alpha threshold bitmap shader

include/effects/SkBitmapAlphaThresholdShader.h

+/*
+ * Copyright 2013 Google Inc.
+ *
+ * Use of this source code is governed by a BSD-style license that can be
+ * found in the LICENSE file.
+ */
+
+#include "SkShader.h"
+#include "SkBitmap.h"
+#include "SkRegion.h"
+#include "SkString.h"
+
+class SK_API SkBitmapAlphaThresholdShader : public SkShader {
+public:
+    /**
+     * Creates a shader that samples a bitmap and a region. If the sample is inside the region
+     * the alpha of the bitmap color is boosted up to a threshold value. If it is
+     * outside the region then the bitmap alpha is decreased to the threshold value.
+     * The 0,0 point of the region corresponds to the upper left corner of the bitmap
+     * Currently, this only has a GPU implementation, doesn't respect the paint's bitmap
+     * filter setting, and always uses clamp mode.
+     */
+    static SkShader* Create(const SkBitmap& bitmap, const SkRegion& region, U8CPU threshold);
+};