Use even rounding for better results when converting from scalar to fdot6

src/core/SkFDot6.h

 
 /*
  * Copyright 2006 The Android Open Source Project
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 
 #ifndef SkFDot6_DEFINED
 #define SkFDot6_DEFINED
 
 #include "SkScalar.h"
 #include "SkMath.h"
 
 typedef int32_t SkFDot6;
 
+/* This uses the magic number approach suggested here:
+ * http://stereopsis.com/sree/fpu2006.html and used in
+ * _cairo_fixed_from_double. It does banker's rounding
+ * (i.e. round to nearest even)
+ */
+inline SkFDot6 SkScalarRoundToFDot6(SkScalar x, int shift = 0)
+{
+    union {
+        double  fDouble;
+        int32_t fBits[2];
+    } tmp;
+    int fractionalBits = 6 + shift;
+    double magic = (1LL << (52 - (fractionalBits))) * 1.5;
+
+    tmp.fDouble = SkScalarToDouble(x) + magic;
+#ifdef SK_CPU_BENDIAN
+    return tmp.fBits[1];
+#else
+    return tmp.fBits[0];
+#endif
+}
+
 #define SK_FDot6One         (64)
 #define SK_FDot6Half        (32)
 
 #ifdef SK_DEBUG
     inline SkFDot6 SkIntToFDot6(S16CPU x) {
         SkASSERT(SkToS16(x) == x);
         return x << 6;
     }
 #else
     #define SkIntToFDot6(x) ((x) << 6)
@@ skipping 18 matching lines @@
     SkASSERT(b != 0);
 
     if (a == (int16_t)a) {
         return (a << 16) / b;
     } else {
         return SkFixedDiv(a, b);
     }
 }
 
 #endif