Add rounding to InterpolateRow for improved quality and consistency.

source/row_common.cc

 /*
  *  Copyright 2011 The LibYuv Project Authors. All rights reserved.
  *
  *  Use of this source code is governed by a BSD-style license
  *  that can be found in the LICENSE file in the root of the source
  *  tree. An additional intellectual property rights grant can be found
  *  in the file PATENTS. All contributing project authors may
  *  be found in the AUTHORS file in the root of the source tree.
  */
 
@@ skipping 2193 matching lines @@
   int x;
   for (x = 0; x < width; ++x) {
     dst_uv[x] = (src_uv[x] + src_uv[src_uv_stride + x] + 1) >> 1;
   }
 }
 
 // C version 2x2 -> 2x1.
 void InterpolateRow_C(uint8* dst_ptr, const uint8* src_ptr,
                       ptrdiff_t src_stride,
                       int width, int source_y_fraction) {
-  int y1_fraction = source_y_fraction;
-  int y0_fraction = 256 - y1_fraction;
+  int y1_fraction = source_y_fraction >> 1;
+  int y0_fraction = 128 - y1_fraction;
   const uint8* src_ptr1 = src_ptr + src_stride;
   int x;
-  if (source_y_fraction == 0) {
+  if (y1_fraction == 0) {
     memcpy(dst_ptr, src_ptr, width);
     return;
   }
-  if (source_y_fraction == 128) {
+  if (y1_fraction == 64) {
     HalfRow_C(src_ptr, (int)(src_stride), dst_ptr, width);
     return;
   }
   for (x = 0; x < width - 1; x += 2) {
-    dst_ptr[0] = (src_ptr[0] * y0_fraction + src_ptr1[0] * y1_fraction) >> 8;
-    dst_ptr[1] = (src_ptr[1] * y0_fraction + src_ptr1[1] * y1_fraction) >> 8;
+    dst_ptr[0] = 
+                (src_ptr[0] * y0_fraction + src_ptr1[0] * y1_fraction + 64) >> 7;
+    dst_ptr[1] = 
+                (src_ptr[1] * y0_fraction + src_ptr1[1] * y1_fraction + 64) >> 7;
     src_ptr += 2;
     src_ptr1 += 2;
     dst_ptr += 2;
   }
   if (width & 1) {
-    dst_ptr[0] = (src_ptr[0] * y0_fraction + src_ptr1[0] * y1_fraction) >> 8;
+    dst_ptr[0] = 
+                (src_ptr[0] * y0_fraction + src_ptr1[0] * y1_fraction + 64) >> 7;
   }
 }
 
 void InterpolateRow_16_C(uint16* dst_ptr, const uint16* src_ptr,
                          ptrdiff_t src_stride,
                          int width, int source_y_fraction) {
   int y1_fraction = source_y_fraction;
   int y0_fraction = 256 - y1_fraction;
   const uint16* src_ptr1 = src_ptr + src_stride;
   int x;
@@ skipping 385 matching lines @@
     dst_rgb565 += twidth * 2;
     width -= twidth;
   }
 }
 #endif
 
 #ifdef __cplusplus
 }  // extern "C"
 }  // namespace libyuv
 #endif