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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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5
src/webp/decode.h
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@ -20,7 +20,7 @@
extern "C" {
#endif
#define WEBP_DECODER_ABI_VERSION 0x0202 // MAJOR(8b) + MINOR(8b)
#define WEBP_DECODER_ABI_VERSION 0x0203 // MAJOR(8b) + MINOR(8b)
// Note: forward declaring enumerations is not allowed in (strict) C and C++,
// the types are left here for reference.
@ -441,11 +441,12 @@ struct WebPDecoderOptions {
int use_scaling; // if true, scaling is applied _afterward_
int scaled_width, scaled_height; // final resolution
int use_threads; // if true, use multi-threaded decoding
int dithering_strength; // dithering strength (0=Off, 100=full)
// Unused for now:
int force_rotation; // forced rotation (to be applied _last_)
int no_enhancement; // if true, discard enhancement layer
uint32_t pad[6]; // padding for later use
uint32_t pad[5]; // padding for later use
};
// Main object storing the configuration for advanced decoding.