From fb0f3526ce46070bbf71f4c6bf745d6ef1f5e4c1 Mon Sep 17 00:00:00 2001
From: Jeff Muizelaar <jmuizelaar@mozilla.com>
Date: Tue, 2 Sep 2014 12:15:27 -0400
Subject: [PATCH] Bug 996108 - Use even rounding for better results when
 converting from scalar to fdot6 r=upstream

---
 gfx/skia/trunk/src/core/SkEdge.cpp | 27 +++++++++++++++++++++++++++
 gfx/skia/trunk/src/core/SkEdge.h   |  7 +++++++
 gfx/skia/trunk/src/core/SkFDot6.h  | 22 ++++++++++++++++++++++
 3 files changed, 56 insertions(+)

diff --git a/gfx/skia/trunk/src/core/SkEdge.cpp b/gfx/skia/trunk/src/core/SkEdge.cpp
index 9ce255843f66..dd5ab5fbc62f 100644
--- a/gfx/skia/trunk/src/core/SkEdge.cpp
+++ b/gfx/skia/trunk/src/core/SkEdge.cpp
@@ -36,11 +36,18 @@ int SkEdge::setLine(const SkPoint& p0, const SkPoint& p1, const SkIRect* clip,
     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 @@ int SkQuadraticEdge::setQuadratic(const SkPoint pts[3], int shift)
     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 @@ int SkQuadraticEdge::setQuadratic(const SkPoint pts[3], int shift)
         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 @@ int SkCubicEdge::setCubic(const SkPoint pts[4], const SkIRect* clip, int shift)
     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 @@ int SkCubicEdge::setCubic(const SkPoint pts[4], const SkIRect* clip, int shift)
         y2 = int(pts[2].fY * scale);
         x3 = int(pts[3].fX * scale);
         y3 = int(pts[3].fY * scale);
+#endif
     }
 
     int winding = 1;
diff --git a/gfx/skia/trunk/src/core/SkEdge.h b/gfx/skia/trunk/src/core/SkEdge.h
index 091223631fed..67a0ee787604 100644
--- a/gfx/skia/trunk/src/core/SkEdge.h
+++ b/gfx/skia/trunk/src/core/SkEdge.h
@@ -89,11 +89,18 @@ int SkEdge::setLine(const SkPoint& p0, const SkPoint& p1, int shift) {
     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;
diff --git a/gfx/skia/trunk/src/core/SkFDot6.h b/gfx/skia/trunk/src/core/SkFDot6.h
index 5a0ec57f5954..3da753da4131 100644
--- a/gfx/skia/trunk/src/core/SkFDot6.h
+++ b/gfx/skia/trunk/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)