vp9: Adjustment to scene-cut detection.

Change recursive weight for average_source_sad and
put some constraint on spacing between detected scene-cuts.

Change only affects 1 pass real-time mode.

Change-Id: I1917e748d845e244812d11aec2a9d755372ec182

vp9/encoder/vp9_encoder.c

@@ -1687,6 +1687,7 @@ VP9_COMP *vp9_create_compressor(VP9EncoderConfig *oxcf,
   cpi->common.buffer_pool = pool;
 
   cpi->rc.high_source_sad = 0;
+  cpi->rc.count_last_scene_change = 0;
 
   init_config(cpi, oxcf);
   vp9_rc_init(&cpi->oxcf, oxcf->pass, &cpi->rc);

vp9/encoder/vp9_ratectrl.c

@@ -2084,10 +2084,11 @@ void vp9_avg_source_sad(VP9_COMP *cpi) {
     else
       rc->high_source_sad = 0;
     if (avg_sad > 0 || cpi->oxcf.rc_mode == VPX_CBR)
-      rc->avg_source_sad = (rc->avg_source_sad + avg_sad) >> 1;
+      rc->avg_source_sad = (3 * rc->avg_source_sad + avg_sad) >> 2;
     // For VBR, under scene change/high content change, force golden refresh.
     if (cpi->oxcf.rc_mode == VPX_VBR &&
         rc->high_source_sad &&
+        rc->count_last_scene_change > 4 &&
         cpi->ext_refresh_frame_flags_pending == 0) {
       int target;
       cpi->refresh_golden_frame = 1;
@@ -2099,6 +2100,9 @@ void vp9_avg_source_sad(VP9_COMP *cpi) {
         rc->frames_till_gf_update_due = rc->frames_to_key;
       target = calc_pframe_target_size_one_pass_vbr(cpi);
       vp9_rc_set_frame_target(cpi, target);
+      rc->count_last_scene_change = 0;
+    } else {
+      rc->count_last_scene_change++;
     }
   }
 }

vp9/encoder/vp9_ratectrl.h

@@ -161,6 +161,7 @@ typedef struct {
 
   uint64_t avg_source_sad;
   int high_source_sad;
+  int count_last_scene_change;
 } RATE_CONTROL;
 
 struct VP9_COMP;