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vp8 denoiser: fix to zero_mv mode selection.

In the current logic, if the sse for zero motion is smaller
than the sse for new_mv (i.e., best_sse), we may still end up
using the non-zero mv for denoising (if the magnitude of new_mv is above threshold).
This can happen for very noisy content, and can lead to artifacts.

This change ensures that we always use zero_mv (over new_mv) for
denoisng if sse_zero_mv <= best_sse.

Change-Id: I8ef9294d837b077013b77a46c9a71d17c648b48a

--- a/vp8/encoder/denoising.c

+++ b/vp8/encoder/denoising.c

@@ -201,6 +201,7 @@

     int mv_col;

     unsigned int motion_magnitude2;

     unsigned int sse_thresh;

+    int sse_diff_thresh = 0;

     MV_REFERENCE_FRAME frame = x->best_reference_frame;

     MV_REFERENCE_FRAME zero_frame = x->best_zeromv_reference_frame;

@@ -225,11 +226,16 @@

         mbmi->need_to_clamp_mvs = x->need_to_clamp_best_mvs;

         mv_col = x->best_sse_mv.as_mv.col;

         mv_row = x->best_sse_mv.as_mv.row;

+        // Bias to zero_mv if small amount of motion.

+        // Note sse_diff_thresh is intialized to zero, so this ensures

+        // we will always choose zero_mv for denoising if

+        // zero_mv_see <= best_sse (i.e., sse_diff <= 0).

+        if ((unsigned int)(mv_row * mv_row + mv_col * mv_col)

+            <= NOISE_MOTION_THRESHOLD)

+            sse_diff_thresh = (int)SSE_DIFF_THRESHOLD;

         if (frame == INTRA_FRAME ||

-            ((unsigned int)(mv_row *mv_row + mv_col *mv_col)

-              <= NOISE_MOTION_THRESHOLD &&

-             sse_diff < (int)SSE_DIFF_THRESHOLD))

+            sse_diff <= sse_diff_thresh)

         {

             /*

              * Handle intra blocks as referring to last frame with zero motion