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vp9_firstpass.c: limit mv_limits with MV_MAX in motion_search

Currently, in rare cases on big videos (> 5K), best_mv may differ from ref_mv by more than the allowable MV_MAX. Intersect mv_limits with those bound by MV_MAX before diamond search.

We could use vp9_set_mv_search_range, but that seems a bit more constrained than the bug I encountered (e.g., MAX_FULL_PEL_VAL < MV_MAX / 8).

Change-Id: I2c6563c05039d6ee05edf642665faaccf51787d4

--- a/vp9/encoder/vp9_firstpass.c

+++ b/vp9/encoder/vp9_firstpass.c

@@ -389,6 +389,29 @@

   return sr;

 }

+// Reduce limits to keep the motion search within MV_MAX of ref_mv. Not doing

+// this can be problematic for big videos (8K) and may cause assert failure

+// (or memory violation) in mv_cost. Limits are only modified if they would

+// be non-empty. Returns 1 if limits are non-empty.

+static int intersect_limits_with_mv_max(MvLimits *mv_limits, const MV *ref_mv) {

+  const int row_min =

+      VPXMAX(mv_limits->row_min, (ref_mv->row + 7 - MV_MAX) >> 3);

+  const int row_max =

+      VPXMIN(mv_limits->row_max, (ref_mv->row - 1 + MV_MAX) >> 3);

+  const int col_min =

+      VPXMAX(mv_limits->col_min, (ref_mv->col + 7 - MV_MAX) >> 3);

+  const int col_max =

+      VPXMIN(mv_limits->col_max, (ref_mv->col - 1 + MV_MAX) >> 3);

+  if (row_min > row_max || col_min > col_max) {

+    return 0;

+  }

+  mv_limits->row_min = row_min;

+  mv_limits->row_max = row_max;

+  mv_limits->col_min = col_min;

+  mv_limits->col_max = col_max;

+  return 1;

+}

+

 static void first_pass_motion_search(VP9_COMP *cpi, MACROBLOCK *x,

                                      const MV *ref_mv, MV *best_mv,

                                      int *best_motion_err) {

@@ -403,9 +426,14 @@

   int step_param = 3;

   int further_steps = (MAX_MVSEARCH_STEPS - 1) - step_param;

   const int sr = get_search_range(cpi);

+  const MvLimits tmp_mv_limits = x->mv_limits;

   step_param += sr;

   further_steps -= sr;

+  if (!intersect_limits_with_mv_max(&x->mv_limits, ref_mv)) {

+    return;

+  }

+

   // Override the default variance function to use MSE.

   v_fn_ptr.vf = get_block_variance_fn(bsize);

 #if CONFIG_VP9_HIGHBITDEPTH

@@ -451,6 +479,7 @@

       }

     }

   }

+  x->mv_limits = tmp_mv_limits;

 }

 static BLOCK_SIZE get_bsize(const VP9_COMMON *cm, int mb_row, int mb_col) {