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Improve motion detection for low complexity regions.

Where there is very subtle motion, especially when combined
with low spatial complexity, the codec sometimes fails to quickly
pick up the ambient motion field.

Once it has been established though the field propagates well using
Nearest and Near MV.

This patch looks specifically at the case where the Nearest and Near
have not been established as non zero vectors and in this case
discounts the cost of searching for a new vector in the rd code.

This will almost certainly have some implications in terms of encode
speed but it should be possible to mitigate the impact in a subsequent
using first pass stats and the local spatial complexity.

Average results for test sets approximately neutral.

Change-Id: I44a29e20f11f7ab10f8c93ffbdc50183d9801524

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

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

@@ -37,6 +37,7 @@

 #include "vp9/encoder/vp9_rd.h"

 #include "vp9/encoder/vp9_rdopt.h"

 #include "vp9/encoder/vp9_variance.h"

+#include "vp9/encoder/vp9_aq_variance.h"

 #define LAST_FRAME_MODE_MASK    ((1 << GOLDEN_FRAME) | (1 << ALTREF_FRAME) | \

                                  (1 << INTRA_FRAME))

@@ -48,6 +49,7 @@

 #define SECOND_REF_FRAME_MASK   ((1 << ALTREF_FRAME) | 0x01)

 #define MIN_EARLY_TERM_INDEX    3

+#define NEW_MV_DISCOUNT_FACTOR  8

 typedef struct {

   PREDICTION_MODE mode;

@@ -75,6 +77,7 @@

   const scan_order *so;

 };

+#define LAST_NEW_MV_INDEX 6

 static const MODE_DEFINITION vp9_mode_order[MAX_MODES] = {

   {NEARESTMV, {LAST_FRAME,   NONE}},

   {NEARESTMV, {ALTREF_FRAME, NONE}},

@@ -2355,6 +2358,27 @@

   }

 }

+// In some situations we want to discount tha pparent cost of a new motion

+// vector. Where there is a subtle motion field and especially where there is

+// low spatial complexity then it can be hard to cover the cost of a new motion

+// vector in a single block, even if that motion vector reduces distortion.

+// However, once established that vector may be usable through the nearest and

+// near mv modes to reduce distortion in subsequent blocks and also improve

+// visual quality.

+static int discount_newmv_test(const VP9_COMP *cpi,

+                               int this_mode,

+                               int_mv this_mv,

+                               int_mv (*mode_mv)[MAX_REF_FRAMES],

+                               int ref_frame) {

+  return (!cpi->rc.is_src_frame_alt_ref &&

+          (this_mode == NEWMV) &&

+          (this_mv.as_int != 0) &&

+          ((mode_mv[NEARESTMV][ref_frame].as_int == 0) ||

+           (mode_mv[NEARESTMV][ref_frame].as_int == INVALID_MV)) &&

+          ((mode_mv[NEARMV][ref_frame].as_int == 0) ||

+           (mode_mv[NEARMV][ref_frame].as_int == INVALID_MV)));

+}

+

 static int64_t handle_inter_mode(VP9_COMP *cpi, MACROBLOCK *x,

                                  BLOCK_SIZE bsize,

                                  int64_t txfm_cache[],

@@ -2464,10 +2488,20 @@

                            &tmp_mv, &rate_mv);

       if (tmp_mv.as_int == INVALID_MV)

         return INT64_MAX;

-      *rate2 += rate_mv;

+

       frame_mv[refs[0]].as_int =

           xd->mi[0].src_mi->bmi[0].as_mv[0].as_int = tmp_mv.as_int;

       single_newmv[refs[0]].as_int = tmp_mv.as_int;

+

+      // Estimate the rate implications of a new mv but discount this

+      // under certain circumstances where we want to help initiate a weak

+      // motion field, where the distortion gain for a single block may not

+      // be enough to overcome the cost of a new mv.

+      if (discount_newmv_test(cpi, this_mode, tmp_mv, mode_mv, refs[0])) {

+        *rate2 += MAX((rate_mv / NEW_MV_DISCOUNT_FACTOR), 1);

+      } else {

+        *rate2 += rate_mv;

+      }

     }

   }

@@ -2492,11 +2526,20 @@

     orig_dst_stride[i] = xd->plane[i].dst.stride;

   }

-  /* We don't include the cost of the second reference here, because there

-   * are only three options: Last/Golden, ARF/Last or Golden/ARF, or in other

-   * words if you present them in that order, the second one is always known

-   * if the first is known */

-  *rate2 += cost_mv_ref(cpi, this_mode, mbmi->mode_context[refs[0]]);

+  // We don't include the cost of the second reference here, because there

+  // are only three options: Last/Golden, ARF/Last or Golden/ARF, or in other

+  // words if you present them in that order, the second one is always known

+  // if the first is known.

+  //

+  // Under some circumstances we discount the cost of new mv mode to encourage

+  // initiation of a motion field.

+  if (discount_newmv_test(cpi, this_mode, frame_mv[refs[0]],

+                          mode_mv, refs[0])) {

+    *rate2 += MIN(cost_mv_ref(cpi, this_mode, mbmi->mode_context[refs[0]]),

+                  cost_mv_ref(cpi, NEARESTMV, mbmi->mode_context[refs[0]]));

+  } else {

+    *rate2 += cost_mv_ref(cpi, this_mode, mbmi->mode_context[refs[0]]);

+  }

   if (RDCOST(x->rdmult, x->rddiv, *rate2, 0) > ref_best_rd &&

       mbmi->mode != NEARESTMV)

@@ -2941,7 +2984,9 @@

   mode_skip_mask[INTRA_FRAME] |=

       ~(sf->intra_y_mode_mask[max_txsize_lookup[bsize]]);

-  for (i = 0; i < MAX_MODES; ++i)

+  for (i = 0; i <= LAST_NEW_MV_INDEX; ++i)

+    mode_threshold[i] = 0;

+  for (i = LAST_NEW_MV_INDEX + 1; i < MAX_MODES; ++i)

     mode_threshold[i] = ((int64_t)rd_threshes[i] * rd_thresh_freq_fact[i]) >> 5;

   midx =  sf->schedule_mode_search ? mode_skip_start : 0;