Fix quickReject computation for blurs

src/effects/SkBlurMaskFilter.cpp

Index: src/effects/SkBlurMaskFilter.cpp
===================================================================
--- src/effects/SkBlurMaskFilter.cpp	(revision 9604)
+++ src/effects/SkBlurMaskFilter.cpp	(working copy)
@@ -15,6 +15,9 @@
 #include "SkRTConf.h"
 #include "SkStringUtils.h"
 
+const SkScalar SkBlurMaskFilter::kBLUR_SIGMA_SCALE = SkFloatToScalar(0.6f);
+const SkScalar SkBlurMaskFilter::kBlurRadiusFudgeFactor = SkFloatToScalar(.57735f);
+
 class SkBlurMaskFilterImpl : public SkMaskFilter {
 public:
     SkBlurMaskFilterImpl(SkScalar radius, SkBlurMaskFilter::BlurStyle,
@@ -300,8 +303,27 @@
 
 void SkBlurMaskFilterImpl::computeFastBounds(const SkRect& src,
                                              SkRect* dst) const {
-    dst->set(src.fLeft - fRadius, src.fTop - fRadius,
-             src.fRight + fRadius, src.fBottom + fRadius);
+    SkScalar gpuPad, rasterPad;
+
+    {
+        // GPU path
+        SkScalar sigma = SkScalarMul(fRadius, SkBlurMaskFilter::kBLUR_SIGMA_SCALE);
+        gpuPad = sigma * 3.0f;
+    }
+
+    {
+        // raster path
+        SkScalar radius = SkScalarMul(fRadius, SkBlurMaskFilter::kBlurRadiusFudgeFactor);
+ 
+        radius = (radius + .5f) * 2.f;
+
+        rasterPad = SkIntToScalar(SkScalarRoundToInt(radius * 3)/2);
+    }
+
+    SkScalar pad = SkMaxScalar(gpuPad, rasterPad);
+
+    dst->set(src.fLeft  - pad, src.fTop    - pad,
+             src.fRight + pad, src.fBottom + pad);
 }
 
 SkBlurMaskFilterImpl::SkBlurMaskFilterImpl(SkFlattenableReadBuffer& buffer)