Enable Near-lossless feature.

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

examples/cwebp.c

@@ -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");
 

man/cwebp.1

@@ -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.

src/enc/near_lossless.c

@@ -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;
 }