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Merge "fixing the findings by Frederic Kayser to the bitstream spec" into 0.2.0

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James Zern 2012-07-18 23:19:34 -07:00 committed by Gerrit Code Review
commit 2390dabcb6
1 changed files with 18 additions and 17 deletions
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35
doc/webp-lossless-bitstream-spec.txt
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@ -130,7 +130,7 @@ internally.
Broadly, a WebP lossless image contains header data, transform Broadly, a WebP lossless image contains header data, transform
information and actual image data. Headers contain width and height of information and actual image data. Headers contain width and height of
the image. A WebP lossless image can go through five different types of the image. A WebP lossless image can go through four different types of
transformation before being entropy encoded. The transform information transformation before being entropy encoded. The transform information
in the bitstream contains the data required to apply the respective in the bitstream contains the data required to apply the respective
inverse transforms. inverse transforms.
@ -781,7 +781,7 @@ 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 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 entropy code to use. First, there is an integer called 'meta Huffman
code' that can be obtained from a subresolution 2D image. This code' that can be obtained from the entropy image. This
meta Huffman code identifies a set of five Huffman codes, one for green 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, (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 blue and alpha, and one for distance. The Huffman codes are identified
@ -886,7 +886,7 @@ codes to the number of meta Huffman codes.
There are two ways to code the meta Huffman codes, indicated by one bit There are two ways to code the meta Huffman codes, indicated by one bit
for the ARGB image and is an implicit zero, i.e., not present in the for the ARGB image and is an implicit zero, i.e., not present in the
stream for all predictor images and Huffman image itself. stream for all transform images and the entropy image itself.
If this bit is zero, there is only one meta Huffman code, using Huffman If this bit is zero, there is only one meta Huffman code, using Huffman
codes 0, 1, 2, 3 and 4 for green, alpha, red, blue and distance, codes 0, 1, 2, 3 and 4 for green, alpha, red, blue and distance,
@ -953,8 +953,8 @@ of pixels (xsize * ysize).
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
<format> ::= <RIFF header><image size><image stream> <format> ::= <RIFF header><image size><image stream>
<image stream> ::= (<optional-transform><image stream>) | <image stream> ::= (<optional-transform><image stream>);
<entropy-coded image> <spatially-coded image>
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
@ -964,23 +964,24 @@ of pixels (xsize * ysize).
<optional-transform> ::= 1-bit <transform> <optional-transform> | 0-bit <optional-transform> ::= 1-bit <transform> <optional-transform> | 0-bit
<transform> ::= <predictor-tx> | <color-tx> | <subtract-green-tx> | <transform> ::= <predictor-tx> | <color-tx> | <subtract-green-tx> |
<color-indexing-tx> <color-indexing-tx>
<predictor-tx> ::= 2-bit value 0; 4-bit sub-pixel code | <entropy-coded image> <predictor-tx> ::= 2-bit value 0; <predictor image>
<color-tx> ::= 2-bit value 1; 4-bit sub-pixel code | <entropy-coded image> <predictor image> ::= 3-bit sub-pixel code | <entropy-coded image>
<color-tx> ::= 2-bit value 1; <color image>
<color image> ::= 3-bit sub-pixel code | <entropy-coded image>
<subtract-green-tx> ::= 2-bit value 2 <subtract-green-tx> ::= 2-bit value 2
<color-indexing-tx> ::= 2-bit value 3; 8-bit color count | <entropy-coded image> <color-indexing-tx> ::= 2-bit value 3; <color-indexing image>
<color-indexing image> ::= 8-bit color count; <entropy-coded image>
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
#### Structure of the Image Data #### Structure of the Image Data
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
<entropy-coded image> ::= <color cache info><optional meta huffman> <spatially-coded image> ::= <meta huffman><entropy-coded image>
<huffman codes><lz77-coded image> <entropy-coded image> ::= <color cache info><huffman codes><lz77-coded image>
<optional meta huffman> ::= 1-bit value 0 | <meta huffman> ::= 1-bit value 0 |
(1-bit value 1; (1-bit value 1; <entropy image>)
<huffman image><meta Huffman size>) <entropy image> ::= 3-bit subsample value; <entropy-coded image>
<huffman image> ::= 4-bit subsample value; <image stream>
<meta huffman size> ::= 4-bit length; meta Huffman size (subtracted by 2).
<color cache info> ::= 1 bit value 0 | <color cache info> ::= 1 bit value 0 |
(1-bit value 1; 4-bit value for color cache size) (1-bit value 1; 4-bit value for color cache size)
<huffman codes> ::= <huffman code> | <huffman code><huffman codes> <huffman codes> ::= <huffman code> | <huffman code><huffman codes>
@ -996,7 +997,7 @@ A possible example sequence:
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
<RIFF header><image size>1-bit value 1<subtract-green-tx> <RIFF header><image size>1-bit value 1<subtract-green-tx>
1-bit value 1<predictor-tx>1-bit value 0<huffman image> 1-bit value 1<predictor-tx>1-bit value 0<meta huffman>
<color cache info><meta huffman code><huffman codes> <color cache info><huffman codes>
<lz77-coded image> <lz77-coded image>
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~