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add alpha dithering for lossy

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new options:
 dwebp -alpha_dither
 vwebp -noalphadither

When the source was marked as quantized, we use a threshold-averaging
filter to smooth the decoded alpha plane.
Note: this option forces the decoding of alpha data in one pass, and
might slow the decoding a bit.

The new field in WebPDecoderOptions struct is 'alpha_dithering_strength'
(0 by default, means: off). Max strength value is '100'.

Change-Id: I218e21af96360d4781587fede95f8ea4e2b7287a
This commit is contained in:
skal
2014-06-14 00:06:16 +02:00
parent 790207679d
commit bbe32df1e3
11 changed files with 308 additions and 21 deletions
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16
src/dec/alpha.c
View File

@ -108,12 +108,6 @@ static int ALPHDecode(VP8Decoder* const dec, int row, int num_rows) {
unfilter_func(width, height, width, row, num_rows, output);
}
if (alph_dec->pre_processing_ == ALPHA_PREPROCESSED_LEVELS) {
if (!DequantizeLevels(output, width, height, row, num_rows)) {
return 0;
}
}
if (row + num_rows == dec->pic_hdr_.height_) {
dec->is_alpha_decoded_ = 1;
}
@ -143,12 +137,22 @@ const uint8_t* VP8DecompressAlphaRows(VP8Decoder* const dec,
dec->alph_dec_ = NULL;
return NULL;
}
// if we allowed use of alpha dithering, check whether it's needed at all
if (dec->alph_dec_->pre_processing_ != ALPHA_PREPROCESSED_LEVELS) {
dec->alpha_dithering_ = 0; // disable dithering
} else {
num_rows = height; // decode everything in one pass
}
}
if (!dec->is_alpha_decoded_) {
int ok = 0;
assert(dec->alph_dec_ != NULL);
ok = ALPHDecode(dec, row, num_rows);
if (ok && dec->alpha_dithering_ > 0) {
ok = WebPDequantizeLevels(dec->alpha_plane_, width, height,
dec->alpha_dithering_);
}
if (!ok || dec->is_alpha_decoded_) {
ALPHDelete(dec->alph_dec_);
dec->alph_dec_ = NULL;

7
src/dec/frame.c
View File

@ -177,6 +177,13 @@ void VP8InitDithering(const WebPDecoderOptions* const options,
dec->dither_ = 1;
}
}
// potentially allow alpha dithering
dec->alpha_dithering_ = options->alpha_dithering_strength;
if (dec->alpha_dithering_ > 100) {
dec->alpha_dithering_ = 100;
} else if (dec->alpha_dithering_ < 0) {
dec->alpha_dithering_ = 0;
}
}
}

3
src/dec/vp8i.h
View File

@ -296,7 +296,8 @@ struct VP8Decoder {
const uint8_t* alpha_data_; // compressed alpha data (if present)
size_t alpha_data_size_;
int is_alpha_decoded_; // true if alpha_data_ is decoded in alpha_plane_
uint8_t* alpha_plane_; // output. Persistent, contains the whole data.
uint8_t* alpha_plane_; // output. Persistent, contains the whole data.
int alpha_dithering_; // derived from decoding options (0=off, 100=full).
};
//------------------------------------------------------------------------------