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add a -dither dithering option to the decoder

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Even at high quality setting, the U/V quantizer step is limited
to 4 which can lead to banding on gradient.
This option allows to selectively apply some randomness to
potentially flattened-out U/V blocks and attenuate the banding.

This option is off by default in 'dwebp', but set to -dither 50
by default in 'vwebp'.

Note: depending on the number of blocks selectively dithered,
we can have up to a 10% slow-down in decoding speed it seems.

Change-Id: Icc2446007f33ddacb60b3a80a9e63f2d5ad162de
This commit is contained in:
skal
2013-11-26 22:59:02 +01:00
committed by Pascal Massimino
parent e812401299
commit cbdd3e6e53
11 changed files with 138 additions and 11 deletions
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8
examples/dwebp.c
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@ -555,6 +555,8 @@ static void Help(void) {
" -version .... print version number and exit.\n"
" -nofancy ..... don't use the fancy YUV420 upscaler.\n"
" -nofilter .... disable in-loop filtering.\n"
" -nodither .... disable dithering.\n"
" -dither <d> .. dithering strength (in 0..100)\n"
" -mt .......... use multi-threading\n"
" -crop <x> <y> <w> <h> ... crop output with the given rectangle\n"
" -scale <w> <h> .......... scale the output (*after* any cropping)\n"
@ -625,6 +627,10 @@ int main(int argc, const char *argv[]) {
format = YUV;
} else if (!strcmp(argv[c], "-mt")) {
config.options.use_threads = 1;
} else if (!strcmp(argv[c], "-nodither")) {
config.options.dithering_strength = 0;
} else if (!strcmp(argv[c], "-dither") && c < argc - 1) {
config.options.dithering_strength = strtol(argv[++c], NULL, 0);
} else if (!strcmp(argv[c], "-crop") && c < argc - 4) {
config.options.use_cropping = 1;
config.options.crop_left = strtol(argv[++c], NULL, 0);
@ -719,7 +725,7 @@ int main(int argc, const char *argv[]) {
if (!incremental) {
status = WebPDecode(data, data_size, &config);
} else {
WebPIDecoder* const idec = WebPINewDecoder(output_buffer);
WebPIDecoder* const idec = WebPIDecode(data, data_size, &config);
if (idec == NULL) {
fprintf(stderr, "Failed during WebPINewDecoder().\n");
status = VP8_STATUS_OUT_OF_MEMORY;