Add Alpha fade GM.

gm/transparency.cpp

+/*
+ * 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 "SkGradientShader.h"
+#include "SkSurface.h"
+#include "gm.h"
+
+static void make_transparency(SkCanvas* canvas,
+                              SkScalar width,
+                              SkScalar height) {
+    SkPoint pts[2];
+    pts[0] = SkPoint::Make(0, 0);
+    pts[1] = SkPoint::Make(width, 0);
+    const SkColor kColors[] = {
+            SK_ColorBLACK,
+            SK_ColorGRAY,
+            SK_ColorWHITE,
+            SK_ColorRED,
+            SK_ColorYELLOW,
+            SK_ColorGREEN,
+            SK_ColorCYAN,
+            SK_ColorBLUE,
+            SK_ColorMAGENTA,
+    };
+    const SkScalar kRowHeight = height / SK_ARRAY_COUNT(kColors);
+    for (size_t i = 0; i < SK_ARRAY_COUNT(kColors); ++i) {
+        SkColor shaderColors[2];
+        shaderColors[0] = SK_AlphaTRANSPARENT;
+        shaderColors[1] = kColors[i];
+        SkAutoTUnref<SkShader> shader(SkGradientShader::CreateLinear(
+                pts, shaderColors, NULL, 2, SkShader::kClamp_TileMode));
+        SkRect r = SkRect::MakeXYWH(0, i * kRowHeight, width, kRowHeight);
+        SkPaint p;
+        p.setShader(shader);
+        canvas->drawRect(r, p);
+    }
+}
+
+// http://crrev.com/834303005
+static SkShader* create_checkerboard_shader(SkColor c1, SkColor c2, int size) {
+    SkBitmap bm;
+    bm.allocN32Pixels(2 * size, 2 * size);
+    bm.eraseColor(c1);
+    bm.eraseArea(SkIRect::MakeLTRB(0, 0, size, size), c2);
+    bm.eraseArea(SkIRect::MakeLTRB(size, size, 2 * size, 2 * size), c2);
+    return SkShader::CreateBitmapShader(bm, SkShader::kRepeat_TileMode,
+                                        SkShader::kRepeat_TileMode);
+}
+
+// http://crrev.com/834303005
+static void checkerboard(SkCanvas* canvas, SkColor c1, SkColor c2, int size) {
+    SkPaint paint;
+    paint.setShader(create_checkerboard_shader(c1, c2, size))->unref();
+    canvas->drawPaint(paint);
+}
+
+/**
+ *  This GM verifies that a transparent bitmap drawn over a
+ *  checkerboard pattern looks correct.
+ */
+DEF_SIMPLE_GM(transparency_check, canvas, 1912, 1080) {
+    checkerboard(canvas, 0xFF999999, 0xFF666666, 8);
+    {
+        SkAutoCanvasRestore autoCanvasRestore(canvas, true);
+        SkAutoTUnref<SkSurface> surface(SkSurface::NewRasterN32Premul(256, 9));
+        make_transparency(surface->getCanvas(), 256.0f, 9.0f);
+        canvas->scale(7.5f, 120.0f);
+        surface->draw(canvas, 0, 0, NULL);
+    }
+}