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* changes:
  webp-lossless-bitstream-spec,cosmetics: reflow abstract
  webp-lossless-bitstream-spec: reword abstract re alpha
  webp-lossless-bitstream-spec,cosmetics: normalize range syntax
  webp-lossless-bitstream-spec: limit dist map lut to 69 cols
  webp-lossless-bitstream-spec: fix dist mapping example
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James Zern 2022-11-18 18:28:15 +00:00 committed by Gerrit Code Review
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doc/webp-lossless-bitstream-spec.txt
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@ -21,16 +21,15 @@ Paragraphs marked as \[AMENDED2\] were amended on 2022-05-13.
Abstract Abstract
-------- --------
WebP lossless is an image format for lossless compression of ARGB WebP lossless is an image format for lossless compression of ARGB images. The
images. The lossless format stores and restores the pixel values lossless format stores and restores the pixel values exactly, including the
exactly, including the color values for zero alpha pixels. The color values for pixels whose alpha value is 0. The format uses subresolution
format uses subresolution images, recursively embedded into the format images, recursively embedded into the format itself, for storing statistical
itself, for storing statistical data about the images, such as the used data about the images, such as the used entropy codes, spatial predictors,
entropy codes, spatial predictors, color space conversion, and color color space conversion, and color table. LZ77, prefix coding, and a color cache
table. LZ77, prefix coding, and a color cache are used for compression are used for compression of the bulk data. Decoding speeds faster than PNG have
of the bulk data. Decoding speeds faster than PNG have been been demonstrated, as well as 25% denser compression than can be achieved using
demonstrated, as well as 25% denser compression than can be achieved today's PNG format.
using today's PNG format.
* TOC placeholder * TOC placeholder
@ -436,7 +435,7 @@ int8 ColorTransformDelta(int8 t, int8 c) {
A conversion from the 8-bit unsigned representation (uint8) to the 8-bit A conversion from the 8-bit unsigned representation (uint8) to the 8-bit
signed one (int8) is required before calling `ColorTransformDelta()`. signed one (int8) is required before calling `ColorTransformDelta()`.
It should be performed using 8-bit two's complement (that is: uint8 range It should be performed using 8-bit two's complement (that is: uint8 range
\[128-255\] is mapped to the \[-128, -1\] range of its converted int8 value). \[128..255\] is mapped to the \[-128..-1\] range of its converted int8 value).
The multiplication is to be done using more precision (with at least The multiplication is to be done using more precision (with at least
16-bit dynamics). The sign extension property of the shift operation 16-bit dynamics). The sign extension property of the shift operation
@ -744,34 +743,37 @@ The mapping between distance code `i` and the neighboring pixel offset
`(xi, yi)` is as follows: `(xi, yi)` is as follows:
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
(0, 1), (1, 0), (1, 1), (-1, 1), (0, 2), (2, 0), (1, 2), (-1, 2), (0, 1), (1, 0), (1, 1), (-1, 1), (0, 2), (2, 0), (1, 2),
(2, 1), (-2, 1), (2, 2), (-2, 2), (0, 3), (3, 0), (1, 3), (-1, 3), (-1, 2), (2, 1), (-2, 1), (2, 2), (-2, 2), (0, 3), (3, 0),
(3, 1), (-3, 1), (2, 3), (-2, 3), (3, 2), (-3, 2), (0, 4), (4, 0), (1, 3), (-1, 3), (3, 1), (-3, 1), (2, 3), (-2, 3), (3, 2),
(1, 4), (-1, 4), (4, 1), (-4, 1), (3, 3), (-3, 3), (2, 4), (-2, 4), (-3, 2), (0, 4), (4, 0), (1, 4), (-1, 4), (4, 1), (-4, 1),
(4, 2), (-4, 2), (0, 5), (3, 4), (-3, 4), (4, 3), (-4, 3), (5, 0), (3, 3), (-3, 3), (2, 4), (-2, 4), (4, 2), (-4, 2), (0, 5),
(1, 5), (-1, 5), (5, 1), (-5, 1), (2, 5), (-2, 5), (5, 2), (-5, 2), (3, 4), (-3, 4), (4, 3), (-4, 3), (5, 0), (1, 5), (-1, 5),
(4, 4), (-4, 4), (3, 5), (-3, 5), (5, 3), (-5, 3), (0, 6), (6, 0), (5, 1), (-5, 1), (2, 5), (-2, 5), (5, 2), (-5, 2), (4, 4),
(1, 6), (-1, 6), (6, 1), (-6, 1), (2, 6), (-2, 6), (6, 2), (-6, 2), (-4, 4), (3, 5), (-3, 5), (5, 3), (-5, 3), (0, 6), (6, 0),
(4, 5), (-4, 5), (5, 4), (-5, 4), (3, 6), (-3, 6), (6, 3), (-6, 3), (1, 6), (-1, 6), (6, 1), (-6, 1), (2, 6), (-2, 6), (6, 2),
(0, 7), (7, 0), (1, 7), (-1, 7), (5, 5), (-5, 5), (7, 1), (-7, 1), (-6, 2), (4, 5), (-4, 5), (5, 4), (-5, 4), (3, 6), (-3, 6),
(4, 6), (-4, 6), (6, 4), (-6, 4), (2, 7), (-2, 7), (7, 2), (-7, 2), (6, 3), (-6, 3), (0, 7), (7, 0), (1, 7), (-1, 7), (5, 5),
(3, 7), (-3, 7), (7, 3), (-7, 3), (5, 6), (-5, 6), (6, 5), (-6, 5), (-5, 5), (7, 1), (-7, 1), (4, 6), (-4, 6), (6, 4), (-6, 4),
(8, 0), (4, 7), (-4, 7), (7, 4), (-7, 4), (8, 1), (8, 2), (6, 6), (2, 7), (-2, 7), (7, 2), (-7, 2), (3, 7), (-3, 7), (7, 3),
(-6, 6), (8, 3), (5, 7), (-5, 7), (7, 5), (-7, 5), (8, 4), (6, 7), (-7, 3), (5, 6), (-5, 6), (6, 5), (-6, 5), (8, 0), (4, 7),
(-6, 7), (7, 6), (-7, 6), (8, 5), (7, 7), (-7, 7), (8, 6), (8, 7) (-4, 7), (7, 4), (-7, 4), (8, 1), (8, 2), (6, 6), (-6, 6),
(8, 3), (5, 7), (-5, 7), (7, 5), (-7, 5), (8, 4), (6, 7),
(-6, 7), (7, 6), (-7, 6), (8, 5), (7, 7), (-7, 7), (8, 6),
(8, 7)
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
For example, distance code `1` indicates an offset of `(0, 1)` for the For example, 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 X-direction and 1 pixel difference in Y-direction). Similarly,
distance code `3` indicates left-top pixel. 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:
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
(xi, yi) = distance_map[i] (xi, yi) = distance_map[i - 1]
dist = x + y * xsize dist = xi + yi * xsize
if (dist < 1) { if (dist < 1) {
dist = 1 dist = 1
} }