Add PlusMergesAA GM
C.f. https://crrev.com/1156453002/
BUG=skia:
Review URL: https://codereview.chromium.org/1149103003
diff --git a/gm/plus.cpp b/gm/plus.cpp
new file mode 100644
index 0000000..ba2a1a0
--- /dev/null
+++ b/gm/plus.cpp
@@ -0,0 +1,44 @@
+/*
+ * Copyright 2015 Google Inc.
+ *
+ * Use of this source code is governed by a BSD-style license that can be
+ * found in the LICENSE file.
+ */
+
+#include "gm.h"
+
+DEF_SIMPLE_GM(PlusMergesAA, canvas, 256, 256) {
+ SkPaint p;
+ p.setColor(SK_ColorRED);
+ p.setAntiAlias(true); // <-- crucial to the test that we use AA
+
+ // Draw a two red squares.
+ canvas->drawRect(SkRect::MakeWH(100, 100), p);
+ canvas->drawRect(SkRect::MakeXYWH(150, 0, 100, 100), p);
+
+ p.setColor(0xf000ff00);
+
+ // We'll draw a green square on top of each using two triangles.
+ SkPath upperLeft;
+ upperLeft.lineTo(100, 0);
+ upperLeft.lineTo(0, 100);
+ upperLeft.lineTo(0, 0);
+
+ SkPath bottomRight;
+ bottomRight.moveTo(100, 0);
+ bottomRight.lineTo(100, 100);
+ bottomRight.lineTo(0, 100);
+ bottomRight.lineTo(100, 0);
+
+ // The left square is drawn simply with SrcOver. It will show a red seam.
+ canvas->drawPath(upperLeft, p);
+ canvas->drawPath(bottomRight, p);
+
+ // Using Plus on the right should merge the AA of seam together completely covering the red.
+ canvas->saveLayer(nullptr, nullptr);
+ p.setXfermodeMode(SkXfermode::kPlus_Mode);
+ canvas->translate(150, 0);
+ canvas->drawPath(upperLeft, p);
+ canvas->drawPath(bottomRight, p);
+ canvas->restore();
+}