diff --git a/src/core/SkAntiRun.h b/src/core/SkAntiRun.h
index 8214e28d5b..5ac0e62a6c 100644
--- a/src/core/SkAntiRun.h
+++ b/src/core/SkAntiRun.h
@@ -21,6 +21,11 @@ public:
     int16_t*    fRuns;
     uint8_t*     fAlpha;
 
+    // Return 0-255 given 0-256
+    static inline SkAlpha CatchOverflow(int alpha) {
+        return alpha - (alpha >> 8);
+    }
+
     /// Returns true if the scanline contains only a single run,
     /// of alpha value 0.
     bool empty() const {
@@ -79,7 +84,7 @@ public:
             runs += x;
             x = 0;
             do {
-                alpha[0] = SkToU8(alpha[0] + maxValue);
+                alpha[0] = SkToU8(CatchOverflow(alpha[0] + maxValue));
                 int n = runs[0];
                 SkASSERT(n <= middleCount);
                 alpha += n;
diff --git a/src/core/SkScan_AAAPath.cpp b/src/core/SkScan_AAAPath.cpp
index 3325249833..91b6e397ae 100644
--- a/src/core/SkScan_AAAPath.cpp
+++ b/src/core/SkScan_AAAPath.cpp
@@ -83,9 +83,9 @@ number of scan lines in our algorithm is only about 3 + H while the
 
 ///////////////////////////////////////////////////////////////////////////////
 
-inline void addAlpha(SkAlpha& alpha, SkAlpha delta) {
-    SkASSERT(alpha + (int)delta <= 0xFF);
-    alpha += delta;
+static inline void addAlpha(SkAlpha& alpha, SkAlpha delta) {
+    SkASSERT(alpha + (int)delta <= 256);
+    alpha = SkAlphaRuns::CatchOverflow(alpha + (int)delta);
 }
 
 class AdditiveBlitter : public SkBlitter {
@@ -481,7 +481,7 @@ static inline SkAlpha partialTriangleToAlpha(SkFixed a, SkFixed b) {
 }
 
 static inline SkAlpha getPartialAlpha(SkAlpha alpha, SkFixed partialHeight) {
-    return (alpha * partialHeight) >> 16;
+    return (alpha * partialHeight + SK_FixedHalf) >> 16;
 }
 
 static inline SkAlpha getPartialAlpha(SkAlpha alpha, SkAlpha fullAlpha) {