Better method for rendering ellipses

src/gpu/GrOvalRenderer.cpp

Index: src/gpu/GrOvalRenderer.cpp
===================================================================
--- src/gpu/GrOvalRenderer.cpp	(revision 9127)
+++ src/gpu/GrOvalRenderer.cpp	(working copy)
@@ -32,14 +32,6 @@
 
 struct EllipseVertex {
     GrPoint  fPos;
-    SkScalar fOuterXRadius;
-    SkScalar fInnerXRadius;
-    GrPoint  fOuterOffset;
-    GrPoint  fInnerOffset;
-};
-
-struct RRectVertex {
-    GrPoint  fPos;
     GrPoint  fOffset;
     GrPoint  fOuterRadii;
     GrPoint  fInnerRadii;
@@ -163,7 +155,10 @@
 
 /**
  * The output of this effect is a modulation of the input color and coverage for an axis-aligned
- * ellipse, specified as  outer and inner radii, and outer and inner offsets from center.
+ * ellipse, specified as a 2D offset from center, and the reciprocals of the outer and inner radii, 
+ * in both x and y directions. 
+ * 
+ * We are using an implicit function of x^2/a^2 + y^2/b^2 - 1 = 0.
  */
 
 class EllipseEdgeEffect : public GrEffect {
@@ -209,32 +204,33 @@
                               const TextureSamplerArray& samplers) SK_OVERRIDE {
             const EllipseEdgeEffect& ellipseEffect = drawEffect.castEffect<EllipseEdgeEffect>();
 
+            const char *vsOffsetName, *fsOffsetName;
             const char *vsRadiiName, *fsRadiiName;
-            const char *vsOffsetsName, *fsOffsetsName;
 
-            builder->addVarying(kVec2f_GrSLType, "EllipseRadii", &vsRadiiName, &fsRadiiName);
+            builder->addVarying(kVec2f_GrSLType, "EllipseOffsets", &vsOffsetName, &fsOffsetName);
             const SkString* attr0Name =
                 builder->getEffectAttributeName(drawEffect.getVertexAttribIndices()[0]);
-            builder->vsCodeAppendf("\t%s = %s;\n", vsRadiiName, attr0Name->c_str());
+            builder->vsCodeAppendf("\t%s = %s;\n", vsOffsetName, attr0Name->c_str());
 
-            builder->addVarying(kVec4f_GrSLType, "EllipseOffsets", &vsOffsetsName, &fsOffsetsName);
+            builder->addVarying(kVec4f_GrSLType, "EllipseRadii", &vsRadiiName, &fsRadiiName);
             const SkString* attr1Name =
                 builder->getEffectAttributeName(drawEffect.getVertexAttribIndices()[1]);
-            builder->vsCodeAppendf("\t%s = %s;\n", vsOffsetsName, attr1Name->c_str());
+            builder->vsCodeAppendf("\t%s = %s;\n", vsRadiiName, attr1Name->c_str());
 
-            // get length of offset
-            builder->fsCodeAppendf("\tfloat dOuter = length(%s.xy);\n", fsOffsetsName);
-            // compare outer lengths against xOuterRadius
-            builder->fsCodeAppendf("\tfloat edgeAlpha = clamp(%s.x-dOuter, 0.0, 1.0);\n",
-                                   fsRadiiName);
+            // for outer curve 
+            builder->fsCodeAppendf("\tvec2 scaledOffset = %s*%s.xy;\n", fsOffsetName, fsRadiiName);
+            builder->fsCodeAppend("\tfloat test = dot(scaledOffset, scaledOffset) - 1.0;\n");
+            builder->fsCodeAppendf("\tvec2 grad = 2.0*scaledOffset*%s.xy;\n", fsRadiiName);
+            builder->fsCodeAppend("\tfloat invlen = inversesqrt(dot(grad, grad));\n");
+            builder->fsCodeAppend("\tfloat edgeAlpha = clamp(0.5-test*invlen, 0.0, 1.0);\n");
 
+            // for inner curve
             if (ellipseEffect.isStroked()) {
-                builder->fsCodeAppendf("\tfloat dInner = length(%s.zw);\n", fsOffsetsName);
-
-                // compare inner lengths against xInnerRadius
-                builder->fsCodeAppendf("\tfloat innerAlpha = clamp(dInner-%s.y, 0.0, 1.0);\n",
-                                       fsRadiiName);
-                builder->fsCodeAppend("\tedgeAlpha *= innerAlpha;\n");
+                builder->fsCodeAppendf("\tscaledOffset = %s*%s.zw;\n", fsOffsetName, fsRadiiName);
+                builder->fsCodeAppend("\ttest = dot(scaledOffset, scaledOffset) - 1.0;\n");
+                builder->fsCodeAppendf("\tgrad = 2.0*scaledOffset*%s.zw;\n", fsRadiiName);
+                builder->fsCodeAppend("\tinvlen = inversesqrt(dot(grad, grad));\n");
+                builder->fsCodeAppend("\tedgeAlpha *= clamp(0.5+test*invlen, 0.0, 1.0);\n");
             }
 
             SkString modulate;
@@ -285,141 +281,6 @@
 
 ///////////////////////////////////////////////////////////////////////////////
 
-/**
- * The output of this effect is a modulation of the input color and coverage for an axis-aligned
- * ellipse, specified as an offset vector from center and reciprocals of outer and inner radii in
- * both x and y directions.
- *
- * This uses a slightly different algorithm than the EllipseEdgeEffect, above. Rather than
- * scaling an ellipse to be a circle, it attempts to find the distance from the offset point to the
- * ellipse by determining where the line through the origin and offset point would cross the
- * ellipse, and computing the distance to that. This is slower but works better for roundrects
- * because the straight edges will be more accurate.
- */
-
-class AltEllipseEdgeEffect : public GrEffect {
-public:
-    static GrEffectRef* Create(bool stroke) {
-        // we go through this so we only have one copy of each effect (stroked/filled)
-        GR_CREATE_STATIC_EFFECT(gAltEllipseStrokeEdge, AltEllipseEdgeEffect, (true));
-        GR_CREATE_STATIC_EFFECT(gAltEllipseFillEdge, AltEllipseEdgeEffect, (false));
-
-        if (stroke) {
-            gAltEllipseStrokeEdge->ref();
-            return gAltEllipseStrokeEdge;
-        } else {
-            gAltEllipseFillEdge->ref();
-            return gAltEllipseFillEdge;
-        }
-    }
-
-    virtual void getConstantColorComponents(GrColor* color,
-                                            uint32_t* validFlags) const SK_OVERRIDE {
-        *validFlags = 0;
-    }
-
-    virtual const GrBackendEffectFactory& getFactory() const SK_OVERRIDE {
-        return GrTBackendEffectFactory<AltEllipseEdgeEffect>::getInstance();
-    }
-
-    virtual ~AltEllipseEdgeEffect() {}
-
-    static const char* Name() { return "RRectEdge"; }
-
-    inline bool isStroked() const { return fStroke; }
-
-    class GLEffect : public GrGLEffect {
-    public:
-        GLEffect(const GrBackendEffectFactory& factory, const GrDrawEffect&)
-            : INHERITED (factory) {}
-
-        virtual void emitCode(GrGLShaderBuilder* builder,
-                              const GrDrawEffect& drawEffect,
-                              EffectKey key,
-                              const char* outputColor,
-                              const char* inputColor,
-                              const TextureSamplerArray& samplers) SK_OVERRIDE {
-            const AltEllipseEdgeEffect& rrectEffect = drawEffect.castEffect<AltEllipseEdgeEffect>();
-
-            const char *vsOffsetName, *fsOffsetName;
-            const char *vsRadiiName, *fsRadiiName;
-
-            builder->addVarying(kVec2f_GrSLType, "EllipseOffsets", &vsOffsetName, &fsOffsetName);
-            const SkString* attr0Name =
-                builder->getEffectAttributeName(drawEffect.getVertexAttribIndices()[0]);
-            builder->vsCodeAppendf("\t%s = %s;\n", vsOffsetName, attr0Name->c_str());
-
-            builder->addVarying(kVec4f_GrSLType, "EllipseRadii", &vsRadiiName, &fsRadiiName);
-            const SkString* attr1Name =
-                builder->getEffectAttributeName(drawEffect.getVertexAttribIndices()[1]);
-            builder->vsCodeAppendf("\t%s = %s;\n", vsRadiiName, attr1Name->c_str());
-
-            builder->fsCodeAppend("\tfloat edgeAlpha;\n");
-            // get length of offset
-            builder->fsCodeAppendf("\tfloat len = length(%s.xy);\n", fsOffsetName);
-
-            // for outer curve
-            builder->fsCodeAppendf("\tvec2 offset = %s.xy*%s.xy;\n",
-                                   fsOffsetName, fsRadiiName);
-            builder->fsCodeAppendf("\tfloat t = inversesqrt(dot(offset.xy, offset.xy));\n");
-            builder->fsCodeAppend("\tedgeAlpha = clamp(len*t - len, 0.0, 1.0);\n");
-
-            // for inner curve
-            if (rrectEffect.isStroked()) {
-                builder->fsCodeAppendf("\toffset = %s.xy*%s.zw;\n",
-                                       fsOffsetName, fsRadiiName);
-                builder->fsCodeAppendf("\tt = inversesqrt(dot(offset.xy, offset.xy));\n");
-                builder->fsCodeAppend("\tedgeAlpha *= clamp(len - len*t, 0.0, 1.0);\n");
-            }
-
-            SkString modulate;
-            GrGLSLModulatef<4>(&modulate, inputColor, "edgeAlpha");
-            builder->fsCodeAppendf("\t%s = %s;\n", outputColor, modulate.c_str());
-        }
-
-        static inline EffectKey GenKey(const GrDrawEffect& drawEffect, const GrGLCaps&) {
-            const AltEllipseEdgeEffect& rrectEffect = drawEffect.castEffect<AltEllipseEdgeEffect>();
-
-            return rrectEffect.isStroked() ? 0x1 : 0x0;
-        }
-
-        virtual void setData(const GrGLUniformManager&, const GrDrawEffect&) SK_OVERRIDE {
-        }
-
-    private:
-        typedef GrGLEffect INHERITED;
-    };
-
-private:
-    AltEllipseEdgeEffect(bool stroke) : GrEffect() {
-        this->addVertexAttrib(kVec2f_GrSLType);
-        this->addVertexAttrib(kVec4f_GrSLType);
-        fStroke = stroke;
-    }
-
-    virtual bool onIsEqual(const GrEffect& other) const SK_OVERRIDE {
-        const AltEllipseEdgeEffect& aeee = CastEffect<AltEllipseEdgeEffect>(other);
-        return aeee.fStroke == fStroke;
-    }
-
-    bool fStroke;
-
-    GR_DECLARE_EFFECT_TEST;
-
-    typedef GrEffect INHERITED;
-};
-
-GR_DEFINE_EFFECT_TEST(AltEllipseEdgeEffect);
-
-GrEffectRef* AltEllipseEdgeEffect::TestCreate(SkMWCRandom* random,
-                                           GrContext* context,
-                                           const GrDrawTargetCaps&,
-                                           GrTexture* textures[]) {
-    return AltEllipseEdgeEffect::Create(random->nextBool());
-}
-
-///////////////////////////////////////////////////////////////////////////////
-
 bool GrOvalRenderer::drawOval(GrDrawTarget* target, const GrContext* context, bool useAA,
                               const GrRect& oval, const SkStrokeRec& stroke)
 {
@@ -589,9 +450,6 @@
                                    vm[SkMatrix::kMSkewY]*ellipseYRadius);
     SkScalar yRadius = SkScalarAbs(vm[SkMatrix::kMSkewX]*ellipseXRadius +
                                    vm[SkMatrix::kMScaleY]*ellipseYRadius);
-    if (SkScalarDiv(xRadius, yRadius) > 2 || SkScalarDiv(yRadius, xRadius) > 2) {
-        return false;
-    }
 
     // do (potentially) anisotropic mapping of stroke
     SkVector scaledStroke;
@@ -599,6 +457,39 @@
     scaledStroke.fX = SkScalarAbs(strokeWidth*(vm[SkMatrix::kMScaleX] + vm[SkMatrix::kMSkewY]));
     scaledStroke.fY = SkScalarAbs(strokeWidth*(vm[SkMatrix::kMSkewX] + vm[SkMatrix::kMScaleY]));
 
+    SkStrokeRec::Style style = stroke.getStyle();
+    bool isStroked = (SkStrokeRec::kStroke_Style == style || SkStrokeRec::kHairline_Style == style);
+
+    SkScalar innerXRadius = 0.0f;
+    SkScalar innerYRadius = 0.0f;
+    if (SkStrokeRec::kFill_Style != style) {
+        if (SkScalarNearlyZero(scaledStroke.length())) {
+            scaledStroke.set(SK_ScalarHalf, SK_ScalarHalf);
+        } else {
+            scaledStroke.scale(0.5f);
+        }
+
+        // we only handle thick strokes for near-circular ellipses
+        if (scaledStroke.length() > SK_ScalarHalf && (xRadius/yRadius > 2 || yRadius/xRadius > 2)) {
+            return false;
+        }
+
+        // we don't handle it if curvature of the stroke is less than curvature of the ellipse

[robertphillips, 2013/05/14 22:44:30]

The compiler should do this - but rejigger to remove /'s?

[jvanverth1, 2013/05/15 18:34:12]

Good catch, thanks.

+        if (scaledStroke.fX/(scaledStroke.fY*scaledStroke.fY) < xRadius/(yRadius*yRadius) ||
+            scaledStroke.fY/(scaledStroke.fX*scaledStroke.fX) < yRadius/(xRadius*xRadius)) {
+            return false;
+        }
+
+        // this is legit only if scale & translation (which should be the case at the moment)

[robertphillips, 2013/05/14 22:44:30]

isn't this if test redundant since we're in this block?

[jvanverth1, 2013/05/15 18:34:12]

No, because it could also be stroked and filled. However, I did already compute
this boolean (for isStroked).

+        if (SkStrokeRec::kStroke_Style == style || SkStrokeRec::kHairline_Style == style) {

[robertphillips, 2013/05/14 22:44:30]

Do innerXRadius first to match xRadius/yRadius order after this block?

[jvanverth1, 2013/05/15 18:34:12]

Done.

+            innerYRadius = SkMaxScalar(0, yRadius - scaledStroke.fY);
+            innerXRadius = SkMaxScalar(0, xRadius - scaledStroke.fX);
+        }
+
+        xRadius += scaledStroke.fX;
+        yRadius += scaledStroke.fY;
+    }
+
     GrDrawState::AutoDeviceCoordDraw adcd(drawState);
     if (!adcd.succeeded()) {
         return false;
@@ -615,50 +506,31 @@
 
     EllipseVertex* verts = reinterpret_cast<EllipseVertex*>(geo.vertices());
 
-    SkStrokeRec::Style style = stroke.getStyle();
-    bool isStroked = (SkStrokeRec::kStroke_Style == style || SkStrokeRec::kHairline_Style == style);
     enum {
         // the edge effects share this stage with glyph rendering
         // (kGlyphMaskStage in GrTextContext) && SW path rendering
         // (kPathMaskStage in GrSWMaskHelper)
         kEdgeEffectStage = GrPaint::kTotalStages,
     };
+    GrEffectRef* effect = EllipseEdgeEffect::Create(isStroked && 
+                                                    innerXRadius > 0.0 && innerYRadius > 0.0);
 
-    GrEffectRef* effect = EllipseEdgeEffect::Create(isStroked);
     static const int kEllipseCenterAttrIndex = 1;
     static const int kEllipseEdgeAttrIndex = 2;
     drawState->setEffect(kEdgeEffectStage, effect,
                          kEllipseCenterAttrIndex, kEllipseEdgeAttrIndex)->unref();
 
-    SkScalar innerXRadius = 0.0f;
-    SkScalar innerRatio = 1.0f;
+    // Compute the reciprocals of the squares of the radii here to save time in the shader

[robertphillips, 2013/05/14 22:44:30]

I think there is a scalar macro for this

[jvanverth1, 2013/05/15 18:34:12]

Done.

+    SkScalar xRadRecip = SK_Scalar1/xRadius;
+    SkScalar yRadRecip = SK_Scalar1/yRadius;

[robertphillips, 2013/05/14 22:44:30]

What happens when innerXRadius is zero?

[jvanverth1, 2013/05/15 18:34:12]

Added a check above. If it's zero, then we'll use the fill shader, and that
won't use the inner parameters.

+    SkScalar xInnerRadRecip = SK_Scalar1/innerXRadius;
+    SkScalar yInnerRadRecip = SK_Scalar1/innerYRadius;
 
-    if (SkStrokeRec::kFill_Style != style) {
-        if (SkScalarNearlyZero(scaledStroke.length())) {
-            scaledStroke.set(SK_ScalarHalf, SK_ScalarHalf);
-        } else {
-            scaledStroke.scale(0.5f);
-        }
-
-        // this is legit only if scale & translation (which should be the case at the moment)
-        if (SkStrokeRec::kStroke_Style == style || SkStrokeRec::kHairline_Style == style) {
-            SkScalar innerYRadius = SkMaxScalar(0, yRadius - scaledStroke.fY);
-            if (innerYRadius > SK_ScalarNearlyZero) {
-                innerXRadius = SkMaxScalar(0, xRadius - scaledStroke.fX);
-                innerRatio = innerXRadius/innerYRadius;
-            }
-        }
-        xRadius += scaledStroke.fX;
-        yRadius += scaledStroke.fY;
-    }
-
-    SkScalar outerRatio = SkScalarDiv(xRadius, yRadius);
-
     // We've extended the outer x radius out half a pixel to antialias.
     // This will also expand the rect so all the pixels will be captured.
+    // TODO: Consider if we should use sqrt(2)/2 instead
     xRadius += SK_ScalarHalf;
     yRadius += SK_ScalarHalf;
-    innerXRadius -= SK_ScalarHalf;
 
     SkRect bounds = SkRect::MakeLTRB(
         center.fX - xRadius,
@@ -667,33 +539,25 @@
         center.fY + yRadius
     );
 
-    // The offsets are created by scaling the y radius by the appropriate ratio. This way we end up
-    // with a circle equation which can be checked quickly in the shader. We need one offset for
-    // outer and one for inner because they have different scale factors -- otherwise we end up with
-    // non-uniform strokes.
     verts[0].fPos = SkPoint::Make(bounds.fLeft,  bounds.fTop);
-    verts[0].fOuterXRadius = xRadius;
-    verts[0].fInnerXRadius = innerXRadius;
-    verts[0].fOuterOffset = SkPoint::Make(-xRadius, -outerRatio*yRadius);
-    verts[0].fInnerOffset = SkPoint::Make(-xRadius, -innerRatio*yRadius);
+    verts[0].fOffset = SkPoint::Make(-xRadius, -yRadius);
+    verts[0].fOuterRadii = SkPoint::Make(xRadRecip, yRadRecip);
+    verts[0].fInnerRadii = SkPoint::Make(xInnerRadRecip, yInnerRadRecip);
 
     verts[1].fPos = SkPoint::Make(bounds.fRight, bounds.fTop);
-    verts[1].fOuterXRadius = xRadius;
-    verts[1].fInnerXRadius = innerXRadius;
-    verts[1].fOuterOffset = SkPoint::Make(xRadius, -outerRatio*yRadius);
-    verts[1].fInnerOffset = SkPoint::Make(xRadius, -innerRatio*yRadius);
+    verts[1].fOffset = SkPoint::Make(xRadius, -yRadius);
+    verts[1].fOuterRadii = SkPoint::Make(xRadRecip, yRadRecip);
+    verts[1].fInnerRadii = SkPoint::Make(xInnerRadRecip, yInnerRadRecip);
 
     verts[2].fPos = SkPoint::Make(bounds.fLeft,  bounds.fBottom);
-    verts[2].fOuterXRadius = xRadius;
-    verts[2].fInnerXRadius = innerXRadius;
-    verts[2].fOuterOffset = SkPoint::Make(-xRadius, outerRatio*yRadius);
-    verts[2].fInnerOffset = SkPoint::Make(-xRadius, innerRatio*yRadius);
+    verts[2].fOffset = SkPoint::Make(-xRadius, yRadius);
+    verts[2].fOuterRadii = SkPoint::Make(xRadRecip, yRadRecip);
+    verts[2].fInnerRadii = SkPoint::Make(xInnerRadRecip, yInnerRadRecip);
 
     verts[3].fPos = SkPoint::Make(bounds.fRight, bounds.fBottom);
-    verts[3].fOuterXRadius = xRadius;
-    verts[3].fInnerXRadius = innerXRadius;
-    verts[3].fOuterOffset = SkPoint::Make(xRadius, outerRatio*yRadius);
-    verts[3].fInnerOffset = SkPoint::Make(xRadius, innerRatio*yRadius);
+    verts[3].fOffset = SkPoint::Make(xRadius, yRadius);
+    verts[3].fOuterRadii = SkPoint::Make(xRadRecip, yRadRecip);
+    verts[3].fInnerRadii = SkPoint::Make(xInnerRadRecip, yInnerRadRecip);
 
     target->drawNonIndexed(kTriangleStrip_GrPrimitiveType, 0, 4, &bounds);
 
@@ -763,10 +627,7 @@
                                    vm[SkMatrix::kMSkewY]*radii.fY);
     SkScalar yRadius = SkScalarAbs(vm[SkMatrix::kMSkewX]*radii.fX +
                                    vm[SkMatrix::kMScaleY]*radii.fY);
-    // tall or wide quarter-ellipse corners aren't handled
-    if (SkScalarDiv(xRadius, yRadius) > 2 || SkScalarDiv(yRadius, xRadius) > 2) {
-        return false;
-    }
+
     // if hairline stroke is greater than radius, we don't handle that right now
     SkStrokeRec::Style style = stroke.getStyle();
     if (SkStrokeRec::kHairline_Style == style &&
@@ -893,28 +754,13 @@
         target->setIndexSourceToBuffer(indexBuffer);
         target->drawIndexed(kTriangles_GrPrimitiveType, 0, 0, 16, indexCnt, &bounds);
 
-    // otherwise we use the special ellipse renderer
+    // otherwise we use the ellipse renderer
     } else {
-
         drawState->setVertexAttribs<gEllipseVertexAttribs>(SK_ARRAY_COUNT(gEllipseVertexAttribs));
-        GrAssert(sizeof(RRectVertex) == drawState->getVertexSize());
+        GrAssert(sizeof(EllipseVertex) == drawState->getVertexSize());
 
-        GrDrawTarget::AutoReleaseGeometry geo(target, 16, 0);
-        if (!geo.succeeded()) {
-            GrPrintf("Failed to get space for vertices!\n");
-            return false;
-        }
-        RRectVertex* verts = reinterpret_cast<RRectVertex*>(geo.vertices());
-
-        GrEffectRef* effect = AltEllipseEdgeEffect::Create(isStroked);
-        static const int kEllipseOffsetAttrIndex = 1;
-        static const int kEllipseRadiiAttrIndex = 2;
-        drawState->setEffect(kEdgeEffectStage, effect,
-                             kEllipseOffsetAttrIndex, kEllipseRadiiAttrIndex)->unref();
-
         SkScalar innerXRadius = 0.0f;
         SkScalar innerYRadius = 0.0f;
-
         if (SkStrokeRec::kFill_Style != style) {
             if (SkScalarNearlyZero(scaledStroke.length())) {
                 scaledStroke.set(SK_ScalarHalf, SK_ScalarHalf);
@@ -922,6 +768,18 @@
                 scaledStroke.scale(0.5f);
             }
 
+            // we only handle thick strokes for near-circular ellipses
+            if (scaledStroke.length() > SK_ScalarHalf && 
+                (xRadius/yRadius > 2 || yRadius/xRadius > 2)) {
+                return false;
+            }
+
+            // we don't handle it if curvature of the stroke is less than curvature of the ellipse
+            if (scaledStroke.fX/(scaledStroke.fY*scaledStroke.fY) < xRadius/(yRadius*yRadius) ||
+                scaledStroke.fY/(scaledStroke.fX*scaledStroke.fX) < yRadius/(xRadius*xRadius)) {
+                return false;
+            }
+
             // this is legit only if scale & translation (which should be the case at the moment)
             if (SkStrokeRec::kStroke_Style == style || SkStrokeRec::kHairline_Style == style) {
                 innerXRadius = SkMaxScalar(0, xRadius - scaledStroke.fX);
@@ -932,11 +790,28 @@
             bounds.outset(scaledStroke.fX, scaledStroke.fY);
         }
 
+        GrDrawTarget::AutoReleaseGeometry geo(target, 16, 0);
+        if (!geo.succeeded()) {
+            GrPrintf("Failed to get space for vertices!\n");
+            return false;
+        }
+        EllipseVertex* verts = reinterpret_cast<EllipseVertex*>(geo.vertices());
+
+        GrEffectRef* effect = EllipseEdgeEffect::Create(isStroked);
+        static const int kEllipseOffsetAttrIndex = 1;
+        static const int kEllipseRadiiAttrIndex = 2;
+        drawState->setEffect(kEdgeEffectStage, effect,
+                             kEllipseOffsetAttrIndex, kEllipseRadiiAttrIndex)->unref();
+
+        // Compute the reciprocals of the radii here to save time in the shader
+        SkScalar xRadRecip = SK_Scalar1/xRadius;
+        SkScalar yRadRecip = SK_Scalar1/yRadius;
+        SkScalar xInnerRadRecip = SK_Scalar1/innerXRadius;
+        SkScalar yInnerRadRecip = SK_Scalar1/innerYRadius;
+
         // Extend the radii out half a pixel to antialias.
         SkScalar xOuterRadius = xRadius + SK_ScalarHalf;
         SkScalar yOuterRadius = yRadius + SK_ScalarHalf;
-        SkScalar xInnerRadius = SkMaxScalar(innerXRadius - SK_ScalarHalf, 0);
-        SkScalar yInnerRadius = SkMaxScalar(innerYRadius - SK_ScalarHalf, 0);
 
         // Expand the rect so all the pixels will be captured.
         bounds.outset(SK_ScalarHalf, SK_ScalarHalf);
@@ -954,34 +829,29 @@
             yOuterRadius
         };
 
-        SkScalar recipOuterX = SK_Scalar1/xOuterRadius;
-        SkScalar recipOuterY = SK_Scalar1/yOuterRadius;
-        SkScalar recipInnerX = SK_Scalar1/xInnerRadius;
-        SkScalar recipInnerY = SK_Scalar1/yInnerRadius;
-
         for (int i = 0; i < 4; ++i) {
             verts->fPos = SkPoint::Make(bounds.fLeft, yCoords[i]);
             verts->fOffset = SkPoint::Make(-xOuterRadius, yOuterOffsets[i]);
-            verts->fOuterRadii = SkPoint::Make(recipOuterX, recipOuterY);
-            verts->fInnerRadii = SkPoint::Make(recipInnerX, recipInnerY);
+            verts->fOuterRadii = SkPoint::Make(xRadRecip, yRadRecip);
+            verts->fInnerRadii = SkPoint::Make(xInnerRadRecip, yInnerRadRecip);
             verts++;
 
             verts->fPos = SkPoint::Make(bounds.fLeft + xOuterRadius, yCoords[i]);
             verts->fOffset = SkPoint::Make(SK_ScalarNearlyZero, yOuterOffsets[i]);
-            verts->fOuterRadii = SkPoint::Make(recipOuterX, recipOuterY);
-            verts->fInnerRadii = SkPoint::Make(recipInnerX, recipInnerY);
+            verts->fOuterRadii = SkPoint::Make(xRadRecip, yRadRecip);
+            verts->fInnerRadii = SkPoint::Make(xInnerRadRecip, yInnerRadRecip);
             verts++;
 
             verts->fPos = SkPoint::Make(bounds.fRight - xOuterRadius, yCoords[i]);
             verts->fOffset = SkPoint::Make(SK_ScalarNearlyZero, yOuterOffsets[i]);
-            verts->fOuterRadii = SkPoint::Make(recipOuterX, recipOuterY);
-            verts->fInnerRadii = SkPoint::Make(recipInnerX, recipInnerY);
+            verts->fOuterRadii = SkPoint::Make(xRadRecip, yRadRecip);
+            verts->fInnerRadii = SkPoint::Make(xInnerRadRecip, yInnerRadRecip);
             verts++;
 
             verts->fPos = SkPoint::Make(bounds.fRight, yCoords[i]);
             verts->fOffset = SkPoint::Make(xOuterRadius, yOuterOffsets[i]);
-            verts->fOuterRadii = SkPoint::Make(recipOuterX, recipOuterY);
-            verts->fInnerRadii = SkPoint::Make(recipInnerX, recipInnerY);
+            verts->fOuterRadii = SkPoint::Make(xRadRecip, yRadRecip);
+            verts->fInnerRadii = SkPoint::Make(xInnerRadRecip, yInnerRadRecip);
             verts++;
         }