[Reland] Repeating SkBitmapProcState rounding bias

1) observe the bias in repeat matrix procs also.
2) add utility for device space -> bitmap space mapping.
3) remove unneeded filter bias

This is a reland of https://codereview.chromium.org/1529833003/. The
main difference (and the fix) vs. the prev version is increased
precision: the mapper now operates with SkFractionalInts.

R=reed@google.com
BUG=skia:4680, skia:4649
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1539083002

Review URL: https://codereview.chromium.org/1539083002

1 files changed, 30 insertions, 11 deletions

diff --git a/src/core/SkBitmapProcState.h b/src/core/SkBitmapProcState.h
index 4d7e7f32dc..24c6db6552 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 {
@@ -210,4 +199,34 @@ 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);
+        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