add rounding-using-doubles methods on SkScalar and SkRect

Inspired by the excellent repro case for https://crbug.com/364224

patch from issue 265933010

BUG=skia:
R=bungeman@google.com

Author: reed@google.com

Review URL: https://codereview.chromium.org/267003002

git-svn-id: http://skia.googlecode.com/svn/trunk/include@14566 2bbb7eff-a529-9590-31e7-b0007b416f81

2 files changed, 38 insertions, 0 deletions

diff --git a/core/SkRect.h b/core/SkRect.h
index 397e4a0..fd8cb16 100644
--- a/core/SkRect.h
+++ b/core/SkRect.h
@@ -732,6 +732,24 @@ struct SK_API SkRect {
     }
 
     /**
+     *  Variant of round() that explicitly performs the rounding step (i.e. floor(x + 0.5)) using
+     *  double instead of SkScalar (float). It does this by calling SkDScalarRoundToInt(), which
+     *  may be slower than calling SkScalarRountToInt(), but gives slightly more accurate results.
+     *
+     *  e.g.
+     *      SkScalar x = 0.49999997f;
+     *      int ix = SkScalarRoundToInt(x);
+     *      SkASSERT(0 == ix);  // <--- fails
+     *      ix = SkDScalarRoundToInt(x);
+     *      SkASSERT(0 == ix);  // <--- succeeds
+     */
+    void dround(SkIRect* dst) const {
+        SkASSERT(dst);
+        dst->set(SkDScalarRoundToInt(fLeft), SkDScalarRoundToInt(fTop),
+                 SkDScalarRoundToInt(fRight), SkDScalarRoundToInt(fBottom));
+    }
+
+    /**
      *  Set the dst rectangle by rounding "out" this rectangle, choosing the
      *  SkScalarFloor of top and left, and the SkScalarCeil of right and bottom.
      */
diff --git a/core/SkScalar.h b/core/SkScalar.h
index b9256ba..b37cf5c 100644
--- a/core/SkScalar.h
+++ b/core/SkScalar.h
@@ -83,6 +83,26 @@ static inline bool SkScalarIsFinite(float x) {
 #define SkScalarRoundToInt(x)       sk_float_round2int(x)
 #define SkScalarTruncToInt(x)       static_cast<int>(x)
 
+/**
+ *  Variant of SkScalarRoundToInt, that performs the rounding step (adding 0.5) explicitly using
+ *  double, to avoid possibly losing the low bit(s) of the answer before calling floor().
+ *
+ *  This routine will likely be slower than SkScalarRoundToInt(), and should only be used when the
+ *  extra precision is known to be valuable.
+ *
+ *  In particular, this catches the following case:
+ *      SkScalar x = 0.49999997;
+ *      int ix = SkScalarRoundToInt(x);
+ *      SkASSERT(0 == ix);    // <--- fails
+ *      ix = SkDScalarRoundToInt(x);
+ *      SkASSERT(0 == ix);    // <--- succeeds
+ */
+static inline int SkDScalarRoundToInt(SkScalar x) {
+    double xx = x;
+    xx += 0.5;
+    return (int)floor(xx);
+}
+
 /** Returns the absolute value of the specified SkScalar
 */
 #define SkScalarAbs(x)          sk_float_abs(x)