Updated the near-lossless level mapping.

Updated the near-lossless level mapping and make it correlated to lossy
quality i.e 100 => minimum loss (in-fact no-loss) and the visual-quality loss
increases with decrease in near-lossless level (quality) till value 0.

The new mapping implies following (PSNR) loss-metric:
-near_lossless 100: No-loss (bit-stream same as -lossless).
-near_lossless  80: Very very high PSNR (around 54dB).
-near_lossless  60: Very high PSNR (around 48dB).
-near_lossless  40: High PSNR (around 42dB).
-near_lossless  20: Moderate PSNR (around 36dB).
-near_lossless   0: Low PSNR (around 30dB).

Change-Id: I930de4b18950faf2868c97d42e9e49ba0b642960

examples/cwebp.c

@@ -616,7 +616,7 @@ static void HelpLong(void) {
   printf("  -noalpha ............... discard any transparency information\n");
   printf("  -lossless .............. encode image losslessly\n");
   printf("  -near_lossless <int> ... use near-lossless image\n"
-         "                           preprocessing (0=off..100)\n");
+         "                           preprocessing (0..100=off)\n");
   printf("  -hint <string> ......... specify image characteristics hint,\n");
   printf("                           one of: photo, picture or graph\n");
 

man/cwebp.1

@@ -1,5 +1,5 @@
 .\"                                      Hey, EMACS: -*- nroff -*-
-.TH CWEBP 1 "July 30, 2014"
+.TH CWEBP 1 "Feb 4, 2015"
 .SH NAME
 cwebp \- compress an image file to a WebP file
 .SH SYNOPSIS
@@ -222,7 +222,7 @@ Encode the image without any loss.
 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.
+Range is 0 (maximum preprocessing) to 100 (no preprocessing, the default).
 .TP
 .BI \-hint " string
 Specify the hint about input image type. Possible values are:

src/enc/config.c

@@ -47,7 +47,7 @@ int WebPConfigInitInternal(WebPConfig* config,
   config->emulate_jpeg_size = 0;
   config->thread_level = 0;
   config->low_memory = 0;
-  config->near_lossless = 0;
+  config->near_lossless = 100;
 
   // TODO(skal): tune.
   switch (preset) {

src/enc/near_lossless.c

@@ -125,9 +125,14 @@ static void NearLossless(int xsize, int ysize, uint32_t* argb,
 }
 
 static int QualityToLimitBits(int quality) {
-  // quality mapping 0..12 -> 5
-  //                 13..100 -> 4..1
-  return MAX_LIMIT_BITS - (quality + 12) / 25;
+  // quality mapping:
+  //  0..19 -> 5
+  //  0..39 -> 4
+  //  0..59 -> 3
+  //  0..79 -> 2
+  //  0..99 -> 1
+  //  100   -> 0
+  return MAX_LIMIT_BITS - quality / 20;
 }
 
 int VP8ApplyNearLossless(int xsize, int ysize, uint32_t* argb, int quality) {

src/enc/vp8l.c

@@ -1254,7 +1254,7 @@ WebPEncodingError VP8LEncodeStream(const WebPConfig* const config,
   }
 
   // Apply near-lossless preprocessing.
-  use_near_lossless = !enc->use_palette_ && config->near_lossless;
+  use_near_lossless = !enc->use_palette_ && (config->near_lossless < 100);
   if (use_near_lossless) {
     if (!VP8ApplyNearLossless(width, height, picture->argb,
                               config->near_lossless)) {