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More corrections/clarifications in lossless spec:

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- Rectify a few BNF descriptions
- Corrections in "Decoding Flow of Image Data" section:
  1. The sequence in case of "S < 256" should be green, red, blue, alpha.
  2. In case of "S >= 256 + 24", the index should be "S - (256 + 24)".
- Provide more description to clarify "Decoding Flow of Image Data" section.
- Some cosmetics: use '1's instead of '1, 2, 3...' sequence, as kramdown takes
  care of sequencing.

Change-Id: I2b76caf72f67aae813522dc1a4115f8ec8ea6db7
This commit is contained in:
Urvang Joshi 2012-12-17 11:40:23 -08:00
parent df7aa07605
commit d82a3e3391
1 changed files with 35 additions and 21 deletions
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56
doc/webp-lossless-bitstream-spec.txt
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@ -784,28 +784,42 @@ prefixes, and the result of this coding is further Huffman coded.
For every pixel (x, y) in the image, there is a definition of which
entropy code to use. First, there is an integer called 'meta Huffman
code' that can be obtained from the entropy image. This
meta Huffman code identifies a set of five Huffman codes, one for green
(along with length codes and color cache codes), one for each of red,
blue and alpha, and one for distance. The Huffman codes are identified
by their position in a table by an integer.
meta Huffman code identifies a set of five Huffman codes:
* Huffman code #1: used for green channel, backward-reference length and
color cache
* Huffman code #2, #3 and #4: used for red, blue and alpha channels
respectively.
* Huffman code #5: used for backward-reference distance.
### Decoding Flow of Image Data
Read next symbol S
Read next symbol S from the bitsteam using Huffman code #1. Note that S is any
integer in the range `0` to `(256 + 24 + `
[`color_cache_size`](#color-cache-code)`- 1)`.
1. S < 256
1. Use S as green component
2. read alpha
3. read red
4. read blue
2. S < 256 + 24
The interpretation of S depends on its value:
1. if S < 256
1. Use S as the green component
1. Read red from the bitstream using Huffman code #2
1. Read blue from the bitstream using Huffman code #3
1. Read alpha from the bitstream using Huffman code #4
1. if S < 256 + 24
1. Use S - 256 as a length prefix code
2. read length extra bits
3. read distance prefix code
4. read distance extra bits
3. S >= 256 + 24
1. Use ARGB color from the color cache, at index S - 256 + 24
1. Read extra bits for length from the bitstream
1. Determine backward-reference length L from length prefix code and the
extra bits read.
1. Read distance prefix code from the bitstream using Huffman code #5
1. Read extra bits for distance from the bitstream
1. Determine backward-reference distance D from distance prefix code and
the extra bits read.
1. Copy the L pixels (in scan-line order) from the sequence of pixels
prior to them by D pixels.
1. if S >= 256 + 24
1. Use S - (256 + 24) as the index into the color cache.
1. Get ARGB color from the color cache at that index.
### Decoding the Code Lengths
@ -955,15 +969,15 @@ of pixels (xsize * ysize).
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
<format> ::= <RIFF header><image size><image stream>
<image stream> ::= (<optional-transform><image stream>);
<spatially-coded image>
<image stream> ::= <optional-transform><spatially-coded image>
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
#### Structure of Transforms
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
<optional-transform> ::= 1-bit <transform> <optional-transform> | 0-bit
<optional-transform> ::= (1-bit value 1; <transform> <optional-transform>) |
1-bit value 0
<transform> ::= <predictor-tx> | <color-tx> | <subtract-green-tx> |
<color-indexing-tx>
<predictor-tx> ::= 2-bit value 0; <predictor image>
@ -991,8 +1005,8 @@ of pixels (xsize * ysize).
<simple huffman code> ::= see "Simple code length code" for details
<normal huffman code> ::= <code length code>; encoded code lengths
<code length code> ::= see section "Normal code length code"
<lz77-coded image> ::= (<argb-pixel> | <color-cache-code> | <lz77-copy>) |
(<lz77-coded image> | "")
<lz77-coded image> ::= ((<argb-pixel> | <lz77-copy> | <color-cache-code>)
<lz77-coded image>) | ""
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
A possible example sequence: