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vp9-svc: Fix to downsampling filter phase_shift.

Set phase_shift = 0 if the scale factors are
above 3/4. Removes artifact for scale factors
close to 1.

phase_shift = 8 is to get an averaging filter
(decimated pixel aligns to 8/16, midway between source pixels),
and only makes sense for scale factors multiples of
2 (1/2, 1/4,...).

Removes artifact for high scaling ratios.

Change-Id: Id0a85869d6c6156dda0032c697ded2de78fad6bd

--- a/vp9/encoder/vp9_svc_layercontext.c

+++ b/vp9/encoder/vp9_svc_layercontext.c

@@ -663,16 +663,17 @@

                        lc->scaling_factor_num, lc->scaling_factor_den, &width,

                        &height);

-  // For resolutions <= VGA: set phase of the filter = 8 (for symmetric

-  // averaging filter), use bilinear for now.

-  if (width * height <= 640 * 480) {

-    cpi->svc.downsample_filter_type[cpi->svc.spatial_layer_id] = BILINEAR;

-    // Use Eightap_smooth for low resolutions.

-    if (width * height <= 320 * 240)

-      cpi->svc.downsample_filter_type[cpi->svc.spatial_layer_id] =

-          EIGHTTAP_SMOOTH;

-    cpi->svc.downsample_filter_phase[cpi->svc.spatial_layer_id] = 8;

-  }

+  // Use Eightap_smooth for low resolutions.

+  if (width * height <= 320 * 240)

+    cpi->svc.downsample_filter_type[cpi->svc.spatial_layer_id] =

+        EIGHTTAP_SMOOTH;

+  // For scale factors > 0.75, set the phase to 0 (aligns decimated pixel

+  // to source pixel).

+  lc = &cpi->svc.layer_context[cpi->svc.spatial_layer_id *

+                                   cpi->svc.number_temporal_layers +

+                               cpi->svc.temporal_layer_id];

+  if (lc->scaling_factor_num > (3 * lc->scaling_factor_den) >> 2)

+    cpi->svc.downsample_filter_phase[cpi->svc.spatial_layer_id] = 0;

   // The usage of use_base_mv assumes down-scale of 2x2. For now, turn off use

   // of base motion vectors if spatial scale factors for any layers are not 2,