factor out computation for dy in SkEdge lines, so we can later try a fix.

git-svn-id: http://skia.googlecode.com/svn/trunk@6198 2bbb7eff-a529-9590-31e7-b0007b416f81

src/core/SkEdge.cpp

@@ -72,8 +72,9 @@ int SkEdge::setLine(const SkPoint& p0, const SkPoint& p1, const SkIRect* clip,
     }
 
     SkFixed slope = SkFDot6Div(x1 - x0, y1 - y0);
+    const int dy  = SkEdge_Compute_DY(top, y0);
 
-    fX          = SkFDot6ToFixed(x0 + SkFixedMul(slope, (32 - y0) & 63));   // + SK_Fixed1/2
+    fX          = SkFDot6ToFixed(x0 + SkFixedMul(slope, dy));   // + SK_Fixed1/2
     fDX         = slope;
     fFirstY     = top;
     fLastY      = bot - 1;
@@ -112,8 +113,9 @@ int SkEdge::updateLine(SkFixed x0, SkFixed y0, SkFixed x1, SkFixed y1)
     x1 >>= 10;
 
     SkFixed slope = SkFDot6Div(x1 - x0, y1 - y0);
+    const int dy  = SkEdge_Compute_DY(top, y0);
 
-    fX          = SkFDot6ToFixed(x0 + SkFixedMul(slope, (32 - y0) & 63));   // + SK_Fixed1/2
+    fX          = SkFDot6ToFixed(x0 + SkFixedMul(slope, dy));   // + SK_Fixed1/2
     fDX         = slope;
     fFirstY     = top;
     fLastY      = bot - 1;

src/core/SkEdge.h

@@ -14,6 +14,15 @@
 #include "SkFDot6.h"
 #include "SkMath.h"
 
+//#ifdef SK_IGNORE_SETLINE_FIX
+#if 1
+    #define SkEdge_Compute_DY(top, y0)  ((32 - (y0)) & 63)
+#else
+    // This is correct, as it favors the lower-pixel when y0 is on a 1/2 pixel
+    // boundary, returning 64 instead of the old code, which returns 0.
+    #define SkEdge_Compute_DY(top, y0)  ((top << 6) + 32 - (y0))
+#endif
+
 struct SkEdge {
     enum Type {
         kLine_Type,
@@ -118,8 +127,9 @@ int SkEdge::setLine(const SkPoint& p0, const SkPoint& p1, int shift) {
     }
 
     SkFixed slope = SkFDot6Div(x1 - x0, y1 - y0);
+    const int dy  = SkEdge_Compute_DY(top, y0);
 
-    fX          = SkFDot6ToFixed(x0 + SkFixedMul(slope, (32 - y0) & 63));   // + SK_Fixed1/2
+    fX          = SkFDot6ToFixed(x0 + SkFixedMul(slope, dy));   // + SK_Fixed1/2
     fDX         = slope;
     fFirstY     = top;
     fLastY      = bot - 1;