vp9 denoiser: Bias to last reference for temporal filter.

Change-Id: I6a360a12e8da8cdcb8a779647512591612d64f31

vp9/encoder/vp9_context_tree.h

@@ -60,6 +60,7 @@ typedef struct {
 #if CONFIG_VP9_TEMPORAL_DENOISING
   unsigned int newmv_sse;
   unsigned int zeromv_sse;
+  unsigned int zeromv_lastref_sse;
   PREDICTION_MODE best_sse_inter_mode;
   int_mv best_sse_mv;
   MV_REFERENCE_FRAME best_reference_frame;

vp9/encoder/vp9_denoiser.c

@@ -224,14 +224,19 @@ static VP9_DENOISER_DECISION perform_motion_compensation(VP9_DENOISER *denoiser,
   } else {
     // Otherwise, use the zero reference frame.
     frame = ctx->best_zeromv_reference_frame;
-
-    mbmi->ref_frame[0] = ctx->best_zeromv_reference_frame;
+    ctx->newmv_sse = ctx->zeromv_sse;
+    // Bias to last reference.
+    if (frame != LAST_FRAME &&
+        ((ctx->zeromv_lastref_sse < (5 * ctx->zeromv_sse) >> 2) ||
+         denoiser->denoising_level >= kDenHigh)) {
+      frame = LAST_FRAME;
+      ctx->newmv_sse = ctx->zeromv_lastref_sse;
+    }
+    mbmi->ref_frame[0] = frame;
     mbmi->mode = ZEROMV;
     mbmi->mv[0].as_int = 0;
-
     ctx->best_sse_inter_mode = ZEROMV;
     ctx->best_sse_mv.as_int = 0;
-    ctx->newmv_sse = ctx->zeromv_sse;
   }
 
   if (ctx->newmv_sse > sse_thresh(bs, increase_denoising)) {
@@ -462,6 +467,8 @@ void vp9_denoiser_update_frame_stats(MB_MODE_INFO *mbmi, unsigned int sse,
   if (mbmi->mv[0].as_int == 0 && sse < ctx->zeromv_sse) {
     ctx->zeromv_sse = sse;
     ctx->best_zeromv_reference_frame = mbmi->ref_frame[0];
+    if (mbmi->ref_frame[0] == LAST_FRAME)
+      ctx->zeromv_lastref_sse = sse;
   }
 
   if (mbmi->mv[0].as_int != 0 && sse < ctx->newmv_sse) {