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Correct the mode decisions in special cases

The rate costs calculated for inter modes are not precise in some
cases, which causes NEWMV is chosen instead of NEARESTMV, NEARMV,
and ZEROMV. This patch added checks for these cases, and corrected
the mode decisions.

Borg tests at speed 3 showed:
1. stdhd set: 0.102% PSNR gain and 0.088% SSIM gain.
2. derf set:  0.147% PSNR gain and 0.132% SSIM gain.
No speed change.

Change-Id: I35d17684b89ad4734fb610942d707899146426db

--- a/vp9/encoder/vp9_rdopt.c

+++ b/vp9/encoder/vp9_rdopt.c

@@ -2850,6 +2850,8 @@

     // them for this frame.

     mbmi->interp_filter = cm->interp_filter == SWITCHABLE ? EIGHTTAP

                                                           : cm->interp_filter;

+    mbmi->mv[0].as_int = mbmi->mv[1].as_int = 0;

+

     x->skip = 0;

     set_ref_ptrs(cm, xd, ref_frame, second_ref_frame);

@@ -3093,6 +3095,28 @@

     if (x->skip && !comp_pred)

       break;

+  }

+

+  // The inter modes' rate costs are not calculated precisely in some cases.

+  // Therefore, sometimes, NEWMV is chosen instead of NEARESTMV, NEARMV, and

+  // ZEROMV. Here, checks are added for those cases, and the mode decisions

+  // are corrected.

+  if (best_mbmode.mode == NEWMV) {

+    const MV_REFERENCE_FRAME refs[2] = {best_mbmode.ref_frame[0],

+        best_mbmode.ref_frame[1]};

+    int comp_pred_mode = refs[1] > INTRA_FRAME;

+

+    if (frame_mv[NEARESTMV][refs[0]].as_int == best_mbmode.mv[0].as_int &&

+        ((comp_pred_mode && frame_mv[NEARESTMV][refs[1]].as_int ==

+            best_mbmode.mv[1].as_int) || !comp_pred_mode))

+      best_mbmode.mode = NEARESTMV;

+    else if (frame_mv[NEARMV][refs[0]].as_int == best_mbmode.mv[0].as_int &&

+        ((comp_pred_mode && frame_mv[NEARMV][refs[1]].as_int ==

+            best_mbmode.mv[1].as_int) || !comp_pred_mode))

+      best_mbmode.mode = NEARMV;

+    else if (best_mbmode.mv[0].as_int == 0 &&

+        ((comp_pred_mode && best_mbmode.mv[1].as_int == 0) || !comp_pred_mode))

+      best_mbmode.mode = ZEROMV;

   }

   if (best_mode_index < 0 || best_rd >= best_rd_so_far)