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"
+
+/**
+ * This shader 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.
+ */
+class SK_API SkBitmapAlphaThresholdShader : public SkShader {
+public:
+    SK_DECLARE_INST_COUNT(SkThresholdShader);
+
+    static SkShader* Create(SkBitmap bitmap, const SkRegion& region, U8CPU threshold);

[reed1, 2013/09/05 15:14:54]

nit: const SkBitmap&

[bsalomon, 2013/09/05 19:54:50]

Done.

+
+    virtual void shadeSpan(int x, int y, SkPMColor[], int count) SK_OVERRIDE {};

[reed1, 2013/09/05 15:14:54]

Suggestion:

move all of this method stuff into the .cpp, so the public factory just looks
like

class MyFancyShader : public SkShader {
public:
    static SkShader* Create(...);
};

Basically, we just need to scope the Create function, and a class prefix is
convenient/somewhat-self-documenting. See SkBlurMaskFilter.h

[bsalomon, 2013/09/05 19:54:50]

Done.

+
+#if SK_SUPPORT_GPU
+    virtual GrEffectRef* asNewEffect(GrContext* context, const SkPaint& paint) const SK_OVERRIDE;
+#endif
+
+    SK_DEVELOPER_TO_STRING();
+    SK_DECLARE_PUBLIC_FLATTENABLE_DESERIALIZATION_PROCS(SkBitmapAlphaThresholdShader)
+
+private:
+    SkBitmapAlphaThresholdShader(SkBitmap bitmap, SkRegion region, U8CPU);
+    SkBitmapAlphaThresholdShader(SkFlattenableReadBuffer& buffer) : INHERITED(buffer) {}
+
+
+    SkBitmap fBitmap;
+    SkRegion fRegion;
+    U8CPU    fThreshold;
+
+    typedef SkShader INHERITED;
+};