Preventing division by 0 in non-separable blend mode shaders.

gm/mixedxfermodes.cpp

Index: gm/mixedxfermodes.cpp
diff --git a/gm/mixedxfermodes.cpp b/gm/mixedxfermodes.cpp
index a5023e1b0b07218ad6b2acc171568ecae7d35de4..37b8da3c15a240f94d3c1808e01f31b5e7642b05 100644
--- a/gm/mixedxfermodes.cpp
+++ b/gm/mixedxfermodes.cpp
@@ -22,6 +22,15 @@ public:
     }
 
 protected:
+    enum ShapeType {
+        kShapeTypeCircle,
+        kShapeTypeRoundRect,
+        kShapeTypeRect,
+        kShapeTypeConvexPath,
+        kShapeTypeConcavePath,
+        kNumShapeTypes
+    };
+
     virtual SkString onShortName() SK_OVERRIDE {
         return SkString("mixed_xfermodes");
     }
@@ -32,21 +41,20 @@ protected:
 
     void drawShape(SkCanvas* canvas,
                    const SkPaint& paint,
-                   SkRandom* random) {
+                   ShapeType type) {
         static const SkRect kRect = SkRect::MakeXYWH(SkIntToScalar(-50), SkIntToScalar(-50),
                                                      SkIntToScalar(75), SkIntToScalar(105));
-        int shape = random->nextULessThan(5);
-        switch (shape) {
-        case 0:
+        switch (type) {
+        case kShapeTypeCircle:

[egdaniel, 2014/11/20 14:34:15]

we tend to indent the "case" inside the switch statements.

[rosca, 2014/11/20 14:52:10]

Done.

             canvas->drawCircle(0, 0, 50, paint);
             break;
-        case 1:
+        case kShapeTypeRoundRect:
             canvas->drawRoundRect(kRect, SkIntToScalar(10), SkIntToScalar(20), paint);
             break;
-        case 2:
+        case kShapeTypeRect:
             canvas->drawRect(kRect, paint);
             break;
-        case 3:
+        case kShapeTypeConvexPath:
             if (fConvexPath.isEmpty()) {
                 SkPoint points[4];
                 kRect.toQuad(points);
@@ -57,7 +65,7 @@ protected:
             }
             canvas->drawPath(fConvexPath, paint);
             break;
-        case 4:
+        case kShapeTypeConcavePath:
             if (fConcavePath.isEmpty()) {
                 SkPoint points[5] = {{0, SkIntToScalar(-50)} };
                 SkMatrix rot;
@@ -74,6 +82,8 @@ protected:
             }
             canvas->drawPath(fConcavePath, paint);
             break;
+        default:
+            break;
         }
     }
 
@@ -108,6 +118,7 @@ protected:
             SkColor color = random.nextU();
             SkXfermode::Mode mode =
                 static_cast<SkXfermode::Mode>(random.nextULessThan(SkXfermode::kLastMode + 1));
+            ShapeType shapeType = static_cast<ShapeType>(random.nextULessThan(kNumShapeTypes));
 
             SkPaint p;
             p.setAntiAlias(true);
@@ -117,9 +128,42 @@ protected:
             canvas->translate(dx, dy);
             canvas->scale(s, s);
             canvas->rotate(r);
-            this->drawShape(canvas, p, &random);
+            this->drawShape(canvas, p, shapeType);
             canvas->restore();
         }
+
+        // This draw should not affect the test's result.
+        drawWithHueOnWhite(canvas);
+    }
+
+    /**
+     * Draws white color into a white square using the hue blend mode.
+     * This will test a divide by 0 bug in the setLum function in shaders,
+     * being fixed with https://codereview.chromium.org/666043003/.

[egdaniel, 2014/11/18 14:57:16]

No need to put link to cl in the comment. The comment itself should describe the
bug enough and if need be the cl can always be found through history/blame.

[rosca, 2014/11/20 14:52:10]

Done.

+     * Without the fix, the result would be black (caused by the faulty code).
+     * With this fix the result color is white, so drawing on white should not
+     * change the expected result.
+     */
+    void drawWithHueOnWhite(SkCanvas* canvas) {
+        SkColor color = SkColorSetARGBMacro(225, 255, 255, 255);
+        SkXfermode::Mode mode = SkXfermode::kHue_Mode;
+        ShapeType shapeType = kShapeTypeConvexPath;
+
+        // Make it fit into a square.
+        SkScalar s = 0.15f;
+        // Look for a clean white square.
+        SkScalar dx = 30.f;
+        SkScalar dy = 350.f;
+
+        SkPaint p;
+        p.setAntiAlias(true);
+        p.setColor(color);
+        p.setXfermodeMode(mode);
+        canvas->save();
+        canvas->translate(dx, dy);
+        canvas->scale(s, s);
+        this->drawShape(canvas, p, shapeType);
+        canvas->restore();
     }
 
     virtual uint32_t onGetFlags() const {