[Reland] Repeating SkBitmapProcState rounding bias

src/core/SkBitmapProcState.h

Index: src/core/SkBitmapProcState.h
diff --git a/src/core/SkBitmapProcState.h b/src/core/SkBitmapProcState.h
index e6e7a3f393661ff729a2abecde11d62950b5e6a6..9c50eb4d3d15f48c3ef8f3cdc9c36a3f59928f08 100644
--- a/src/core/SkBitmapProcState.h
+++ b/src/core/SkBitmapProcState.h
@@ -25,17 +25,6 @@ typedef SkFixed3232    SkFractionalInt;
 #define SkFixedToFractionalInt(x)   SkFixedToFixed3232(x)
 #define SkFractionalIntToInt(x)     SkFixed3232ToInt(x)
 
-// Applying a fixed point (SkFixed, SkFractionalInt) epsilon bias ensures that the inverse-mapped
-// bitmap coordinates are rounded consistently WRT geometry.  Note that we only have to do this
-// when the scale is positive - for negative scales we're already rounding in the right direction.
-static inline int bitmap_sampler_inv_bias(SkScalar scale) {
-#ifndef SK_SUPPORT_LEGACY_BITMAP_SAMPLER_BIAS
-    return -(scale > 0);
-#else
-    return 0;
-#endif
-}
-
 class SkPaint;
 
 struct SkBitmapProcState {
@@ -212,4 +201,34 @@ void S32_D16_filter_DX(const SkBitmapProcState& s,
 void S32_D16_filter_DXDY(const SkBitmapProcState& s,
                          const uint32_t* xy, int count, uint16_t* colors);
 
+// Helper class for mapping the middle of pixel (x, y) into SkFractionalInt bitmap space.
+// TODO: filtered version which applies a fFilterOne{X,Y}/2 bias instead of epsilon?
+class SkBitmapProcStateAutoMapper {
+public:
+    SkBitmapProcStateAutoMapper(const SkBitmapProcState& s, int x, int y) {
+        SkPoint pt;
+        s.fInvProc(s.fInvMatrix,
+                   SkIntToScalar(x) + SK_ScalarHalf,
+                   SkIntToScalar(y) + SK_ScalarHalf, &pt);
+
+#ifndef SK_SUPPORT_LEGACY_BITMAP_SAMPLER_BIAS
+        // SkFixed epsilon bias to ensure inverse-mapped bitmap coordinates are rounded
+        // consistently WRT geometry.  Note that we only need the bias for positive scales:
+        // for negative scales, the rounding is intrinsically correct.
+        // We scale it to persist SkFractionalInt -> SkFixed conversions.
+        fX = SkScalarToFractionalInt(pt.x()) - SkFixedToFractionalInt(s.fInvMatrix.getScaleX() > 0);

[reed1, 2016/01/04 17:55:57]

Now that is a tricky expression: SkFixedToFractionalInt(s.fInvMatrix.getScaleX()
> 0)

I presume its the same as:

bias = 0;
if (s.fInvMatrix.getScaleX() > 0) {
    bias = 1 << 16;
}

[f(malita), 2016/01/04 18:04:27]

Right.  Maybe something like this would look more straightforward?

SkFixed biasX = (s.fInvMatrix.getScaleX() > 0);
fX = SkScalarToFractionalInt(pt.x()) - SkFixedToFractionalInt(biasX);

+        fY = SkScalarToFractionalInt(pt.y()) - SkFixedToFractionalInt(s.fInvMatrix.getScaleY() > 0);
+#else
+        fX = SkScalarToFractionalInt(pt.x());
+        fY = SkScalarToFractionalInt(pt.y());
+#endif
+    }
+
+    SkFractionalInt x() const { return fX; }
+    SkFractionalInt y() const { return fY; }
+
+private:
+    SkFractionalInt fX, fY;
+};
+
 #endif