fix round_asymmetric_to_int to also bias bottom and right

BUG=skia:6294

Change-Id: I1ee1031f1fb7cb035916e8945ce5396759955885
Reviewed-on: https://skia-review.googlesource.com/9700
Reviewed-by: Cary Clark <caryclark@google.com>
Commit-Queue: Cary Clark <caryclark@google.com>

1 files changed, 24 insertions, 22 deletions

diff --git a/src/core/SkScan_Path.cpp b/src/core/SkScan_Path.cpp
index 11f9c403c8..0415b2f99a 100644
--- a/src/core/SkScan_Path.cpp
+++ b/src/core/SkScan_Path.cpp
@@ -562,28 +562,35 @@ static bool clip_to_limit(const SkRegion& orig, SkRegion* reduced) {
 }
 
 /**
-  * Variant of SkScalarRoundToInt, identical to SkDScalarRoundToInt except when the input fraction
-  * is 0.5. In this case only, round the value down. This is used to round the top and left
-  * of a rectangle, and corresponds to the way the scan converter treats the top and left edges.
+  * Variants of SkScalarRoundToInt, identical to SkDScalarRoundToInt except when the input fraction
+  * is 0.5. When SK_RASTERIZE_EVEN_ROUNDING is enabled, we must bias the result before rounding to
+  * account for potential FDot6 rounding edge-cases.
+  */
+#ifdef SK_RASTERIZE_EVEN_ROUNDING
+static const double kRoundBias = 0.5 / SK_FDot6One;
+#else
+static const double kRoundBias = 0.0;
+#endif
+
+/**
+  * Round the value down. This is used to round the top and left of a rectangle,
+  * and corresponds to the way the scan converter treats the top and left edges.
   */
 static inline int round_down_to_int(SkScalar x) {
     double xx = x;
-    xx -= 0.5;
+    xx -= 0.5 + kRoundBias;
     return (int)ceil(xx);
 }
 
-#ifdef SK_RASTERIZE_EVEN_ROUNDING
 /**
-  * Variant of SkDScalarRoundToInt that biases the input up by a half an FDot6 unit to account
-  * for potential FDot6 rounding in downstream FDot6 calculations.
+  * Round the value up. This is used to round the bottom and right of a rectangle,
+  * and corresponds to the way the scan converter treats the bottom and right edges.
   */
-static inline int round_biased_to_int(SkScalar x) {
-    const double bias = 0.5 / SK_FDot6One;
+static inline int round_up_to_int(SkScalar x) {
     double xx = x;
-    xx += 0.5 + bias;
+    xx += 0.5 + kRoundBias;
     return (int)floor(xx);
 }
-#endif
 
 /**
   *  Variant of SkRect::round() that explicitly performs the rounding step (i.e. floor(x + 0.5))
@@ -604,26 +611,21 @@ static inline int round_biased_to_int(SkScalar x) {
   *      SkASSERT(0 == iright);  // <--- succeeds
   *
   *
-  *  If using SK_RASTERIZE_EVEN_ROUNDING, we need to ensure that bottom and right account for
-  *  edges bounded by this rect being rounded to FDot6 format before being later rounded to an
-  *  integer. For example, a value like 0.499 can be below 0.5, but round to 0.5 as FDot6, which
-  *  would finally round to the integer 1, instead of just rounding to 0.
+  *  If using SK_RASTERIZE_EVEN_ROUNDING, we need to ensure we account for edges bounded by this
+  *  rect being rounded to FDot6 format before being later rounded to an integer. For example, a
+  *  value like 0.499 can be below 0.5, but round to 0.5 as FDot6, which would finally round to
+  *  the integer 1, instead of just rounding to 0.
   *
   *  To handle this, a small bias of half an FDot6 increment is added before actually rounding to
   *  an integer value. This simulates the rounding of SkScalarRoundToFDot6 without incurring the
   *  range loss of converting to FDot6 format first, preserving the integer range for the SkIRect.
   *  Thus, bottom and right are rounded in this manner (biased up), ensuring the rect is large
-  *  enough. Top and left can round as normal since they will round (biased down) to values less
-  *  or equal to the desired rect origin.
+  *  enough.
   */
 static void round_asymmetric_to_int(const SkRect& src, SkIRect* dst) {
     SkASSERT(dst);
     dst->set(round_down_to_int(src.fLeft), round_down_to_int(src.fTop),
-#ifdef SK_RASTERIZE_EVEN_ROUNDING
-             round_biased_to_int(src.fRight), round_biased_to_int(src.fBottom));
-#else
-             SkDScalarRoundToInt(src.fRight), SkDScalarRoundToInt(src.fBottom));
-#endif
+             round_up_to_int(src.fRight), round_up_to_int(src.fBottom));
 }
 
 void SkScan::FillPath(const SkPath& path, const SkRegion& origClip,