1 files changed, 17 insertions, 18 deletions

diff --git a/src/utils/SkShadowTessellator.cpp b/src/utils/SkShadowTessellator.cpp
index 3d84e8fdad..873e6d2687 100755
--- a/src/utils/SkShadowTessellator.cpp
+++ b/src/utils/SkShadowTessellator.cpp
@@ -92,7 +92,6 @@ protected:
     SkPoint             fCentroid;
     SkScalar            fArea;
     SkScalar            fLastArea;
-    int                 fAreaSignFlips;
     SkScalar            fLastCross;
 
     int                 fFirstVertexIndex;
@@ -137,18 +136,16 @@ static bool duplicate_pt(const SkPoint& p0, const SkPoint& p1) {
 
 static SkScalar perp_dot(const SkPoint& p0, const SkPoint& p1, const SkPoint& p2) {
     SkVector v0 = p1 - p0;
-    SkVector v1 = p2 - p0;
+    SkVector v1 = p2 - p1;
     return v0.cross(v1);
 }
 
-
 SkBaseShadowTessellator::SkBaseShadowTessellator(const SkPoint3& zPlaneParams, bool transparent)
         : fZPlaneParams(zPlaneParams)
         , fZOffset(0)
         , fCentroid({0, 0})
         , fArea(0)
         , fLastArea(0)
-        , fAreaSignFlips(0)
         , fLastCross(0)
         , fFirstVertexIndex(-1)
         , fSucceeded(false)
@@ -189,15 +186,20 @@ bool SkBaseShadowTessellator::accumulateCentroid(const SkPoint& curr, const SkPo
         return false;
     }
 
-    SkScalar quadArea = curr.cross(next);
-    fCentroid.fX += (curr.fX + next.fX) * quadArea;
-    fCentroid.fY += (curr.fY + next.fY) * quadArea;
+    SkASSERT(fPathPolygon.count() > 0);
+    SkVector v0 = curr - fPathPolygon[0];
+    SkVector v1 = next - fPathPolygon[0];
+    SkScalar quadArea = v0.cross(v1);
+    fCentroid.fX += (v0.fX + v1.fX) * quadArea;
+    fCentroid.fY += (v0.fY + v1.fY) * quadArea;
     fArea += quadArea;
     // convexity check
     if (quadArea*fLastArea < 0) {
-        ++fAreaSignFlips;
+        fIsConvex = false;
+    }
+    if (0 != quadArea) {
+        fLastArea = quadArea;
     }
-    fLastArea = quadArea;
 
     return true;
 }
@@ -215,7 +217,9 @@ bool SkBaseShadowTessellator::checkConvexity(const SkPoint& p0,
     if (fLastCross*cross < 0) {
         fIsConvex = false;
     }
-    fLastCross = cross;
+    if (0 != cross) {
+        fLastCross = cross;
+    }
 
     return true;
 }
@@ -229,6 +233,9 @@ void SkBaseShadowTessellator::finishPathPolygon() {
     }
 
     if (fPathPolygon.count() > 2) {
+        // do this before the final convexity check, so we use the correct fPathPolygon[0]
+        fCentroid *= sk_ieee_float_divide(1, 3 * fArea);
+        fCentroid += fPathPolygon[0];
         if (!checkConvexity(fPathPolygon[fPathPolygon.count() - 2],
                             fPathPolygon[fPathPolygon.count() - 1],
                             fPathPolygon[0])) {
@@ -238,14 +245,6 @@ void SkBaseShadowTessellator::finishPathPolygon() {
         }
     }
 
-    fCentroid *= sk_ieee_float_divide(1, 3 * fArea);
-    // It's possible to have a concave path that self-intersects but also passes the
-    // cross-product check (e.g., a star). In that case, the signed area will change signs more
-    // than twice, so we check for that here.
-    if (fAreaSignFlips > 2) {
-        fIsConvex = false;
-    }
-
     // if area is positive, winding is ccw
     fDirection = fArea > 0 ? -1 : 1;
 }