vp9-real-time mode: Fix condition for allowing reference masking.

Add frame-level condition for reference masking: under external or
internal dynamic resize, allow for reference masking if none of
the references have been scaled.
Peviously, reference masking was turned off for the stream if dynamic
resize feature was enabled or an external resize event occurred.

reference_masking gives speed up with little/no loss in compression.
For speed 7 on rtc set: encoding time decreases by about 5-7%,
avgPSNR/SSIM goes down ~0.2%.

Change-Id: Ie4444577451ef954414d8fb4b2c99d65cadf1746

vp9/encoder/vp9_speed_features.c

@@ -303,14 +303,26 @@ static void set_rt_speed_feature(VP9_COMP *cpi, SPEED_FEATURES *sf,
                                  FLAG_SKIP_INTRA_LOWVAR;
     sf->adaptive_pred_interp_filter = 2;
 
-    // Disable reference masking if using spatial scaling or for dynamic
-    // resizing (internal or external) since pred_mv_sad will not be set
-    // (since vp9_mv_pred will not be called).
-    // TODO(marpan): Fix this condition to allow reference masking for when
-    // all references have same resolution as source frame.
-    sf->reference_masking = (cpi->external_resize == 0 &&
-                             cpi->oxcf.resize_mode != RESIZE_DYNAMIC &&
-                             cpi->svc.number_spatial_layers == 1) ? 1 : 0;
+    // Reference masking only enabled for 1 spatial layer, and if none of the
+    // references have been scaled. The latter condition needs to be checked
+    // for external or internal dynamic resize.
+    sf->reference_masking = (cpi->svc.number_spatial_layers == 1);
+    if (sf->reference_masking == 1 &&
+        (cpi->external_resize == 1 ||
+         cpi->oxcf.resize_mode == RESIZE_DYNAMIC)) {
+      MV_REFERENCE_FRAME ref_frame;
+      static const int flag_list[4] =
+          {0, VP9_LAST_FLAG, VP9_GOLD_FLAG, VP9_ALT_FLAG};
+      for (ref_frame = LAST_FRAME; ref_frame <= ALTREF_FRAME; ++ref_frame) {
+        const YV12_BUFFER_CONFIG *yv12 = get_ref_frame_buffer(cpi, ref_frame);
+        if (yv12 != NULL && (cpi->ref_frame_flags & flag_list[ref_frame])) {
+          const struct scale_factors *const scale_fac =
+              &cm->frame_refs[ref_frame - 1].sf;
+          if (vp9_is_scaled(scale_fac))
+            sf->reference_masking = 0;
+        }
+      }
+    }
 
     sf->disable_filter_search_var_thresh = 50;
     sf->comp_inter_joint_search_thresh = BLOCK_SIZES;