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Enable Near-lossless feature.

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Enable the WebP near-lossless feature by pre-processing the image to smoothen
the pixels.

On a 1000 PNG image corpus, for which WebP lossless (default settings) gets
25% compression gains, following is the performance of near-lossless feature
at various '-near_lossless' levels:
-near_lossless 90: 30% (very very high PSNR 54-60dB)
-near_lossless 75: 38% (very high PSNR 48-54dB)
-near_lossless 50: 45% (high PSNR 42-48dB)
-near_lossless 25: 48% (moderate PSNR 36-42dB)
-near_lossless 10: 50% (PSNR 30-36dB)

WebP near-lossless is specifically useful for discrete-tone images like
line-art, icons etc.

Change-Id: I7d12a2c9362ccd076d09710ea05c85fa64664c38
This commit is contained in:
Vikas Arora 2015-01-29 16:02:09 -08:00
parent c6b24543fc
commit 98c8138663
3 changed files with 7 additions and 19 deletions
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4
examples/cwebp.c
View File

@ -615,10 +615,8 @@ static void HelpLong(void) {
" green=0xe0 and blue=0xd0\n");
printf(" -noalpha ............... discard any transparency information\n");
printf(" -lossless .............. encode image losslessly\n");
#ifdef WEBP_EXPERIMENTAL_FEATURES
printf(" -near_lossless ......... use near-lossless image\n"
printf(" -near_lossless <int> ... use near-lossless image\n"
" preprocessing (0=off..100)\n");
#endif
printf(" -hint <string> ......... specify image characteristics hint,\n");
printf(" one of: photo, picture or graph\n");

12
man/cwebp.1
View File

@ -218,12 +218,12 @@ Using this option will discard the alpha channel.
.B \-lossless
Encode the image without any loss.
.TP
.\" .B \-near_lossless " int
.\" Use near-lossless image preprocessing. This option adjusts pixel values
.\" to help compressibility, but has minimal impact on the visual quality.
.\" It triggers lossless compression mode automatically.
.\" Range is 0 (no preprocessing, the default) to 100.
.\" .TP
.BI \-near_lossless " int
Use near-lossless image preprocessing. This option adjusts pixel values
to help compressibility, but has minimal impact on the visual quality.
It triggers lossless compression mode automatically.
Range is 0 (no preprocessing, the default) to 100.
.TP
.BI \-hint " string
Specify the hint about input image type. Possible values are:
\fBphoto\fP, \fBpicture\fP or \fBgraph\fP.

10
src/enc/near_lossless.c
View File

@ -20,8 +20,6 @@
#include "../utils/utils.h"
#include "./vp8enci.h"
#ifdef WEBP_EXPERIMENTAL_FEATURES
#define MIN_DIM_FOR_NEAR_LOSSLESS 64
#define MAX_LIMIT_BITS 5
@ -131,15 +129,8 @@ static int QualityToLimitBits(int quality) {
// 13..100 -> 4..1
return MAX_LIMIT_BITS - (quality + 12) / 25;
}
#endif // WEBP_EXPERIMENTAL_FEATURES
int VP8ApplyNearLossless(int xsize, int ysize, uint32_t* argb, int quality) {
#ifndef WEBP_EXPERIMENTAL_FEATURES
(void)xsize;
(void)ysize;
(void)argb;
(void)quality;
#else
int i;
uint32_t* const copy_buffer =
(uint32_t*)WebPSafeMalloc(xsize * 3, sizeof(*copy_buffer));
@ -160,6 +151,5 @@ int VP8ApplyNearLossless(int xsize, int ysize, uint32_t* argb, int quality) {
NearLossless(xsize, ysize, argb, i, copy_buffer);
}
WebPSafeFree(copy_buffer);
#endif // WEBP_EXPERIMENTAL_FEATURES
return 1;
}