commit 2f26528e59908856e36e88aa3be94d84014e9a58
author george <george@mozilla.com> Fri Aug 29 08:47:55 2014 -0700
committer Commit bot <commit-bot@chromium.org> Fri Aug 29 08:47:55 2014 -0700
tree a8d2ad9ccd9a38a26139481d1ff74c4d9bbc24c1
parent b75b2c05b999e26343e095eabff646705a0091ae

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

Originally from https://bugzilla.mozilla.org/show_bug.cgi?id=996108, patch by Jeff Muizelaar

R=reed@google.com, reed1
BUG=skia:

Author: george@mozilla.com

Review URL: https://codereview.chromium.org/270263005

src/core/SkEdge.cpp

diff --git a/src/core/SkEdge.cpp b/src/core/SkEdge.cpp
index 9ce2558..dd5ab5f 100644
--- a/src/core/SkEdge.cpp
+++ b/src/core/SkEdge.cpp
@@ -36,11 +36,18 @@
     SkFDot6 x0, y0, x1, y1;
 
     {
+#ifdef SK_RASTERIZE_EVEN_ROUNDING
+        x0 = SkScalarRoundToFDot6(p0.fX, shift);
+        y0 = SkScalarRoundToFDot6(p0.fY, shift);
+        x1 = SkScalarRoundToFDot6(p1.fX, shift);
+        y1 = SkScalarRoundToFDot6(p1.fY, shift);
+#else
         float scale = float(1 << (shift + 6));
         x0 = int(p0.fX * scale);
         y0 = int(p0.fY * scale);
         x1 = int(p1.fX * scale);
         y1 = int(p1.fY * scale);
+#endif
     }
 
     int winding = 1;
@@ -171,6 +178,14 @@
     SkFDot6 x0, y0, x1, y1, x2, y2;
 
     {
+#ifdef SK_RASTERIZE_EVEN_ROUNDING
+        x0 = SkScalarRoundToFDot6(pts[0].fX, shift);
+        y0 = SkScalarRoundToFDot6(pts[0].fY, shift);
+        x1 = SkScalarRoundToFDot6(pts[1].fX, shift);
+        y1 = SkScalarRoundToFDot6(pts[1].fY, shift);
+        x2 = SkScalarRoundToFDot6(pts[2].fX, shift);
+        y2 = SkScalarRoundToFDot6(pts[2].fY, shift);
+#else
         float scale = float(1 << (shift + 6));
         x0 = int(pts[0].fX * scale);
         y0 = int(pts[0].fY * scale);
@@ -178,6 +193,7 @@
         y1 = int(pts[1].fY * scale);
         x2 = int(pts[2].fX * scale);
         y2 = int(pts[2].fY * scale);
+#endif
     }
 
     int winding = 1;
@@ -321,6 +337,16 @@
     SkFDot6 x0, y0, x1, y1, x2, y2, x3, y3;
 
     {
+#ifdef SK_RASTERIZE_EVEN_ROUNDING
+        x0 = SkScalarRoundToFDot6(pts[0].fX, shift);
+        y0 = SkScalarRoundToFDot6(pts[0].fY, shift);
+        x1 = SkScalarRoundToFDot6(pts[1].fX, shift);
+        y1 = SkScalarRoundToFDot6(pts[1].fY, shift);
+        x2 = SkScalarRoundToFDot6(pts[2].fX, shift);
+        y2 = SkScalarRoundToFDot6(pts[2].fY, shift);
+        x3 = SkScalarRoundToFDot6(pts[3].fX, shift);
+        y3 = SkScalarRoundToFDot6(pts[3].fY, shift);
+#else
         float scale = float(1 << (shift + 6));
         x0 = int(pts[0].fX * scale);
         y0 = int(pts[0].fY * scale);
@@ -330,6 +356,7 @@
         y2 = int(pts[2].fY * scale);
         x3 = int(pts[3].fX * scale);
         y3 = int(pts[3].fY * scale);
+#endif
     }
 
     int winding = 1;

src/core/SkEdge.h

diff --git a/src/core/SkEdge.h b/src/core/SkEdge.h
index 0912236..67a0ee7 100644
--- a/src/core/SkEdge.h
+++ b/src/core/SkEdge.h
@@ -89,11 +89,18 @@
     SkFDot6 x0, y0, x1, y1;
 
     {
+#ifdef SK_RASTERIZE_EVEN_ROUNDING
+        x0 = SkScalarRoundToFDot6(p0.fX, shift);
+        y0 = SkScalarRoundToFDot6(p0.fY, shift);
+        x1 = SkScalarRoundToFDot6(p1.fX, shift);
+        y1 = SkScalarRoundToFDot6(p1.fY, shift);
+#else
         float scale = float(1 << (shift + 6));
         x0 = int(p0.fX * scale);
         y0 = int(p0.fY * scale);
         x1 = int(p1.fX * scale);
         y1 = int(p1.fY * scale);
+#endif
     }
 
     int winding = 1;

src/core/SkFDot6.h

diff --git a/src/core/SkFDot6.h b/src/core/SkFDot6.h
index 5a0ec57..3da753d 100644
--- a/src/core/SkFDot6.h
+++ b/src/core/SkFDot6.h
@@ -15,6 +15,28 @@
 
 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)