YUV scale filter columns improved filtering accuracy

source/scale_common.cc

Index: source/scale_common.cc
diff --git a/source/scale_common.cc b/source/scale_common.cc
index d3992df2e6a9f5bd97fb34c72f65e50446f7ff1c..baed70b9da1f9c6d705feff6f04bdb4e1c93cc92 100644
--- a/source/scale_common.cc
+++ b/source/scale_common.cc
@@ -417,8 +417,16 @@ void ScaleColsUp2_16_C(uint16* dst_ptr, const uint16* src_ptr,
 }
 
 // (1-f)a + fb can be replaced with a + f(b-a)
+#if defined(__arm__)
+// arm uses 16 bit math with truncation.
+// TODO(fbarchard): add rounding.
 #define BLENDER(a, b, f) (uint8)((int)(a) + \
-    ((int)(f) * ((int)(b) - (int)(a)) >> 16))
+    (((int)((f)) * ((int)(b) - (int)(a))) >> 16))
+#else
+// inteluses 7 bit math with rounding.
+#define BLENDER(a, b, f) (uint8)((int)(a) + \
+    (((int)((f) >> 9) * ((int)(b) - (int)(a)) + 0x40) >> 7))
+#endif
 
 void ScaleFilterCols_C(uint8* dst_ptr, const uint8* src_ptr,
                        int dst_width, int x, int dx) {
@@ -470,8 +478,9 @@ void ScaleFilterCols64_C(uint8* dst_ptr, const uint8* src_ptr,
 }
 #undef BLENDER
 
+// Same as 8 bit arm blender but return is cast to uint16
 #define BLENDER(a, b, f) (uint16)((int)(a) + \
-    ((int)(f) * ((int)(b) - (int)(a)) >> 16))
+    (((int)((f)) * ((int)(b) - (int)(a))) >> 16))
 
 void ScaleFilterCols_16_C(uint16* dst_ptr, const uint16* src_ptr,
                        int dst_width, int x, int dx) {
@@ -809,6 +818,7 @@ void ScaleARGBColsUp2_C(uint8* dst_argb, const uint8* src_argb,
   }
 }
 
+// TODO(fbarchard): Replace 0x7f ^ f with 128-f.  bug=605.
 // Mimics SSSE3 blender
 #define BLENDER1(a, b, f) ((a) * (0x7f ^ f) + (b) * f) >> 7
 #define BLENDERC(a, b, f, s) (uint32)( \