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webp-lossless-bitstream-spec,cosmetics: fix some typos

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Bug: webp:551
Change-Id: I9ff90601b77deb9034ed7bef9cfd421d1f6e2e2b
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James Zern 2022-03-11 13:09:27 -08:00
parent 5ccbd6ed8c
commit f0e9351cce
1 changed files with 32 additions and 31 deletions
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doc/webp-lossless-bitstream-spec.txt
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@ -235,7 +235,7 @@ transform, the current pixel value is predicted from the pixels already
decoded (in scan-line order) and only the residual value (actual - decoded (in scan-line order) and only the residual value (actual -
predicted) is encoded. The _prediction mode_ determines the type of predicted) is encoded. The _prediction mode_ determines the type of
prediction to use. We divide the image into squares and all the pixels prediction to use. We divide the image into squares and all the pixels
in a square use same prediction mode. in a square use the same prediction mode.
The first 3 bits of prediction data define the block width and height in The first 3 bits of prediction data define the block width and height in
number of bits. The number of block columns, `block_xsize`, is used in number of bits. The number of block columns, `block_xsize`, is used in
@ -590,12 +590,12 @@ The values are packed into the green component as follows:
4 most-significant bits of the green value at x / 2. 4 most-significant bits of the green value at x / 2.
* `width_bits` = 2: for every x value where x ≡ 0 (mod 4), a green * `width_bits` = 2: for every x value where x ≡ 0 (mod 4), a green
value at x is positioned into the 2 least-significant bits of the value at x is positioned into the 2 least-significant bits of the
green value at x / 4, green values at x + 1 to x + 3 in order to the green value at x / 4, green values at x + 1 to x + 3 are positioned in order
more significant bits of the green value at x / 4. to the more significant bits of the green value at x / 4.
* `width_bits` = 3: for every x value where x ≡ 0 (mod 8), a green * `width_bits` = 3: for every x value where x ≡ 0 (mod 8), a green
value at x is positioned into the least-significant bit of the green value at x is positioned into the least-significant bit of the green
value at x / 8, green values at x + 1 to x + 7 in order to the more value at x / 8, green values at x + 1 to x + 7 are positioned in order to
significant bits of the green value at x / 8. the more significant bits of the green value at x / 8.
4 Image Data 4 Image Data
@ -621,7 +621,7 @@ We use image data in five different roles:
the image. Each `ColorTransformElement` `'cte'` is treated as a pixel whose the image. Each `ColorTransformElement` `'cte'` is treated as a pixel whose
alpha component is `255`, red component is `cte.red_to_blue`, green alpha component is `255`, red component is `cte.red_to_blue`, green
component is `cte.green_to_blue` and blue component is `cte.green_to_red`. component is `cte.green_to_blue` and blue component is `cte.green_to_red`.
1. Color indexing image: An array of of size `color_table_size` (up to 256 1. Color indexing image: An array of size `color_table_size` (up to 256
ARGB values) storing the metadata for the ARGB values) storing the metadata for the
[Color Indexing Transform](#color-indexing-transform). This is stored as an [Color Indexing Transform](#color-indexing-transform). This is stored as an
image of width `color_table_size` and height `1`. image of width `color_table_size` and height `1`.
@ -678,7 +678,7 @@ very few values in the image. Thus, this approach results in a 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
different range of values. different ranges of values.
Note: The maximum backward reference length is limited to 4096. Hence, only the Note: The maximum backward reference length is limited to 4096. Hence, only the
first 24 prefix codes (with the respective extra bits) are meaningful for length first 24 prefix codes (with the respective extra bits) are meaningful for length
@ -753,13 +753,13 @@ The mapping between distance code `i` and the neighboring pixel offset
(-6, 7), (7, 6), (-7, 6), (8, 5), (7, 7), (-7, 7), (8, 6), (8, 7) (-6, 7), (7, 6), (-7, 6), (8, 5), (7, 7), (-7, 7), (8, 6), (8, 7)
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
For example, distance code `1` indicates offset of `(0, 1)` for the neighboring For example, distance code `1` indicates an offset of `(0, 1)` for the
pixel, that is, the pixel above the current pixel (0-pixel difference in neighboring pixel, that is, the pixel above the current pixel (0 pixel
X-direction and 1 pixel difference in Y-direction). Similarly, distance code difference in X-direction and 1 pixel difference in Y-direction). Similarly,
`3` indicates left-top pixel. distance code `3` indicates left-top pixel.
The decoder can convert a distances 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:
~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
(xi, yi) = distance_map[i] (xi, yi) = distance_map[i]
@ -769,7 +769,7 @@ if (dist < 1) {
} }
~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
where 'distance_map' is the mapping noted above and `xsize` is the width of the where `distance_map` is the mapping noted above and `xsize` is the width of the
image in pixels. image in pixels.
@ -778,7 +778,7 @@ image in pixels.
Color cache stores a set of colors that have been recently used in the image. Color cache stores a set of colors that have been recently used in the image.
**Rationale:** This way, the recently used colors can sometimes be referred to **Rationale:** This way, the recently used colors can sometimes be referred to
more efficiently than emitting them using other two methods (described in more efficiently than emitting them using the other two methods (described in
[4.2.1](#huffman-coded-literals) and [4.2.2](#lz77-backward-reference)). [4.2.1](#huffman-coded-literals) and [4.2.2](#lz77-backward-reference)).
Color cache codes are stored as follows. First, there is a 1-bit value that Color cache codes are stored as follows. First, there is a 1-bit value that
@ -822,8 +822,9 @@ In particular, the format uses **spatially-variant Huffman coding**. In other
words, different blocks of the image can potentially use different entropy words, different blocks of the image can potentially use different entropy
codes. codes.
**Rationale**: Different areas of the image may have different characteristics. So, allowing them to use different entropy codes provides more flexibility and **Rationale**: Different areas of the image may have different characteristics.
potentially a better compression. So, allowing them to use different entropy codes provides more flexibility and
potentially better compression.
### 5.2 Details ### 5.2 Details
@ -865,7 +866,7 @@ int huffman_ysize = DIV_ROUND_UP(ysize, 1 << huffman_bits);
where `DIV_ROUND_UP` is as defined [earlier](#predictor-transform). where `DIV_ROUND_UP` is as defined [earlier](#predictor-transform).
Next bits contain an entropy image of width `huffman_xsize` and height The next bits contain an entropy image of width `huffman_xsize` and height
`huffman_ysize`. `huffman_ysize`.
**Interpretation of Meta Huffman Codes:** **Interpretation of Meta Huffman Codes:**
@ -890,7 +891,7 @@ int num_huff_groups = max(entropy image) + 1;
where `max(entropy image)` indicates the largest Huffman code stored in the where `max(entropy image)` indicates the largest Huffman code stored in the
entropy image. entropy image.
As each Huffman code groups contains five Huffman codes, the total number of As each Huffman code group contains five Huffman codes, the total number of
Huffman codes is: Huffman codes is:
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
@ -922,22 +923,22 @@ Huffman code group, the pixel is read and decoded as follows:
Read next symbol S from the bitstream using Huffman code #1. \[See Read next symbol S from the bitstream using Huffman code #1. \[See
[next section](#decoding-the-code-lengths) for details on decoding the Huffman [next section](#decoding-the-code-lengths) for details on decoding the Huffman
code lengths\]. Note that S is any integer in the range `0` to code lengths\]. Note that S is 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:
1. if S < 256 1. if S < 256
1. Use S as the green component 1. Use S as the green component.
1. Read red from the bitstream using Huffman code #2 1. Read red from the bitstream using Huffman code #2.
1. Read blue from the bitstream using Huffman code #3 1. Read blue from the bitstream using Huffman code #3.
1. Read alpha from the bitstream using Huffman code #4 1. Read alpha from the bitstream using Huffman code #4.
1. if S < 256 + 24 1. if S < 256 + 24
1. Use S - 256 as a length prefix code 1. Use S - 256 as a length prefix code.
1. Read extra bits for length from the bitstream 1. Read extra bits for length from the bitstream.
1. Determine backward-reference length L from length prefix code and the 1. Determine backward-reference length L from length prefix code and the
extra bits read. extra bits read.
1. Read distance prefix code from the bitstream using Huffman code #5 1. Read distance prefix code from the bitstream using Huffman code #5.
1. Read extra bits for distance from the bitstream 1. Read extra bits for distance from the bitstream.
1. Determine backward-reference distance D from distance prefix code and 1. Determine backward-reference distance D from distance prefix code and
the extra bits read. the extra bits read.
1. Copy the L pixels (in scan-line order) from the sequence of pixels 1. Copy the L pixels (in scan-line order) from the sequence of pixels
@ -1013,12 +1014,12 @@ for (i = 0; i < num_code_lengths; ++i) {
* Value 0 means no symbols have been coded. * Value 0 means no symbols have been coded.
* Values \[1..15\] indicate the bit length of the respective code. * Values \[1..15\] indicate the bit length of the respective code.
* Code 16 repeats the previous non-zero value \[3..6\] times, i.e., * Code 16 repeats the previous non-zero value \[3..6\] times, i.e.,
3 + `ReadBits(2)` times. If code 16 is used before a non-zero `3 + ReadBits(2)` times. If code 16 is used before a non-zero
value has been emitted, a value of 8 is repeated. value has been emitted, a value of 8 is repeated.
* Code 17 emits a streak of zeros \[3..10\], i.e., 3 + `ReadBits(3)` * Code 17 emits a streak of zeros \[3..10\], i.e., `3 + ReadBits(3)`
times. times.
* Code 18 emits a streak of zeros of length \[11..138\], i.e., * Code 18 emits a streak of zeros of length \[11..138\], i.e.,
11 + `ReadBits(7)` times. `11 + ReadBits(7)` times.
6 Overall Structure of the Format 6 Overall Structure of the Format