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container doc: move RIFF description to own section

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- remove some little used terms
- condense file header description
- add description of FourCC

Change-Id: Ia873652f41789811f3fb2ec97182a068ac727961
This commit is contained in:
James Zern 2013-03-26 14:48:51 -07:00
parent 4299f39852
commit e3e339497e
1 changed files with 27 additions and 39 deletions
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66
doc/webp-container-spec.txt
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@ -93,11 +93,21 @@ _uint32_
: A 32-bit, little-endian, unsigned integer.
_FourCC_
: A _FourCC_ (four-character code) is a _uint32_ created by concatenating four
ASCII characters in little-endian order.
_1-based_
: An unsigned integer field storing values offset by `-1`. e.g., Such a field
would store value _25_ as _24_.
RIFF file format
----------------
The WebP file format is based on the RIFF (resource interchange file format)
document format.
The basic element of a RIFF file is a _chunk_. It consists of:
0 1 2 3
@ -112,12 +122,12 @@ The basic element of a RIFF file is a _chunk_. It consists of:
Chunk FourCC: 32 bits
: ASCII four character code or _chunk tag_ used for chunk identification.
: ASCII four-character code used for chunk identification.
Chunk Size: 32 bits (_uint32_)
: The size of the chunk (_ckSize_) not including this field, the chunk
identifier and padding.
: The size of the chunk not including this field, the chunk identifier or
padding.
Chunk Payload: _Chunk Size_ bytes
@ -126,43 +136,13 @@ Chunk Payload: _Chunk Size_ bytes
_ChunkHeader('ABCD')_
: This is used to describe the fourcc and size header of individual
chunks, where 'ABCD' is the fourcc for the chunk. This element's
: This is used to describe the _FourCC_ and _Chunk Size_ header of individual
chunks, where 'ABCD' is the FourCC for the chunk. This element's
size is 8 bytes.
: Note that, in this specification, all chunk tag characters are in
file order, not in byte order of a uint32 of any particular
architecture.
_list of chunks_
: A concatenation of multiple chunks.
: We will refer to the first chunk as having _position_ 0, the second
as position 1, etc. By _chunk with index 0 among "ABCD"_ we mean
the first chunk among the chunks of type "ABCD" in the list, the
_chunk with index 1 among "ABCD"_ is the second such chunk, etc.
A WebP file MUST begin with a single chunk with a tag 'RIFF'. All
other defined chunks are contained within this chunk. The file SHOULD
NOT contain anything after it.
The maximum size of RIFF's _ckSize_ is 2^32 minus 10 bytes. The size
of the whole file is at most 4GiB minus 2 bytes.
**Note:** some RIFF libraries are said to have bugs when handling files
larger than 1GiB or 2GiB. If you are using an existing library, check
that it handles large files correctly.
The first four bytes of the RIFF chunk contents (i.e., bytes 8-11 of the file)
MUST be the ASCII string "WEBP". They are followed by a list of chunks. As the
size of any chunk is even, the size of the RIFF chunk is also even. The
contents of the chunks in that list will be described in the following sections.
**Note:** RIFF has a convention that all-uppercase chunks are standard
chunks that apply to any RIFF file format, while chunks specific to a
file format are all lowercase. WebP does not follow this convention.
**Note:** RIFF has a convention that all-uppercase chunk FourCCs are standard
chunks that apply to any RIFF file format, while FourCCs specific to a file
format are all lowercase. WebP does not follow this convention.
WebP file header
----------------
@ -183,12 +163,20 @@ WebP file header
File Size: 32 bits (_uint32_)
: The size of the file in bytes starting at offset 8.
: The size of the file in bytes starting at offset 8. The maximum value of
this field is 2^32 minus 10 bytes and thus the size of the whole file is at
most 4GiB minus 2 bytes.
'WEBP': 32 bits
: The ASCII characters 'W' 'E' 'B' 'P'.
A WebP file MUST begin with a RIFF header with the FourCC 'WEBP'. The file size
in the header is the total size of the chunks that follow plus `4` bytes for
the 'WEBP' FourCC. The file SHOULD NOT contain anything after it. As the size
of any chunk is even, the size given by the RIFF header is also even. The
contents of individual chunks will be described in the following sections.
Simple file format (lossy)
--------------------------