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Skip the partition search for the frame with no motion

This patch allows the encoder to skip the partition search for the
frame if it is an inter frame and only zero motion vectors have
been detected in the first pass. The partition size is directly
assigned according to the difference variance.

Borg tests show overall little performance changes in term of PSNR
(derf -0.027%, yt 0.152%, hd 0.078%, stdhd 0%). The worst case of
PSNR loss is -0.514% from yt. The best PSNR gain is 4.293% from yt.
The second pass encoding speedup for slideshow clips is 15%-40%.

Change-Id: I881f347d286553ee5594a9ea09ba1a61ac684045

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

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

@@ -2309,7 +2309,8 @@

                                sf->always_this_block_size);

         rd_use_partition(cpi, tile, mi, tp, mi_row, mi_col, BLOCK_64X64,

                          &dummy_rate, &dummy_dist, 1, cpi->pc_root);

-      } else if (sf->partition_search_type == VAR_BASED_FIXED_PARTITION) {

+      } else if (cpi->skippable_frame ||

+                 sf->partition_search_type == VAR_BASED_FIXED_PARTITION) {

         BLOCK_SIZE bsize;

         set_offsets(cpi, tile, mi_row, mi_col, BLOCK_64X64);

         bsize = get_rd_var_based_fixed_partition(cpi, mi_row, mi_col);

--- a/vp9/encoder/vp9_encoder.c

+++ b/vp9/encoder/vp9_encoder.c

@@ -743,6 +743,8 @@

   cpi->alt_is_last = 0;

   cpi->gold_is_alt = 0;

+  cpi->skippable_frame = 0;

+

   // Create the encoder segmentation map and set all entries to 0

   CHECK_MEM_ERROR(cm, cpi->segmentation_map,

                   vpx_calloc(cm->mi_rows * cm->mi_cols, 1));

@@ -1972,6 +1974,29 @@

   }

 }

+static void configure_skippable_frame(VP9_COMP *cpi) {

+  // If the current frame does not have non-zero motion vector detected in the

+  // first  pass, and so do its previous and forward frames, then this frame

+  // can be skipped for partition check, and the partition size is assigned

+  // according to the variance

+

+  SVC *const svc = &cpi->svc;

+  const int is_spatial_svc = (svc->number_spatial_layers > 1) &&

+                             (svc->number_temporal_layers == 1);

+  TWO_PASS *const twopass = is_spatial_svc ?

+                            &svc->layer_context[svc->spatial_layer_id].twopass

+                            : &cpi->twopass;

+

+  cpi->skippable_frame = (!frame_is_intra_only(&cpi->common) &&

+    twopass->stats_in - 2 > twopass->stats_in_start &&

+    twopass->stats_in < twopass->stats_in_end &&

+    (twopass->stats_in - 1)->pcnt_inter - (twopass->stats_in - 1)->pcnt_motion

+    == 1 &&

+    (twopass->stats_in - 2)->pcnt_inter - (twopass->stats_in - 2)->pcnt_motion

+    == 1 &&

+    twopass->stats_in->pcnt_inter - twopass->stats_in->pcnt_motion == 1);

+}

+

 static void encode_frame_to_data_rate(VP9_COMP *cpi,

                                       size_t *size,

                                       uint8_t *dest,

@@ -2066,6 +2091,13 @@

   // and if the relevant speed feature flag is set.

   if (cpi->pass == 2 && cpi->sf.static_segmentation)

     configure_static_seg_features(cpi);

+

+  // Check if the current frame is skippable for the partition search in the

+  // second pass according to the first pass stats

+  if (cpi->pass == 2 &&

+      (!cpi->use_svc || cpi->svc.number_temporal_layers == 1)) {

+    configure_skippable_frame(cpi);

+  }

   // For 1 pass CBR, check if we are dropping this frame.

   // Never drop on key frame.

--- a/vp9/encoder/vp9_encoder.h

+++ b/vp9/encoder/vp9_encoder.h

@@ -266,6 +266,8 @@

   int alt_is_last;  // Alt same as last ( short circuit altref search)

   int gold_is_alt;  // don't do both alt and gold search ( just do gold).

+  int skippable_frame;

+

   int scaled_ref_idx[3];

   int lst_fb_idx;

   int gld_fb_idx;