libwebp-0.5.2-rc2

third_party/libwebp/enc/analysis.c

 // Copyright 2011 Google Inc. All Rights Reserved.
 //
 // Use of this source code is governed by a BSD-style license
 // that can be found in the COPYING file in the root of the source
 // tree. An additional intellectual property rights grant can be found
 // in the file PATENTS. All contributing project authors may
 // be found in the AUTHORS file in the root of the source tree.
 // -----------------------------------------------------------------------------
 //
 // Macroblock analysis
@@ skipping 289 matching lines @@
   i4_alpha = GetAlpha(&total_histo);
   if (IS_BETTER_ALPHA(i4_alpha, best_alpha)) {
     VP8SetIntra4Mode(it, modes);
     best_alpha = i4_alpha;
   }
   return best_alpha;
 }
 
 static int MBAnalyzeBestUVMode(VP8EncIterator* const it) {
   int best_alpha = DEFAULT_ALPHA;
+  int smallest_alpha = 0;
   int best_mode = 0;
   const int max_mode = MAX_UV_MODE;
   int mode;
 
   VP8MakeChroma8Preds(it);
   for (mode = 0; mode < max_mode; ++mode) {
     VP8Histogram histo;
     int alpha;
     InitHistogram(&histo);
     VP8CollectHistogram(it->yuv_in_ + U_OFF_ENC,
                         it->yuv_p_ + VP8UVModeOffsets[mode],
                         16, 16 + 4 + 4, &histo);
     alpha = GetAlpha(&histo);
     if (IS_BETTER_ALPHA(alpha, best_alpha)) {
       best_alpha = alpha;
+    }
+    // The best prediction mode tends to be the one with the smallest alpha.
+    if (mode == 0 || alpha < smallest_alpha) {
+      smallest_alpha = alpha;
       best_mode = mode;
     }
   }
   VP8SetIntraUVMode(it, best_mode);
   return best_alpha;
 }
 
 static void MBAnalyze(VP8EncIterator* const it,
                       int alphas[MAX_ALPHA + 1],
                       int* const alpha, int* const uv_alpha) {
@@ skipping 157 matching lines @@
       enc->alpha_ = main_job.alpha / total_mb;
       enc->uv_alpha_ = main_job.uv_alpha / total_mb;
       AssignSegments(enc, main_job.alphas);
     }
   } else {   // Use only one default segment.
     ResetAllMBInfo(enc);
   }
   return ok;
 }