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Modify the setting of transform skip flags in non-rd mode

While searching for the best mode in non-rd case, SSE of
a partition block is calculated and the transform size is set.
This patch rewrites the skip checking conditions based on
transform size instead of partition size to be more precise.

Small gains were seen in rtc set borg test (speed 6).
AVG PSNR: 0.087%, overall PSNR: 0.073%, SSIM: 0.146%.
No noticeable speed change.

Change-Id: I5603ca5339c784dfa02263f4005988ccd8c32f6e

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

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

@@ -224,15 +224,6 @@

   *var_y = var;

   *sse_y = sse;

-  x->skip_txfm[0] = 0;

-  // Check if all ac coefficients can be quantized to zero.

-  if (var < ac_thr || var == 0) {

-    x->skip_txfm[0] = 2;

-    // Check if dc coefficient can be quantized to zero.

-    if (sse - var < dc_thr || sse == var)

-      x->skip_txfm[0] = 1;

-  }

-

   if (cpi->common.tx_mode == TX_MODE_SELECT) {

     if (sse > (var << 2))

       xd->mi[0].src_mi->mbmi.tx_size =

@@ -254,6 +245,32 @@

             tx_mode_to_biggest_tx_size[cpi->common.tx_mode]);

   }

+  // Evaluate if the partition block is a skippable block in Y plane.

+  {

+    const BLOCK_SIZE unit_size =

+        txsize_to_bsize[xd->mi[0].src_mi->mbmi.tx_size];

+    const unsigned int num_blk_log2 =

+        (b_width_log2_lookup[bsize] - b_width_log2_lookup[unit_size]) +

+        (b_height_log2_lookup[bsize] - b_height_log2_lookup[unit_size]);

+    const unsigned int sse_tx = sse >> num_blk_log2;

+    const unsigned int var_tx = var >> num_blk_log2;

+

+    x->skip_txfm[0] = 0;

+    // Check if all ac coefficients can be quantized to zero.

+    if (var_tx < ac_thr || var == 0) {

+      x->skip_txfm[0] = 2;

+      // Check if dc coefficient can be quantized to zero.

+      if (sse_tx - var_tx < dc_thr || sse == var)

+        x->skip_txfm[0] = 1;

+    }

+  }

+

+  if (x->skip_txfm[0] == 1) {

+    *out_rate_sum = 0;

+    *out_dist_sum = sse << 4;

+    return;

+  }

+

 #if CONFIG_VP9_HIGHBITDEPTH

   if (xd->cur_buf->flags & YV12_FLAG_HIGHBITDEPTH) {

     vp9_model_rd_from_var_lapndz(sse - var, num_pels_log2_lookup[bsize],

@@ -285,9 +302,6 @@

   *out_rate_sum += rate;

   *out_dist_sum += dist << 4;

-

-  if (*out_rate_sum == 0)

-    x->skip_txfm[0] = 1;

 }

 static void model_rd_for_sb_uv(VP9_COMP *cpi, BLOCK_SIZE bsize,

-- 

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