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changed mode_context update strategy

Previously, the mode context is always udpated based on stats of current
frame, when there is no count, 50% is used for both left and right branch.
However, it is observed that with such strategy, a small count or no count
at all can skew the probability distribution significantly. This commmit
changed the mode_context update strategy to prevent small counts from
skewing the probability distributions.

Tests on derf set showed a small gain:  .06% in psnr and .09% in ssim

Change-Id: Ic812e64ae5f70251c170b0717f7b7fa587055488

--- a/vp8/common/entropymode.c

+++ b/vp8/common/entropymode.c

@@ -442,17 +442,13 @@

         {

             int this_prob;

             int count = mv_ref_ct[j][i][0] + mv_ref_ct[j][i][1];

-

-            if (count)

+            /* preventing rare occurances from skewing the probs */

+            if (count>=4)

+            {

                 this_prob = 256 * mv_ref_ct[j][i][0] / count;

-            else

-                this_prob = 128;

-            this_prob = this_prob? (this_prob<255?this_prob:255):1;

-            if (this_prob == 0)

-                this_prob = 1;

-            if (this_prob == 256)

-                this_prob = 255;

-            mode_context[j][i] = this_prob;

+                this_prob = this_prob? (this_prob<255?this_prob:255):1;

+                mode_context[j][i] = this_prob;

+            }

         }

     }

 }

-- 

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