Merge "Tune Lossless compression for lower qualities."

src/enc/alpha.c

@@ -80,7 +80,7 @@ static int EncodeLossless(const uint8_t* const data, int width, int height,
   config.lossless = 1;
   config.method = effort_level;  // impact is very small
   // Set a moderate default quality setting for alpha.
-  config.quality = 6.f * effort_level;
+  config.quality = 5.f * effort_level;
   assert(config.quality >= 0 && config.quality <= 100.f);
 
   ok = VP8LBitWriterInit(&tmp_bw, (width * height) >> 3);

src/enc/backward_references.c

@@ -555,10 +555,10 @@ static int BackwardReferencesHashChainDistanceOnly(
           }
           // 2) Add to the hash_chain (but cannot add the last pixel)
           {
-            const int last = (len < pix_count - 1 - i) ? len
-                                                       : pix_count - 1 - i;
-            for (k = 0; k < last; ++k) {
-              HashChainInsert(hash_chain, &argb[i + k], i + k);
+            const int last = (len + i < pix_count - 1) ? len + i
+                                                       : pix_count - 1;
+            for (k = i; k < last; ++k) {
+              HashChainInsert(hash_chain, &argb[k], k);
             }
           }
           // 3) jump.

src/enc/histogram.c

@@ -390,17 +390,12 @@ int VP8LGetHistoImageSymbols(int xsize, int ysize,
   // Heuristic params for HistogramCombine().
   const int num_tries_no_success = 8 + (quality >> 1);
   const int iter_mult = (quality < 27) ? 1 : 1 + ((quality - 27) >> 4);
-  int num_pairs = (quality >> 1);
+  const int num_pairs = (quality < 25) ? 10 : (5 * quality) >> 3;
 
   VP8LHistogramSet* const image_out =
       VP8LAllocateHistogramSet(histo_image_raw_size, cache_bits);
   if (image_out == NULL) return 0;
 
-  if (num_pairs > (histo_image_raw_size >> 2)) {
-    num_pairs = histo_image_raw_size >> 2;
-  }
-  num_pairs += 10;
-
   // Build histogram image.
   HistogramBuildImage(xsize, histo_bits, refs, image_out);
   // Collapse similar histograms.