Change gm_get_motion_vector

Since gm_get_motion_vector is trying to give a motion vector for
"the frame as a whole", it makes more sense for it to calculate
the motion of the *center* of the block rather than the top-left
corner of the frame.

In theory, this change should also help the encoder make better
decisions on when to use global motion. It avoids an issue
where, early in the frame, NEARESTMV looks like a good way to use
the global motion vector *without* paying the rate cost applied
to the first few global motion blocks in each frame. This seems
to lead to a better overall result.

Change-Id: Ia5c6259ceb8b4ff3d00a5d553e1d18bdb802da59

av1/decoder/decodemv.c

diff --git a/av1/decoder/decodemv.c b/av1/decoder/decodemv.c
index 39cb2cc..fc21bc4 100644
--- a/av1/decoder/decodemv.c
+++ b/av1/decoder/decodemv.c
@@ -1271,8 +1271,9 @@
                             PREDICTION_MODE mode,
                             MV_REFERENCE_FRAME ref_frame[2], int block,
                             int_mv mv[2], int_mv ref_mv[2],
-                            int_mv nearest_mv[2], int_mv near_mv[2],
-                            int is_compound, int allow_hp, aom_reader *r) {
+                            int_mv nearest_mv[2], int_mv near_mv[2], int mi_row,
+                            int mi_col, int is_compound, int allow_hp,
+                            aom_reader *r) {
   int i;
   int ret = 1;
 #if CONFIG_EC_ADAPT
@@ -1295,6 +1296,8 @@
 #endif  // CONFIG_REF_MV
   (void)ref_frame;
   (void)cm;
+  (void)mi_row;
+  (void)mi_col;
 
   switch (mode) {
 #if CONFIG_EXT_INTER
@@ -1350,11 +1353,15 @@
     case ZEROMV: {
 #if CONFIG_GLOBAL_MOTION
       mv[0].as_int = gm_get_motion_vector(&cm->global_motion[ref_frame[0]],
-                                          cm->allow_high_precision_mv)
+                                          cm->allow_high_precision_mv,
+                                          mi_col * MI_SIZE + MI_SIZE / 2,
+                                          mi_row * MI_SIZE + MI_SIZE / 2)
                          .as_int;
       if (is_compound)
         mv[1].as_int = gm_get_motion_vector(&cm->global_motion[ref_frame[1]],
-                                            cm->allow_high_precision_mv)
+                                            cm->allow_high_precision_mv,
+                                            mi_col * MI_SIZE + MI_SIZE / 2,
+                                            mi_row * MI_SIZE + MI_SIZE / 2)
                            .as_int;
 #else
       mv[0].as_int = 0;
@@ -1503,10 +1510,14 @@
       assert(is_compound);
 #if CONFIG_GLOBAL_MOTION
       mv[0].as_int = gm_get_motion_vector(&cm->global_motion[ref_frame[0]],
-                                          cm->allow_high_precision_mv)
+                                          cm->allow_high_precision_mv,
+                                          mi_col * MI_SIZE + MI_SIZE / 2,
+                                          mi_row * MI_SIZE + MI_SIZE / 2)
                          .as_int;
       mv[1].as_int = gm_get_motion_vector(&cm->global_motion[ref_frame[1]],
-                                          cm->allow_high_precision_mv)
+                                          cm->allow_high_precision_mv,
+                                          mi_col * MI_SIZE + MI_SIZE / 2,
+                                          mi_row * MI_SIZE + MI_SIZE / 2)
                          .as_int;
 #else
       mv[0].as_int = 0;
@@ -1613,13 +1624,18 @@
       av1_set_ref_frame(rf, ref_frame);
 #if CONFIG_GLOBAL_MOTION
       zeromv[0].as_int = gm_get_motion_vector(&cm->global_motion[rf[0]],
-                                              cm->allow_high_precision_mv)
+                                              cm->allow_high_precision_mv,
+                                              mi_col * MI_SIZE + MI_SIZE / 2,
+                                              mi_row * MI_SIZE + MI_SIZE / 2)
                              .as_int;
-      zeromv[1].as_int = (rf[1] != NONE_FRAME)
-                             ? gm_get_motion_vector(&cm->global_motion[rf[1]],
-                                                    cm->allow_high_precision_mv)
-                                   .as_int
-                             : 0;
+      zeromv[1].as_int =
+          (rf[1] != NONE_FRAME)
+              ? gm_get_motion_vector(&cm->global_motion[rf[1]],
+                                     cm->allow_high_precision_mv,
+                                     mi_col * MI_SIZE + MI_SIZE / 2,
+                                     mi_row * MI_SIZE + MI_SIZE / 2)
+                    .as_int
+              : 0;
 #else
       zeromv[0].as_int = zeromv[1].as_int = 0;
 #endif
@@ -1843,7 +1859,8 @@
 #else
                        ref_mv_s8,
 #endif  // CONFIG_EXT_INTER
-                       nearest_sub8x8, near_sub8x8, is_compound, allow_hp, r)) {
+                       nearest_sub8x8, near_sub8x8, mi_row, mi_col, is_compound,
+                       allow_hp, r)) {
           xd->corrupted |= 1;
           break;
         };
@@ -1892,7 +1909,7 @@
 #else
                    ref_mv,
 #endif  // CONFIG_EXT_INTER
-                   nearestmv, nearmv, is_compound, allow_hp, r);
+                   nearestmv, nearmv, mi_row, mi_col, is_compound, allow_hp, r);
   }
 
 #if CONFIG_EXT_INTER