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Improve the second-level sub-pixel motion search

Re-investigated the second-level sub-pixel motion search. Improved the
way of choosing search points. Rewrote the second-level search code.

At speed 0, the borg tests showed:
1. for stdhd set, Avg PSNR gain: 0.216%; Overall PSNR gain: 0.196%;
   SSIM gain: 0.206%. Only 1 out of 15 clips showed PSNR loss.
2. for derf set, Avg PSNR gain: 0.171%; Overall PSNR gain: 0.192%;
   SSIM gain: 0.207%. Only 3 out of 30 clips showed PSNR losses.
Added the condition for third-point checking, namely, less points
were checked. Speed tests showed no speed loss(Avg 0.3% speedup at
speed 0).

Change-Id: I6284ebb3fa7ba63be8528184c49e06757211a7f1

--- a/vp9/encoder/vp9_mcomp.c

+++ b/vp9/encoder/vp9_mcomp.c

@@ -256,6 +256,27 @@

     }                                                   \

   }

+// TODO(yunqingwang): SECOND_LEVEL_CHECKS_BEST was a rewrote of

+// SECOND_LEVEL_CHECKS, and SECOND_LEVEL_CHECKS should be rewritten

+// later in the same way.

+#define SECOND_LEVEL_CHECKS_BEST                        \

+  {                                                     \

+    unsigned int second;                                \

+    int br0 = br;                                       \

+    int bc0 = bc;                                       \

+    assert(tr == br || tc == bc);                       \

+    if (tr == br && tc != bc) {                         \

+      kc = bc - tc;                                     \

+    } else if (tr != br && tc == bc) {                  \

+      kr = br - tr;                                     \

+    }                                                   \

+    CHECK_BETTER(second, br0 + kr, bc0);                \

+    CHECK_BETTER(second, br0, bc0 + kc);                \

+    if (br0 != br || bc0 != bc) {                       \

+      CHECK_BETTER(second, br0 + kr, bc0 + kc);         \

+    }                                                   \

+  }

+

 #define SETUP_SUBPEL_SEARCH                                                \

   const uint8_t *const z = x->plane[0].src.buf;                            \

   const int src_stride = x->plane[0].src.stride;                           \

@@ -636,7 +657,6 @@

   const MACROBLOCKD *xd = &x->e_mbd;

   unsigned int besterr = INT_MAX;

   unsigned int sse;

-  unsigned int whichdir = 0;

   int thismse;

   const int y_stride = xd->plane[0].pre[0].stride;

   const int offset = bestmv->row * y_stride + bestmv->col;

@@ -657,6 +677,7 @@

   const MV *search_step = search_step_table;

   int idx, best_idx = -1;

   unsigned int cost_array[5];

+  int kr, kc;

   if (!(allow_hp && vp9_use_mv_hp(ref_mv)))

     if (round == 3)

@@ -703,8 +724,11 @@

     }

     // Check diagonal sub-pixel position

-    tc = bc + (cost_array[0] <= cost_array[1] ? -hstep : hstep);

-    tr = br + (cost_array[2] <= cost_array[3] ? -hstep : hstep);

+    kc = (cost_array[0] <= cost_array[1] ? -hstep : hstep);

+    kr = (cost_array[2] <= cost_array[3] ? -hstep : hstep);

+

+    tc = bc + kc;

+    tr = br + kr;

     if (tc >= minc && tc <= maxc && tr >= minr && tr <= maxr) {

       const uint8_t *const pre_address = y + (tr >> 3) * y_stride + (tc >> 3);

       MV this_mv = {tr, tc};

@@ -736,7 +760,7 @@

     }

     if (iters_per_step > 1 && best_idx != -1)

-      SECOND_LEVEL_CHECKS;

+      SECOND_LEVEL_CHECKS_BEST;

     tr = br;

     tc = bc;