SkBitmapProcState rounding bias

Epsilon bias to keep bitmap sample rounding consistent with geometry
rounding.

Also update the GM to draw an outer border + drop uninteresting
scales in favor of negative scale variants.

BUG=skia:4680,skia:4649
R=reed@google.com,caryclark@google.com,mtklein@google.com

Review URL: https://codereview.chromium.org/1527633002

3 files changed, 24 insertions, 6 deletions

diff --git a/src/core/SkBitmapProcState.cpp b/src/core/SkBitmapProcState.cpp
index 5ec6721436..487bd80402 100644
--- a/src/core/SkBitmapProcState.cpp
+++ b/src/core/SkBitmapProcState.cpp
@@ -823,10 +823,12 @@ void  Clamp_S32_opaque_D32_nofilter_DX_shaderproc(const SkBitmapProcState& s, in
         SkPoint pt;
         s.fInvProc(s.fInvMatrix, SkIntToScalar(x) + SK_ScalarHalf, SkIntToScalar(y) + SK_ScalarHalf,
                    &pt);
-        fx = SkScalarToFractionalInt(pt.fY);
+        fx = SkScalarToFractionalInt(pt.fY)
+            + bitmap_sampler_inv_bias(s.fInvMatrix.getScaleY());
         const unsigned maxY = s.fPixmap.height() - 1;
         dstY = SkClampMax(SkFractionalIntToInt(fx), maxY);
-        fx = SkScalarToFractionalInt(pt.fX);
+        fx = SkScalarToFractionalInt(pt.fX)
+            + bitmap_sampler_inv_bias(s.fInvMatrix.getScaleX());
     }
 
     const SkPMColor* SK_RESTRICT src = s.fPixmap.addr32(0, dstY);
diff --git a/src/core/SkBitmapProcState.h b/src/core/SkBitmapProcState.h
index 2ce76bc83e..e6e7a3f393 100644
--- a/src/core/SkBitmapProcState.h
+++ b/src/core/SkBitmapProcState.h
@@ -12,6 +12,7 @@
 #include "SkBitmapController.h"
 #include "SkBitmapFilter.h"
 #include "SkBitmapProvider.h"
+#include "SkFloatBits.h"
 #include "SkMatrix.h"
 #include "SkMipMap.h"
 #include "SkPaint.h"
@@ -24,6 +25,17 @@ 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 {
diff --git a/src/core/SkBitmapProcState_matrix.h b/src/core/SkBitmapProcState_matrix.h
index bdab846496..42a5ab6e22 100644
--- a/src/core/SkBitmapProcState_matrix.h
+++ b/src/core/SkBitmapProcState_matrix.h
@@ -64,12 +64,14 @@ void SCALE_FILTER_NAME(const SkBitmapProcState& s,
         SkPoint pt;
         s.fInvProc(s.fInvMatrix, SkIntToScalar(x) + SK_ScalarHalf,
                                   SkIntToScalar(y) + SK_ScalarHalf, &pt);
-        const SkFixed fy = SkScalarToFixed(pt.fY) - (s.fFilterOneY >> 1);
+        const SkFixed fy = SkScalarToFixed(pt.fY) - (s.fFilterOneY >> 1)
+            + bitmap_sampler_inv_bias(s.fInvMatrix.getScaleY());
         const unsigned maxY = s.fPixmap.height() - 1;
         // compute our two Y values up front
         *xy++ = PACK_FILTER_Y_NAME(fy, maxY, s.fFilterOneY PREAMBLE_ARG_Y);
         // now initialize fx
-        fx = SkScalarToFractionalInt(pt.fX) - (SkFixedToFractionalInt(one) >> 1);
+        fx = SkScalarToFractionalInt(pt.fX) - (SkFixedToFractionalInt(one) >> 1)
+            + bitmap_sampler_inv_bias(s.fInvMatrix.getScaleX());
     }
 
 #ifdef CHECK_FOR_DECAL
@@ -102,8 +104,10 @@ void AFFINE_FILTER_NAME(const SkBitmapProcState& s,
 
     SkFixed oneX = s.fFilterOneX;
     SkFixed oneY = s.fFilterOneY;
-    SkFixed fx = SkScalarToFixed(srcPt.fX) - (oneX >> 1);
-    SkFixed fy = SkScalarToFixed(srcPt.fY) - (oneY >> 1);
+    SkFixed fx = SkScalarToFixed(srcPt.fX) - (oneX >> 1)
+        + bitmap_sampler_inv_bias(s.fInvMatrix.getScaleX());
+    SkFixed fy = SkScalarToFixed(srcPt.fY) - (oneY >> 1)
+        + bitmap_sampler_inv_bias(s.fInvMatrix.getScaleY());
     SkFixed dx = s.fInvSx;
     SkFixed dy = s.fInvKy;
     unsigned maxX = s.fPixmap.width() - 1;