From 532215dd29dc8c5192cac4fc3c1d7769864205ff Mon Sep 17 00:00:00 2001
From: hui su <huisu@google.com>
Date: Wed, 14 Sep 2016 10:34:25 -0700
Subject: [PATCH] Change the rule of picking UV mode in MBAnalyzeBestUVMode()

Pick the mode with the smallest alpha.
It only affects m0, in which case the mode decision is not re-examined
later in VP8Decimate(). Tests on some natural content png images show
PSNR increase as well as visual quality improvement.

Change-Id: Iea997e718cd7477160fa05eb7cfb35f4cec2fa9a
(cherry picked from commit 1377ac2ec1cb81e4a74fa6294ff30a9e4cc584aa)
---
 src/enc/analysis.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/src/enc/analysis.c b/src/enc/analysis.c
index b55128fd..136c3312 100644
--- a/src/enc/analysis.c
+++ b/src/enc/analysis.c
@@ -307,6 +307,7 @@ static int MBAnalyzeBestIntra4Mode(VP8EncIterator* const it,
 
 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;
@@ -322,6 +323,10 @@ static int MBAnalyzeBestUVMode(VP8EncIterator* const it) {
     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;
     }
   }