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webp-lossless-bitstream-spec: fix ColorTransform impl

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and the corresponding InverseTransform(); the operations were swapped.
The forward transform subtracts the deltas, the inverse adds them.

Bug: webp:551
Change-Id: Ieb90a24f843cee7cdfe46cbb15ec7d716ca9d269
This commit is contained in:
James Zern 2022-03-11 13:51:51 -08:00
parent 7a7e33e977
commit 44dd765def
1 changed files with 20 additions and 16 deletions
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36
doc/webp-lossless-bitstream-spec.txt
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@ -395,8 +395,8 @@ typedef struct {
The actual color transformation is done by defining a color transform
delta. The color transform delta depends on the `ColorTransformElement`,
which is the same for all the pixels in a particular block. The delta is
added during color transform. The inverse color transform then is just
subtracting those deltas.
subtracted during color transform. The inverse color transform then is just
adding those deltas.
The color transform function is defined as follows:
@ -405,13 +405,13 @@ void ColorTransform(uint8 red, uint8 blue, uint8 green,
ColorTransformElement *trans,
uint8 *new_red, uint8 *new_blue) {
// Transformed values of red and blue components
uint32 tmp_red = red;
uint32 tmp_blue = blue;
int tmp_red = red;
int tmp_blue = blue;
// Applying transform is just adding the transform deltas
tmp_red += ColorTransformDelta(trans->green_to_red, green);
tmp_blue += ColorTransformDelta(trans->green_to_blue, green);
tmp_blue += ColorTransformDelta(trans->red_to_blue, red);
// Applying the transform is just subtracting the transform deltas
tmp_red -= ColorTransformDelta(p->green_to_red_, green);
tmp_blue -= ColorTransformDelta(p->green_to_blue_, green);
tmp_blue -= ColorTransformDelta(p->red_to_blue_, red);
*new_red = tmp_red & 0xff;
*new_blue = tmp_blue & 0xff;
@ -429,7 +429,7 @@ int8 ColorTransformDelta(int8 t, int8 c) {
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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
\[128-255\] is mapped to the \[-128, -1\] range of its converted int8 value).
@ -467,14 +467,18 @@ channels.
void InverseTransform(uint8 red, uint8 green, uint8 blue,
ColorTransformElement *p,
uint8 *new_red, uint8 *new_blue) {
// Applying inverse transform is just subtracting the
// color transform deltas
red -= ColorTransformDelta(p->green_to_red_, green);
blue -= ColorTransformDelta(p->green_to_blue_, green);
blue -= ColorTransformDelta(p->red_to_blue_, red & 0xff);
// Transformed values of red and blue components
int tmp_red = red;
int tmp_blue = blue;
*new_red = red & 0xff;
*new_blue = blue & 0xff;
// Applying inverse transform is just adding the
// color transform deltas
tmp_red += ColorTransformDelta(trans->green_to_red, green);
tmp_blue += ColorTransformDelta(trans->green_to_blue, green);
tmp_blue += ColorTransformDelta(trans->red_to_blue, tmp_red & 0xff);
*new_red = tmp_red & 0xff;
*new_blue = tmp_blue & 0xff;
}
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~