` -- is added.
_ChunkHeader('ABCD')_
@ -281,6 +281,8 @@ An extended format file consists of:
* 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
an _animated image_, it consists of multiple frames. More details about frames
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.
* An optional list of [unknown chunks](#unknown-chunks).
#### Fragments
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.
* 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
subchunk.
@ -702,8 +707,22 @@ Metadata Working Group's [Guidelines for Handling Metadata][metadata].
#### Unknown Chunks
A file MAY contain other unknown chunks. Readers SHOULD ignore these chunks.
Writers SHOULD preserve them in their original order.
A RIFF chunk (described in [this](#terminology-amp-basics) section) whose _chunk
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