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Merge changes I644d7d39,Icf05491e,Ic02e6652,I63b11258 into main

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* changes:
  webp-lossless-bitstream-spec: fix code blocks
  webp-lossless-bitstream-spec: block -> chunk
  webp-lossless-bitstream-spec: add some missing commas
  webp-lossless-bitstream-spec: normalize item text in 5.1
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James Zern 2023-07-27 19:34:20 +00:00 committed by Gerrit Code Review
commit 6e75054736
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doc/webp-lossless-bitstream-spec.txt
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@ -58,12 +58,12 @@ b = ReadBits(1);
b |= ReadBits(1) << 1; b |= ReadBits(1) << 1;
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
We assume that each color component, that is, alpha, red, blue and green, is We assume that each color component, that is, alpha, red, blue, and green, is
represented using an 8-bit byte. We define the corresponding type as uint8. A represented using an 8-bit byte. We define the corresponding type as uint8. A
whole ARGB pixel is represented by a type called uint32, which is an unsigned whole ARGB pixel is represented by a type called uint32, which is an unsigned
integer consisting of 32 bits. In the code showing the behavior of the integer consisting of 32 bits. In the code showing the behavior of the
transforms, these values are codified in the following bits: alpha in bits transforms, these values are codified in the following bits: alpha in bits
31..24, red in bits 23..16, green in bits 15..8 and blue in bits 7..0; however, 31..24, red in bits 23..16, green in bits 15..8, and blue in bits 7..0; however,
implementations of the format are free to use another representation internally. implementations of the format are free to use another representation internally.
Broadly, a WebP lossless image contains header data, transform information, and Broadly, a WebP lossless image contains header data, transform information, and
@ -136,8 +136,8 @@ The beginning of the header has the RIFF container. This consists of the
following 21 bytes: following 21 bytes:
1. String 'RIFF'. 1. String 'RIFF'.
2. A little-endian, 32-bit value of the block length, which is the whole size 2. A little-endian, 32-bit value of the chunk length, which is the whole size
of the block controlled by the RIFF header. Normally, this equals of the chunk controlled by the RIFF header. Normally, this equals
the payload size (file size minus 8 bytes: 4 bytes for the 'RIFF' the payload size (file size minus 8 bytes: 4 bytes for the 'RIFF'
identifier and 4 bytes for storing the value itself). identifier and 4 bytes for storing the value itself).
3. String 'WEBP' (RIFF container name). 3. String 'WEBP' (RIFF container name).
@ -336,7 +336,7 @@ for each ARGB component as follows:
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
// Clamp the input value between 0 and 255. // Clamp the input value between 0 and 255.
int Clamp(int a) { int Clamp(int a) {
return (a < 0) ? 0 : (a > 255) ? 255 : a; return (a < 0) ? 0 : (a > 255) ? 255 : a;
} }
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
@ -592,11 +592,11 @@ The values are packed into the green component as follows:
After reading this transform, `image_width` is subsampled by `width_bits`. This After reading this transform, `image_width` is subsampled by `width_bits`. This
affects the size of subsequent transforms. The new size can be calculated using affects the size of subsequent transforms. The new size can be calculated using
`DIV_ROUND_UP` as defined [earlier](#predictor-transform). `DIV_ROUND_UP`, as defined [earlier](#predictor-transform).
``` ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
image_width = DIV_ROUND_UP(image_width, 1 << width_bits); image_width = DIV_ROUND_UP(image_width, 1 << width_bits);
``` ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
5 Image Data 5 Image Data
------------ ------------
@ -608,16 +608,16 @@ Image data is an array of pixel values in scan-line order.
We use image data in five different roles: We use image data in five different roles:
1. ARGB image: Stores the actual pixels of the image. 1. ARGB image: Stores the actual pixels of the image.
1. Entropy image: Stores the 1. Entropy image: Stores the meta prefix codes (see
[meta prefix codes](#decoding-of-meta-prefix-codes). ["Decoding of Meta Prefix Codes"](#decoding-of-meta-prefix-codes)).
1. Predictor image: Stores the metadata for the 1. Predictor image: Stores the metadata for the predictor transform (see
[predictor transform](#predictor-transform). ["Predictor Transform"](#predictor-transform)).
1. Color transform image: Created by `ColorTransformElement` values 1. Color transform image: Created by `ColorTransformElement` values
(defined in ["Color Transform"](#color-transform)) for different blocks of (defined in ["Color Transform"](#color-transform)) for different blocks of
the image. the image.
1. Color indexing image: An array of size `color_table_size` (up to 256 1. Color indexing image: An array of size `color_table_size` (up to 256 ARGB
ARGB values) storing the metadata for the values) storing the metadata for the color indexing transform (see
[color indexing transform](#color-indexing-transform). ["Color Indexing Transform"](#color-indexing-transform)).
### 5.2 Encoding of Image Data ### 5.2 Encoding of Image Data
@ -788,14 +788,14 @@ int color_cache_code_bits = ReadBits(4);
int color_cache_size = 1 << color_cache_code_bits; int color_cache_size = 1 << color_cache_code_bits;
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
`color_cache_code_bits` defines the size of the color_cache (1 << `color_cache_code_bits` defines the size of the color_cache (`1 <<
`color_cache_code_bits`). The range of allowed values for color_cache_code_bits`). The range of allowed values for
`color_cache_code_bits` is \[1..11\]. Compliant decoders must indicate a `color_cache_code_bits` is \[1..11\]. Compliant decoders must indicate a
corrupted bitstream for other values. corrupted bitstream for other values.
A color cache is an array of size `color_cache_size`. Each entry stores one ARGB A color cache is an array of size `color_cache_size`. Each entry stores one ARGB
color. Colors are looked up by indexing them by (0x1e35a7bd * `color`) >> (32 - color. Colors are looked up by indexing them by `(0x1e35a7bd * color) >> (32 -
`color_cache_code_bits`). Only one lookup is done in a color cache; there is no color_cache_code_bits)`. Only one lookup is done in a color cache; there is no
conflict resolution. conflict resolution.
In the beginning of decoding or encoding of an image, all entries in all color In the beginning of decoding or encoding of an image, all entries in all color
@ -904,7 +904,7 @@ First, `num_code_lengths` specifies the number of code lengths.
int num_code_lengths = 4 + ReadBits(4); int num_code_lengths = 4 + ReadBits(4);
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
If `num_code_lengths` is > 19, the bitstream is invalid. If `num_code_lengths` is greater than 19, the bitstream is invalid.
The code lengths are themselves encoded using prefix codes; lower-level code The code lengths are themselves encoded using prefix codes; lower-level code
lengths, `code_length_code_lengths`, first have to be read. The rest of those lengths, `code_length_code_lengths`, first have to be read. The rest of those