Scale by 3/8 only if source is multiple of 8 tall.

source/scale.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 1393 matching lines @@
     int dy = FixedDiv(src_height, dst_height);
     // Arbitrary scale vertically, but unscaled horizontally.
     ScalePlaneVertical(src_height,
                        dst_width, dst_height,
                        src_stride, dst_stride, src, dst,
                        0, 0, dy, 1, filtering);
     return;
   }
   if (dst_width <= Abs(src_width) && dst_height <= src_height) {
     // Scale down.
-    if (4 * dst_width == 3 * src_width &&
-        4 * dst_height == 3 * src_height) {
+    if (4 * dst_width == 3 * src_width && 4 * dst_height == 3 * src_height) {
       // optimized, 3/4
       ScalePlaneDown34(src_width, src_height, dst_width, dst_height,
                        src_stride, dst_stride, src, dst, filtering);
       return;
     }
     if (2 * dst_width == src_width && 2 * dst_height == src_height) {
       // optimized, 1/2
       ScalePlaneDown2(src_width, src_height, dst_width, dst_height,
                       src_stride, dst_stride, src, dst, filtering);
       return;
     }
     // 3/8 rounded up for odd sized chroma height.
-    if (8 * dst_width == 3 * src_width &&
-        dst_height == ((src_height * 3 + 7) / 8)) {
+    if (8 * dst_width == 3 * src_width && 8 * dst_height == 3 * src_height) {
       // optimized, 3/8
       ScalePlaneDown38(src_width, src_height, dst_width, dst_height,
                        src_stride, dst_stride, src, dst, filtering);
       return;
     }
     if (4 * dst_width == src_width && 4 * dst_height == src_height &&
         (filtering == kFilterBox || filtering == kFilterNone)) {
       // optimized, 1/4
       ScalePlaneDown4(src_width, src_height, dst_width, dst_height,
                       src_stride, dst_stride, src, dst, filtering);
@@ skipping 61 matching lines @@
                           src_stride, dst_stride, src, dst, filtering);
       return;
     }
     if (2 * dst_width == src_width && 2 * dst_height == src_height) {
       // optimized, 1/2
       ScalePlaneDown2_16(src_width, src_height, dst_width, dst_height,
                          src_stride, dst_stride, src, dst, filtering);
       return;
     }
     // 3/8 rounded up for odd sized chroma height.
-    if (8 * dst_width == 3 * src_width &&
-        dst_height == ((src_height * 3 + 7) / 8)) {
+    if (8 * dst_width == 3 * src_width && 8 * dst_height == 3 * src_height) {
       // optimized, 3/8
       ScalePlaneDown38_16(src_width, src_height, dst_width, dst_height,
                           src_stride, dst_stride, src, dst, filtering);
       return;
     }
     if (4 * dst_width == src_width && 4 * dst_height == src_height &&
                filtering != kFilterBilinear) {
       // optimized, 1/4
       ScalePlaneDown4_16(src_width, src_height, dst_width, dst_height,
                          src_stride, dst_stride, src, dst, filtering);
@@ skipping 140 matching lines @@
                    dst_u, dst_halfwidth,
                    dst_v, dst_halfwidth,
                    dst_width, aheight,
                    interpolate ? kFilterBox : kFilterNone);
 }
 
 #ifdef __cplusplus
 }  // extern "C"
 }  // namespace libyuv
 #endif