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vp9-svc: Fix in pickmode for key frames.

For the fixed/default SVC patterns, GOLDEN is the
spatial reference, except on key frames, where LAST
is labeled as the spatial reference.

The current code was assuming GOLDEN is always the
spatial reference for the purpose of selecting the
subpel motion (due to the downsampling filter).

Fix is make sure flag_svc_subpel is set and used
with spatial_ref, which is labeled as the proper
spatial reference before entering mode check.

Some quality improvement on key frames.
Change-Id: Id236bcd47055b035731cc910ed84449d7e29f50c

--- a/vp9/encoder/vp9_pickmode.c

+++ b/vp9/encoder/vp9_pickmode.c

@@ -1495,6 +1495,7 @@

 #endif

   INTERP_FILTER filter_gf_svc = EIGHTTAP;

   MV_REFERENCE_FRAME best_second_ref_frame = NONE;

+  MV_REFERENCE_FRAME spatial_ref = GOLDEN_FRAME;

   const struct segmentation *const seg = &cm->seg;

   int comp_modes = 0;

   int num_inter_modes = (cpi->use_svc) ? RT_INTER_MODES_SVC : RT_INTER_MODES;

@@ -1615,11 +1616,17 @@

       cpi->svc.spatial_layer_id > 0) {

     if (cpi->ref_frame_flags & flag_list[LAST_FRAME]) {

       struct scale_factors *const sf = &cm->frame_refs[LAST_FRAME - 1].sf;

-      if (vp9_is_scaled(sf)) svc_force_zero_mode[LAST_FRAME - 1] = 1;

+      if (vp9_is_scaled(sf)) {

+        svc_force_zero_mode[LAST_FRAME - 1] = 1;

+        spatial_ref = LAST_FRAME;

+      }

     }

     if (cpi->ref_frame_flags & flag_list[GOLDEN_FRAME]) {

       struct scale_factors *const sf = &cm->frame_refs[GOLDEN_FRAME - 1].sf;

-      if (vp9_is_scaled(sf)) svc_force_zero_mode[GOLDEN_FRAME - 1] = 1;

+      if (vp9_is_scaled(sf)) {

+        svc_force_zero_mode[GOLDEN_FRAME - 1] = 1;

+        spatial_ref = GOLDEN_FRAME;

+      }

     }

   }

@@ -1673,10 +1680,10 @@

   // Set the flag_svc_subpel to 1 for SVC if the lower spatial layer used

   // an averaging filter for downsampling (phase = 8). If so, we will test

-  // a nonzero motion mode on the spatial (goldeen) reference.

+  // a nonzero motion mode on the spatial reference.

   // The nonzero motion is half pixel shifted to left and top (-4, -4).

   if (cpi->use_svc && cpi->svc.spatial_layer_id > 0 &&

-      svc_force_zero_mode[GOLDEN_FRAME - 1] &&

+      svc_force_zero_mode[spatial_ref - 1] &&

       cpi->svc.downsample_filter_phase[cpi->svc.spatial_layer_id - 1] == 8) {

     svc_mv_col = -4;

     svc_mv_row = -4;

@@ -1726,7 +1733,7 @@

         get_segdata(seg, mi->segment_id, SEG_LVL_REF_FRAME) != (int)ref_frame)

       continue;

-    if (flag_svc_subpel && ref_frame == GOLDEN_FRAME) {

+    if (flag_svc_subpel && ref_frame == spatial_ref) {

       force_gf_mv = 1;

       // Only test mode if NEARESTMV/NEARMV is (svc_mv_col, svc_mv_row),

       // otherwise set NEWMV to (svc_mv_col, svc_mv_row).