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vp9: Speed 8: More aggresive golden skip for low res.

Add a new, more aggresive short circuit: short_circuit_low_temp_var = 3 to skip
golden of any mode when variance is lower than threshold for low res.
This change only affects speed = 8, low resolution.

Metrics for avgPSNR/SSIM on rtc_derf (low resolution) show loss of
0.27/0.31%.
On Nexus 6, the encoding time is reduced by ~2.3% on average across all
low-res clips.

Visually little change on rtc_derf clips.

Change-Id: Ia8f7366fc2d49181a96733a380b4dbd7390246ec

--- a/vp9/encoder/vp9_encodeframe.c

+++ b/vp9/encoder/vp9_encodeframe.c

@@ -742,9 +742,12 @@

           continue;

         if ((*this_mi)->sb_type == BLOCK_32X32) {

-          if (vt->split[i].part_variances.none.variance < (thresholds[1] >> 1))

+          int64_t threshold_32x32 = cpi->sf.short_circuit_low_temp_var == 3

+                                        ? ((3 * thresholds[1]) >> 2)

+                                        : (thresholds[1] >> 1);

+          if (vt->split[i].part_variances.none.variance < threshold_32x32)

             x->variance_low[i + 5] = 1;

-        } else if (cpi->sf.short_circuit_low_temp_var == 2) {

+        } else if (cpi->sf.short_circuit_low_temp_var >= 2) {

           // For 32x16 and 16x32 blocks, the flag is set on each 16x16 block

           // inside.

           if ((*this_mi)->sb_type == BLOCK_16X16 ||

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

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

@@ -1490,6 +1490,10 @@

   if (cpi->sf.short_circuit_low_temp_var) {

     force_skip_low_temp_var =

         get_force_skip_low_temp_var(&x->variance_low[0], mi_row, mi_col, bsize);

+    // In the most aggresive short circuit, skip golden in any mode

+    if (cpi->sf.short_circuit_low_temp_var == 3 && force_skip_low_temp_var) {

+      usable_ref_frame = LAST_FRAME;

+    }

   }

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

@@ -1558,7 +1562,7 @@

       continue;

     }

-    if (cpi->sf.short_circuit_low_temp_var == 2 && force_skip_low_temp_var &&

+    if (cpi->sf.short_circuit_low_temp_var >= 2 && force_skip_low_temp_var &&

         ref_frame == LAST_FRAME && this_mode == NEWMV) {

       continue;

     }

--- a/vp9/encoder/vp9_speed_features.c

+++ b/vp9/encoder/vp9_speed_features.c

@@ -506,6 +506,10 @@

         content != VP9E_CONTENT_SCREEN) {

       // More aggressive short circuit for speed 8.

       sf->short_circuit_low_temp_var = 2;

+      // More aggressive short circuit for low resolution

+      if (cm->width <= 352 && cm->height <= 288) {

+        sf->short_circuit_low_temp_var = 3;

+      }

     }

     sf->limit_newmv_early_exit = 0;

   }

--- a/vp9/encoder/vp9_speed_features.h

+++ b/vp9/encoder/vp9_speed_features.h

@@ -457,6 +457,7 @@

   // 2: Skip golden non-zeromv and newmv-last for bsize >= 16x16, skip ALL

   // INTRA for bsize >= 32x32 and vert/horz INTRA for bsize 16x16, 16x32 and

   // 32x16.

+  // 3: Same as (2), also skip golden in any mode for low res

   int short_circuit_low_temp_var;

   // Limits the rd-threshold update for early exit for the newmv-last mode,