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webp-lossless-bitstream-spec,cosmetics: grammar/capitalization

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Bug: webp:448
Change-Id: I2d6cc66b45342716fdba7792c570510601432109
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James Zern 2022-11-21 10:40:33 -08:00
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doc/webp-lossless-bitstream-spec.txt
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@ -40,7 +40,7 @@ today's PNG format.
-------------- --------------
This document describes the compressed data representation of a WebP This document describes the compressed data representation of a WebP
lossless image. It is intended as a detailed reference for WebP lossless lossless image. It is intended as a detailed reference for the WebP lossless
encoder and decoder implementation. encoder and decoder implementation.
In this document, we extensively use C programming language syntax to In this document, we extensively use C programming language syntax to
@ -380,7 +380,7 @@ on the same row as the current pixel is instead used as the TR-pixel.
\[AMENDED2\] \[AMENDED2\]
The goal of the color transform is to decorrelate the R, G and B values The goal of the color transform is to decorrelate the R, G and B values
of each pixel. Color transform keeps the green (G) value as it is, of each pixel. The color transform keeps the green (G) value as it is,
transforms red (R) based on green and transforms blue (B) based on green transforms red (R) based on green and transforms blue (B) based on green
and then based on red. and then based on red.
@ -399,7 +399,7 @@ typedef struct {
The actual color transformation is done by defining a color transform The actual color transformation is done by defining a color transform
delta. The color transform delta depends on the `ColorTransformElement`, delta. The color transform delta depends on the `ColorTransformElement`,
which is the same for all the pixels in a particular block. The delta is which is the same for all the pixels in a particular block. The delta is
subtracted during color transform. The inverse color transform then is just subtracted during the color transform. The inverse color transform then is just
adding those deltas. adding those deltas.
The color transform function is defined as follows: The color transform function is defined as follows:
@ -475,7 +475,7 @@ void InverseTransform(uint8 red, uint8 green, uint8 blue,
int tmp_red = red; int tmp_red = red;
int tmp_blue = blue; int tmp_blue = blue;
// Applying inverse transform is just adding the // Applying the inverse transform is just adding the
// color transform deltas // color transform deltas
tmp_red += ColorTransformDelta(trans->green_to_red, green); tmp_red += ColorTransformDelta(trans->green_to_red, green);
tmp_blue += ColorTransformDelta(trans->green_to_blue, green); tmp_blue += ColorTransformDelta(trans->green_to_blue, green);
@ -517,7 +517,7 @@ create a color index array and replace the pixel values by the array's
indices. The color indexing transform achieves this. (In the context of indices. The color indexing transform achieves this. (In the context of
WebP lossless, we specifically do not call this a palette transform WebP lossless, we specifically do not call this a palette transform
because a similar but more dynamic concept exists in WebP lossless because a similar but more dynamic concept exists in WebP lossless
encoding: color cache.) encoding: color cache).
The color indexing transform checks for the number of unique ARGB values The color indexing transform checks for the number of unique ARGB values
in the image. If that number is below a threshold (256), it creates an in the image. If that number is below a threshold (256), it creates an
@ -556,7 +556,7 @@ color.
argb = color_table[GREEN(argb)]; argb = color_table[GREEN(argb)];
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
If the index is equal or larger than color_table_size, the argb color value If the index is equal or larger than `color_table_size`, the argb color value
should be set to 0x00000000 (transparent black). \[AMENDED\] should be set to 0x00000000 (transparent black). \[AMENDED\]
When the color table is small (equal to or less than 16 colors), several When the color table is small (equal to or less than 16 colors), several
@ -583,7 +583,7 @@ if (color_table_size <= 2) {
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
`width_bits` has a value of 0, 1, 2 or 3. A value of 0 indicates no pixel `width_bits` has a value of 0, 1, 2 or 3. A value of 0 indicates no pixel
bundling to be done for the image. A value of 1 indicates that two pixels are bundling is to be done for the image. A value of 1 indicates that two pixels are
combined, and each pixel has a range of \[0..15\]. A value of 2 indicates that combined, and each pixel has a range of \[0..15\]. A value of 2 indicates that
four pixels are combined, and each pixel has a range of \[0..3\]. A value of 3 four pixels are combined, and each pixel has a range of \[0..3\]. A value of 3
indicates that eight pixels are combined and each pixel has a range of \[0..1\], indicates that eight pixels are combined and each pixel has a range of \[0..1\],
@ -650,7 +650,7 @@ the image.
Each pixel is encoded using one of the three possible methods: Each pixel is encoded using one of the three possible methods:
1. prefix coded literal: each channel (green, red, blue and alpha) is 1. Prefix coded literal: each channel (green, red, blue and alpha) is
entropy-coded independently; entropy-coded independently;
2. LZ77 backward reference: a sequence of pixels are copied from elsewhere 2. LZ77 backward reference: a sequence of pixels are copied from elsewhere
in the image; or in the image; or
@ -681,7 +681,7 @@ while the extra bits are stored as they are (without an entropy code).
**Rationale**: This approach reduces the storage requirement for the entropy **Rationale**: This approach reduces the storage requirement for the entropy
code. Also, large values are usually rare, and so extra bits would be used for code. Also, large values are usually rare, and so extra bits would be used for
very few values in the image. Thus, this approach results in a better very few values in the image. Thus, this approach results in better
compression overall. compression overall.
The following table denotes the prefix codes and extra bits used for storing The following table denotes the prefix codes and extra bits used for storing
@ -722,12 +722,12 @@ return offset + ReadBits(extra_bits) + 1;
**Distance Mapping:** **Distance Mapping:**
{:#distance-mapping} {:#distance-mapping}
As noted previously, distance code is a number indicating the position of a As noted previously, a distance code is a number indicating the position of a
previously seen pixel, from which the pixels are to be copied. This subsection previously seen pixel, from which the pixels are to be copied. This subsection
defines the mapping between a distance code and the position of a previous defines the mapping between a distance code and the position of a previous
pixel. pixel.
The distance codes larger than 120 denote the pixel-distance in scan-line Distance codes larger than 120 denote the pixel-distance in scan-line
order, offset by 120. order, offset by 120.
The smallest distance codes \[1..120\] are special, and are reserved for a close The smallest distance codes \[1..120\] are special, and are reserved for a close
@ -763,10 +763,10 @@ The mapping between distance code `i` and the neighboring pixel offset
(8, 7) (8, 7)
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
For example, distance code `1` indicates an offset of `(0, 1)` for the For example, the distance code `1` indicates an offset of `(0, 1)` for the
neighboring pixel, that is, the pixel above the current pixel (0 pixel neighboring pixel, that is, the pixel above the current pixel (0 pixel
difference in X-direction and 1 pixel difference in Y-direction). Similarly, difference in the X-direction and 1 pixel difference in the Y-direction).
distance code `3` indicates the left-top pixel. Similarly, the distance code `3` indicates the left-top pixel.
The decoder can convert a distance code `i` to a scan-line order distance The decoder can convert a distance code `i` to a scan-line order distance
`dist` as follows: `dist` as follows:
@ -897,7 +897,7 @@ if (num_symbols == 2) {
(an empty prefix code). For example, a prefix code for distance can be empty (an empty prefix code). For example, a prefix code for distance can be empty
if there are no backward references. Similarly, prefix codes for alpha, red, if there are no backward references. Similarly, prefix codes for alpha, red,
and blue can be empty if all pixels within the same meta prefix code are and blue can be empty if all pixels within the same meta prefix code are
produced using the color cache. However, this case doesn't need a special produced using the color cache. However, this case doesn't need special
handling, as empty prefix codes can be coded as those containing a single handling, as empty prefix codes can be coded as those containing a single
symbol `0`. symbol `0`.
@ -913,7 +913,7 @@ int num_code_lengths = 4 + ReadBits(4);
If `num_code_lengths` is > 18, the bitstream is invalid. If `num_code_lengths` is > 18, 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
`code_length_code_lengths` (according to the order in `kCodeLengthCodeOrder`) `code_length_code_lengths` (according to the order in `kCodeLengthCodeOrder`)
are zeros. are zeros.
@ -980,7 +980,7 @@ The entropy image defines which prefix codes are used in different parts of the
image, as described below. image, as described below.
The first 3-bits contain the `prefix_bits` value. The dimensions of the entropy The first 3-bits contain the `prefix_bits` value. The dimensions of the entropy
image are derived from 'prefix_bits'. image are derived from `prefix_bits`.
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
int prefix_bits = ReadBits(3) + 2; int prefix_bits = ReadBits(3) + 2;
@ -998,11 +998,11 @@ The next bits contain an entropy image of width `prefix_xsize` and height
For any given pixel (x, y), there is a set of five prefix codes associated with For any given pixel (x, y), there is a set of five prefix codes associated with
it. These codes are (in bitstream order): it. These codes are (in bitstream order):
* **prefix code #1**: used for green channel, backward-reference length and * **Prefix code #1**: used for green channel, backward-reference length and
color cache color cache.
* **prefix code #2, #3 and #4**: used for red, blue and alpha channels * **Prefix code #2, #3 and #4**: used for red, blue and alpha channels
respectively. respectively.
* **prefix code #5**: used for backward-reference distance. * **Prefix code #5**: used for backward-reference distance.
From here on, we refer to this set as a **prefix code group**. From here on, we refer to this set as a **prefix code group**.
@ -1047,8 +1047,8 @@ For the current position (x, y) in the image, the decoder first identifies the
corresponding prefix code group (as explained in the last section). Given the corresponding prefix code group (as explained in the last section). Given the
prefix code group, the pixel is read and decoded as follows: prefix code group, the pixel is read and decoded as follows:
Read next symbol S from the bitstream using prefix code #1. Note that S is any Read the next symbol S from the bitstream using prefix code #1. Note that S is
integer in the range `0` to any integer in the range `0` to
`(256 + 24 + ` [`color_cache_size`](#color-cache-code)` - 1)`. `(256 + 24 + ` [`color_cache_size`](#color-cache-code)` - 1)`.
The interpretation of S depends on its value: The interpretation of S depends on its value:
@ -1139,9 +1139,9 @@ prefix-code-group =
; codes are for. ; codes are for.
prefix-code = simple-prefix-code / normal-prefix-code prefix-code = simple-prefix-code / normal-prefix-code
simple-prefix-code = ; see "Simple code length code" for details simple-prefix-code = ; see "Simple Code Length Code" for details
normal-prefix-code = code-length-code encoded-code-lengths normal-prefix-code = code-length-code encoded-code-lengths
code-length-code = ; see section "Normal code length code" code-length-code = ; see section "Normal Code Length Code"
lz77-coded-image = lz77-coded-image =
*((argb-pixel / lz77-copy / color-cache-code) lz77-coded-image) *((argb-pixel / lz77-copy / color-cache-code) lz77-coded-image)