diff --git a/doc/webp-lossless-bitstream-spec.txt b/doc/webp-lossless-bitstream-spec.txt index f83063ce..bfb870c2 100644 --- a/doc/webp-lossless-bitstream-spec.txt +++ b/doc/webp-lossless-bitstream-spec.txt @@ -16,6 +16,8 @@ _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. + Abstract -------- @@ -367,6 +369,7 @@ the 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 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 @@ -375,6 +378,8 @@ on the same row as the current pixel is instead used as the TR-pixel. ### Color Transform +\[AMENDED2\] + 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, transforms red (R) based on green and transforms blue (B) based on green @@ -834,7 +839,7 @@ potentially better compression. The encoded image data consists of several parts: - 1. Decoding and building the prefix codes + 1. Decoding and building the prefix codes \[AMENDED2\] 1. Meta Huffman codes 1. Entropy-coded image data @@ -858,6 +863,8 @@ stream. This may be inefficient, but it is allowed by the format. **(i) Simple Code Length Code:** +\[AMENDED2\] + This variant is used in the special case when only 1 or 2 Huffman symbols are in the range \[0, 255\] with code length `1`. All other Huffman code lengths are implicitly zeros. @@ -1031,6 +1038,8 @@ The decoder then uses Huffman code group `huff_group` to decode the pixel #### 5.2.3 Decoding Entropy-coded Image Data +\[AMENDED2\] + For the current position (x, y) in the image, the decoder first identifies the corresponding Huffman code group (as explained in the last section). Given the Huffman code group, the pixel is read and decoded as follows: @@ -1097,6 +1106,8 @@ of pixels (xsize * ysize). #### Structure of the Image Data +\[AMENDED2\] + ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ::= ::=