mirror of
https://github.com/webmproject/libwebp.git
synced 2025-02-15 16:32:52 +01:00
clarify single leaf node trees and use of canonical prefix coding
remove AMENDED-notes (the last functional spec change to match with the implementation is from 2014, other amendments are clarifications) Bug: webp:581 Change-Id: Ic47739be0fd5a975fd734d6813567ca615304f1d
This commit is contained in:
Jyrki Alakuijala
James Zern
parent
8dd80ef814
commit
428588ef90
@ -12,13 +12,7 @@ at the end of this file.
|
||||
Specification for WebP Lossless Bitstream
|
||||
=========================================
|
||||
|
||||
_Jyrki Alakuijala, Ph.D., Google, Inc., 2012-06-19_
|
||||
|
||||
Paragraphs marked as \[AMENDED\] were amended on 2014-09-16.
|
||||
|
||||
Paragraphs marked as \[AMENDED2\] were amended on 2022-05-13.
|
||||
|
||||
Paragraphs marked as \[AMENDED3\] were amended on 2022-11-21.
|
||||
_Jyrki Alakuijala, Ph.D., Google, Inc., 2023-03-09_
|
||||
|
||||
Abstract
|
||||
--------
|
||||
@ -173,7 +167,7 @@ int alpha_is_used = ReadBits(1);
|
||||
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
|
||||
|
||||
The version_number is a 3 bit code that must be set to 0. Any other value should
|
||||
be treated as an error. \[AMENDED\]
|
||||
be treated as an error.
|
||||
|
||||
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
|
||||
int version_number = ReadBits(3);
|
||||
@ -325,7 +319,7 @@ uint32 Select(uint32 L, uint32 T, uint32 TL) {
|
||||
abs(pGreen - GREEN(T)) + abs(pBlue - BLUE(T));
|
||||
|
||||
// Return either left or top, the one closer to the prediction.
|
||||
if (pL < pT) { // \[AMENDED\]
|
||||
if (pL < pT) {
|
||||
return L;
|
||||
} else {
|
||||
return T;
|
||||
@ -361,7 +355,7 @@ predicted value for the left-topmost pixel of the image is 0xff000000, L-pixel
|
||||
for all pixels on the top row, and T-pixel for all pixels on the leftmost
|
||||
column.
|
||||
|
||||
\[AMENDED2\] Addressing the TR-pixel for pixels on the rightmost column is
|
||||
Addressing the TR-pixel for pixels on the rightmost column is
|
||||
exceptional. The pixels on the rightmost column are predicted by using the modes
|
||||
\[0..13\] just like pixels not on the border, but the leftmost pixel on the same
|
||||
row as the current pixel is instead used as the TR-pixel.
|
||||
@ -369,8 +363,6 @@ row as the current pixel is instead used as the TR-pixel.
|
||||
|
||||
### 4.2 Color Transform
|
||||
|
||||
\[AMENDED2\]
|
||||
|
||||
The goal of the color transform is to decorrelate the R, G and B values 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 and then based on red.
|
||||
@ -451,7 +443,7 @@ described in [Chapter 5](#image-data).
|
||||
During decoding, `ColorTransformElement` instances of the blocks are decoded and
|
||||
the inverse color transform is applied on the ARGB values of the pixels. As
|
||||
mentioned earlier, that inverse color transform is just adding
|
||||
`ColorTransformElement` values to the red and blue channels. \[AMENDED3\]
|
||||
`ColorTransformElement` values to the red and blue channels.
|
||||
|
||||
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
|
||||
void InverseTransform(uint8 red, uint8 green, uint8 blue,
|
||||
@ -535,7 +527,7 @@ argb = color_table[GREEN(argb)];
|
||||
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
|
||||
|
||||
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).
|
||||
|
||||
When the color table is small (equal to or less than 16 colors), several pixels
|
||||
are bundled into a single pixel. The pixel bundling packs several (2, 4, or 8)
|
||||
@ -748,8 +740,6 @@ Similarly, the distance code `3` indicates the left-top pixel.
|
||||
The decoder can convert a distance code `i` to a scan-line order distance `dist`
|
||||
as follows:
|
||||
|
||||
\[AMENDED3\]
|
||||
|
||||
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
|
||||
(xi, yi) = distance_map[i - 1]
|
||||
dist = xi + yi * xsize
|
||||
@ -820,7 +810,7 @@ potentially better compression.
|
||||
|
||||
The encoded image data consists of several parts:
|
||||
|
||||
1. Decoding and building the prefix codes \[AMENDED2\]
|
||||
1. Decoding and building the prefix codes
|
||||
1. Meta prefix codes
|
||||
1. Entropy-coded image data
|
||||
|
||||
@ -841,11 +831,16 @@ by a 1-bit value.
|
||||
|
||||
In both cases, there can be unused code lengths that are still part of the
|
||||
stream. This may be inefficient, but it is allowed by the format.
|
||||
The described tree must be a complete binary tree. A single leaf node is
|
||||
considered a complete binary tree and can be encoded using either the simple
|
||||
code length code or the normal code length code. When coding a single leaf
|
||||
node using the _normal code length code_, all but one code length should be
|
||||
zeros, and the single leaf node value is marked with the length of 1 -- even
|
||||
when no bits will be consumed when that single leaf node tree is used.
|
||||
|
||||
|
||||
**(i) Simple Code Length Code:**
|
||||
|
||||
\[AMENDED2\]
|
||||
|
||||
This variant is used in the special case when only 1 or 2 prefix symbols are in
|
||||
the range \[0..255\] with code length `1`. All other prefix code lengths are
|
||||
implicitly zeros.
|
||||
@ -889,7 +884,7 @@ First, `num_code_lengths` specifies the number of code lengths.
|
||||
int num_code_lengths = 4 + ReadBits(4);
|
||||
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
|
||||
|
||||
If `num_code_lengths` is > 19, the bitstream is invalid. \[AMENDED3\]
|
||||
If `num_code_lengths` is > 19, the bitstream is invalid.
|
||||
|
||||
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
|
||||
@ -936,6 +931,11 @@ distance) is formed using their respective alphabet sizes:
|
||||
* other literals (A,R,B): 256
|
||||
* distance code: 40
|
||||
|
||||
The Normal Code Length Code must code a full decision tree, i.e., the sum of
|
||||
`2 ^ (-length)` for all non-zero codes must be exactly one. There is however
|
||||
one exception to this rule, the single leaf node tree, where the leaf node
|
||||
value is marked with value 1 and other values are 0s.
|
||||
|
||||
#### 6.2.2 Decoding of Meta Prefix Codes
|
||||
|
||||
As noted earlier, the format allows the use of different prefix codes for
|
||||
@ -1020,8 +1020,6 @@ The decoder then uses prefix code group `prefix_group` to decode the pixel
|
||||
|
||||
#### 6.2.3 Decoding Entropy-coded Image Data
|
||||
|
||||
\[AMENDED2\]
|
||||
|
||||
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
|
||||
prefix code group, the pixel is read and decoded as follows:
|
||||
@ -1099,8 +1097,6 @@ color-indexing-image = 8BIT ; color count
|
||||
|
||||
#### 7.3 Structure of the Image Data
|
||||
|
||||
\[AMENDED2\]
|
||||
|
||||
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
|
||||
spatially-coded-image = color-cache-info meta-prefix data
|
||||
entropy-coded-image = color-cache-info data
|
||||
|
||||
Loading…
x
Reference in New Issue
Block a user