alpha threshold bitmap shader

Committed: http://code.google.com/p/skia/source/detail?r=11122

R=reed@google.com

Author: bsalomon@google.com

Review URL: https://chromiumcodereview.appspot.com/23707019

git-svn-id: http://skia.googlecode.com/svn/trunk@11131 2bbb7eff-a529-9590-31e7-b0007b416f81

10 files changed, 498 insertions, 6 deletions

diff --git a/gm/bitmapalphathreshold.cpp b/gm/bitmapalphathreshold.cpp
new file mode 100644
index 0000000000..8af49fe351
--- /dev/null
+++ b/gm/bitmapalphathreshold.cpp
@@ -0,0 +1,144 @@
+/*
+ * 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 "gm.h"
+#include "SkGradientShader.h"
+#include "SkBitmapAlphaThresholdShader.h"
+#include "SkTArray.h"
+#include "SkParsePath.h"
+
+class BitmapAlphaThresholdGM : public skiagm::GM {
+public:
+    BitmapAlphaThresholdGM() {
+        this->setBGColor(0xFF000000);
+    }
+
+private:
+    virtual uint32_t onGetFlags() const SK_OVERRIDE {
+        // narrow this flags when the shader has a CPU implementation and
+        // when it serializes.
+        return 
+            kSkipPDF_Flag               |
+            kSkipPicture_Flag           |
+            kSkipPipe_Flag              |
+            kSkipPipeCrossProcess_Flag  |
+            kSkipTiled_Flag             |
+            kSkip565_Flag               |
+            kSkipScaledReplay_Flag      |
+            kSkipPDFRasterization_Flag  |
+
+            kGPUOnly_Flag;
+    }
+
+    virtual void onOnceBeforeDraw() SK_OVERRIDE {
+        fBM.setConfig(SkBitmap::kARGB_8888_Config, 100, 100);
+        if (!fBM.allocPixels()) {
+            return;
+        }
+        SkCanvas canvas(fBM);
+        SkPoint pts[] = { {0, 0}, {SkIntToScalar(fBM.width()), SkIntToScalar(fBM.height())} };
+        SkColor colors[] = {0x00000000, 0xffffffff};
+        SkShader* grad = SkGradientShader::CreateLinear(pts, colors, NULL, 2,
+                                                        SkShader::kClamp_TileMode);
+        SkPaint gradPaint;
+        gradPaint.setShader(grad)->unref();
+        gradPaint.setXfermodeMode(SkXfermode::kSrc_Mode);
+        canvas.drawPaint(gradPaint);
+
+        // Construct the region used as a mask.
+        SkRegion bmpBoundsClip;
+        bmpBoundsClip.setRect(0, 0, fBM.width(), fBM.height());
+        SkPath circlePath;
+        SkScalar radius = SkScalarSqrt(SkIntToScalar(fBM.width() * fBM.height())) / 2;
+        circlePath.addCircle(SkIntToScalar(fBM.width() / 2),
+                             SkIntToScalar(fBM.height() / 2),
+                             radius);
+        fMask.setPath(circlePath, bmpBoundsClip);
+
+        SkPath batPath;
+        SkParsePath::FromSVGString(
+        "M305.214,374.779c2.463,0,3.45,0.493,3.45,0.493l1.478-6.241c0,0,1.15,4.763,1.643,9.034"
+		"c0.493,4.271,8.048,1.479,14.454,0.164c6.405-1.314,7.72-11.662,7.72-11.662h59.294c0,0-35.807,10.841-26.772,34.656"
+		"c0,0-52.889-8.048-61.101,24.967h-0.001c-8.212-33.015-61.101-24.967-61.101-24.967c9.034-23.815-26.772-34.656-26.772-34.656"
+		"h59.294c0,0,1.314,10.348,7.719,11.662c6.406,1.314,13.962,4.106,14.454-0.164c0.493-4.271,1.643-9.034,1.643-9.034l1.479,6.241"
+		"c0,0,0.985-0.493,3.449-0.493H305.214L305.214,374.779z",
+        &batPath);
+
+        SkMatrix matrix;
+        matrix.setTranslate(-208, -280);
+        matrix.postScale(radius / 100, radius / 100);
+        batPath.transform(matrix, &batPath);
+        SkRegion batRegion;
+        batRegion.setPath(batPath, bmpBoundsClip);
+
+        fMask.op(batRegion, SkRegion::kDifference_Op);
+    }
+
+    virtual SkString onShortName() SK_OVERRIDE {
+        return SkString("bat");
+    }
+
+    virtual SkISize onISize() SK_OVERRIDE {
+        return SkISize::Make(518, 735);
+    }
+
+    virtual void onDraw(SkCanvas* canvas) SK_OVERRIDE {
+
+        SkTArray<SkMatrix> lms;
+        lms.push_back().reset();
+        lms.push_back().setScale(SK_Scalar1 / 2, SK_Scalar1);
+        lms.push_back().setScale(SK_Scalar1, 2 * SK_Scalar1);
+        lms.push_back().setRotate(-SK_Scalar1 * 30);
+        lms.push_back().setSkew(0, SK_Scalar1 / 5);
+
+        static const SkScalar kMargin = 5 * SK_Scalar1;
+        canvas->translate(kMargin, kMargin);
+        canvas->save();
+
+        static const U8CPU kThresholds[] = { 0x0, 0x08, 0x40, 0x80, 0xC0, 0xF0, 0xFF };
+
+        for (size_t i = 0; i < SK_ARRAY_COUNT(kThresholds); ++i) {
+            for (int j = 0; j < lms.count(); ++j) {
+                SkRect rect;
+                rect.fLeft = 0;
+                rect.fTop = 0;
+                rect.fRight = SkIntToScalar(fBM.width());
+                rect.fBottom = SkIntToScalar(fBM.height());
+
+                SkShader* thresh;
+                // This SkShader currently only has a GPU implementation.
+                if (canvas->getDevice()->accessRenderTarget()) {
+                    thresh = SkBitmapAlphaThresholdShader::Create(fBM, fMask, kThresholds[i]);
+                } else {
+                    thresh = SkShader::CreateBitmapShader(fBM, SkShader::kClamp_TileMode,
+                                                               SkShader::kClamp_TileMode);
+                }
+
+                thresh->setLocalMatrix(lms[j]);
+
+                SkPaint paint;
+                paint.setShader(thresh)->unref();
+
+                canvas->drawRect(rect, paint);
+                canvas->translate(SkIntToScalar(fBM.width() + kMargin), 0);
+            }
+            canvas->restore();
+            canvas->translate(0, SkIntToScalar(fBM.height() + kMargin));
+            canvas->save();
+        }
+
+    }
+
+    SkBitmap    fBM;
+    SkRegion    fMask;
+
+    typedef skiagm::GM INHERITED;
+};
+
+//////////////////////////////////////////////////////////////////////////////
+
+DEF_GM( return new BitmapAlphaThresholdGM(); )
diff --git a/gyp/effects.gypi b/gyp/effects.gypi
index 56ba59f09b..9105dddfa5 100644
--- a/gyp/effects.gypi
+++ b/gyp/effects.gypi
@@ -12,6 +12,7 @@
     '<(skia_src_path)/effects/SkArithmeticMode.cpp',
     '<(skia_src_path)/effects/SkAvoidXfermode.cpp',
     '<(skia_src_path)/effects/SkBicubicImageFilter.cpp',
+    '<(skia_src_path)/effects/SkBitmapAlphaThresholdShader.cpp',
     '<(skia_src_path)/effects/SkBitmapSource.cpp',
     '<(skia_src_path)/effects/SkBlurDrawLooper.cpp',
     '<(skia_src_path)/effects/SkBlurMask.cpp',
@@ -80,6 +81,7 @@
     '<(skia_include_path)/effects/SkXfermodeImageFilter.h',
     '<(skia_include_path)/effects/SkArithmeticMode.h',
     '<(skia_include_path)/effects/SkAvoidXfermode.h',
+    '<(skia_include_path)/effects/SkBitmapAlphaThresholdShader.h',
     '<(skia_include_path)/effects/SkBitmapSource.h',
     '<(skia_include_path)/effects/SkBlurDrawLooper.h',
     '<(skia_include_path)/effects/SkBlurImageFilter.h',
diff --git a/gyp/gmslides.gypi b/gyp/gmslides.gypi
index 52faae0448..e101baf739 100644
--- a/gyp/gmslides.gypi
+++ b/gyp/gmslides.gypi
@@ -10,6 +10,7 @@
     '../gm/bicubicfilter.cpp',
     '../gm/bigmatrix.cpp',
     '../gm/bigtext.cpp',
+    '../gm/bitmapalphathreshold.cpp',
     '../gm/bitmapcopy.cpp',
     '../gm/bitmapmatrix.cpp',
     '../gm/bitmapfilters.cpp',
diff --git a/include/core/SkShader.h b/include/core/SkShader.h
index e1cab96795..2632830ef0 100644
--- a/include/core/SkShader.h
+++ b/include/core/SkShader.h
@@ -318,9 +318,11 @@ public:
     virtual GradientType asAGradient(GradientInfo* info) const;
 
     /**
-     *  If the shader subclass has a GrEffect implementation, this installs an effect on the stage.
-     *  The GrContext may be used by the effect to create textures. The GPU device does not call
-     *  setContext. Instead we pass the paint here in case the shader needs paint info.
+     *  If the shader subclass has a GrEffect implementation, this resturns the effect to install.
+     *  The incoming color to the effect has r=g=b=a all extracted from the SkPaint's alpha.
+     *  The output color should be the computed SkShader premul color modulated by the incoming
+     *  color. The GrContext may be used by the effect to create textures. The GPU device does not
+     *  call setContext. Instead we pass the SkPaint here in case the shader needs paint info.
      */
     virtual GrEffectRef* asNewEffect(GrContext* context, const SkPaint& paint) const;
 
diff --git a/include/effects/SkBitmapAlphaThresholdShader.h b/include/effects/SkBitmapAlphaThresholdShader.h
new file mode 100644
index 0000000000..61596daff3
--- /dev/null
+++ b/include/effects/SkBitmapAlphaThresholdShader.h
@@ -0,0 +1,24 @@
+/*
+ * 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);
+};
diff --git a/include/gpu/GrEffect.h b/include/gpu/GrEffect.h
index 942e580d4c..a84810c094 100644
--- a/include/gpu/GrEffect.h
+++ b/include/gpu/GrEffect.h
@@ -204,8 +204,9 @@ public:
 protected:
     /**
      * Subclasses call this from their constructor to register GrTextureAccesses. The effect
-     * subclass manages the lifetime of the accesses (this function only stores a pointer). This
-     * must only be called from the constructor because GrEffects are immutable.
+     * subclass manages the lifetime of the accesses (this function only stores a pointer). The
+     * GrTextureAccess is typically a member field of the GrEffet subclass. This must only be
+     * called from the constructor because GrEffects are immutable.
      */
     void addTextureAccess(const GrTextureAccess* textureAccess);
 
diff --git a/include/gpu/GrTextureAccess.h b/include/gpu/GrTextureAccess.h
index 0f44807466..f8bb554326 100644
--- a/include/gpu/GrTextureAccess.h
+++ b/include/gpu/GrTextureAccess.h
@@ -18,7 +18,7 @@ class GrTexture;
  * Represents the filtering and tile modes used to access a texture. It is mostly used with
  * GrTextureAccess (defined below). Also, some of the texture cache methods require knowledge about
  * filtering and tiling to perform a cache lookup. If it wasn't for this latter usage this would
- * be folded into GrTextureAccess.
+ * be folded into GrTextureAccess. The default is clamp tile modes and no filtering.
  */
 class GrTextureParams {
 public:
diff --git a/src/effects/SkBitmapAlphaThresholdShader.cpp b/src/effects/SkBitmapAlphaThresholdShader.cpp
new file mode 100644
index 0000000000..c9275883e3
--- /dev/null
+++ b/src/effects/SkBitmapAlphaThresholdShader.cpp
@@ -0,0 +1,311 @@
+/*
+ * 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 "SkBitmapAlphaThresholdShader.h"
+
+class BATShader : public SkShader {
+public:
+    SK_DECLARE_INST_COUNT(SkThresholdShader);
+
+    BATShader(const SkBitmap& bitmap, SkRegion region, U8CPU);
+    BATShader(SkFlattenableReadBuffer& buffer) : INHERITED(buffer) {
+        // We should probably do something here.
+    }
+    
+    
+    virtual void shadeSpan(int x, int y, SkPMColor[], int count) SK_OVERRIDE {};
+    
+#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(BATShader)
+    
+private:
+    SkBitmap fBitmap;
+    SkRegion fRegion;
+    U8CPU    fThreshold;
+    
+    typedef SkShader INHERITED;
+};
+
+SkShader* SkBitmapAlphaThresholdShader::Create(const SkBitmap& bitmap,
+                                               const SkRegion& region,
+                                               U8CPU threshold) {
+    SkASSERT(threshold < 256);
+    return SkNEW_ARGS(BATShader, (bitmap, region, threshold));
+}
+
+BATShader::BATShader(const SkBitmap& bitmap, SkRegion region, U8CPU threshold)
+: fBitmap(bitmap)
+, fRegion(region)
+, fThreshold(threshold) {
+};
+
+
+#ifdef SK_DEVELOPER
+void BATShader::toString(SkString* str) const {
+    str->append("BATShader: (");
+
+    fBitmap.toString(str);
+
+    this->INHERITED::toString(str);
+
+    str->append(")");
+}
+#endif
+
+#if SK_SUPPORT_GPU
+#include "GrContext.h"
+#include "GrEffect.h"
+#include "gl/GrGLEffect.h"
+#include "gl/GrGLEffectMatrix.h"
+#include "GrTBackendEffectFactory.h"
+#include "GrTextureAccess.h"
+
+#include "SkGr.h"
+
+/**
+ * Could create specializations for some simple cases:
+ *  - The region is empty.
+ *  - The region fully contains the bitmap.
+ *  - The regions is 1 rect (or maybe a small number of rects).
+ */
+class ThresholdEffect : public GrEffect {
+public:
+    virtual const GrBackendEffectFactory& getFactory() const SK_OVERRIDE {
+        return GrTBackendEffectFactory<ThresholdEffect>::getInstance();
+    }
+
+    static GrEffectRef* Create(GrTexture* bmpTexture, const SkMatrix& bmpMatrix,
+                               GrTexture* maskTexture, const SkMatrix& maskMatrix,
+                               U8CPU threshold) {
+        SkScalar thresh = SkIntToScalar(threshold) / 255;
+
+        AutoEffectUnref effect(SkNEW_ARGS(ThresholdEffect, (bmpTexture, bmpMatrix,
+                                                            maskTexture, maskMatrix,
+                                                            thresh)));
+        return CreateEffectRef(effect);
+    }
+
+    virtual void getConstantColorComponents(GrColor* color,
+                                            uint32_t* validFlags) const SK_OVERRIDE {
+        if ((kA_GrColorComponentFlag & *validFlags) && 0 == GrColorUnpackA(*color)) {
+            return;
+        }
+        *validFlags = 0;
+        return;
+    }
+
+    static const char* Name() { return "Bitmap Alpha Threshold"; }
+
+    class GLEffect : public GrGLEffect {
+    public:
+        GLEffect(const GrBackendEffectFactory& factory,
+                    const GrDrawEffect& e)
+        : GrGLEffect(factory)
+        , fBmpMatrix(GrEffect::kLocal_CoordsType)
+        , fMaskMatrix(GrEffect::kLocal_CoordsType)
+        , fPrevThreshold(-SK_Scalar1) {
+        }
+
+        virtual void emitCode(GrGLShaderBuilder* builder,
+                                const GrDrawEffect& drawEffect,
+                                EffectKey key,
+                                const char* outputColor,
+                                const char* inputColor,
+                                const TextureSamplerArray& samplers) SK_OVERRIDE {
+            SkString bmpCoord;
+            SkString maskCoord;
+
+            GrSLType bmpCoordType = fBmpMatrix.emitCode(builder, key, &bmpCoord, NULL, "Bmp");
+            EffectKey maskMatrixKey = key >> GrGLEffectMatrix::kKeyBits;
+            GrSLType maskCoordType = fMaskMatrix.emitCode(builder,
+                                                          maskMatrixKey,
+                                                          &maskCoord,
+                                                          NULL,
+                                                          "Mask");
+
+            // put bitmap color in "color"
+            builder->fsCodeAppend("\t\tvec4 color = ");
+            builder->fsAppendTextureLookup(samplers[0], bmpCoord.c_str(), bmpCoordType);
+            builder->fsCodeAppend(";\n");
+
+            // put alpha from mask texture in "mask"
+            builder->fsCodeAppend("\t\tfloat mask = ");
+            builder->fsAppendTextureLookup(samplers[1], maskCoord.c_str(), maskCoordType);
+            builder->fsCodeAppend(".a;\n");
+
+            const char* threshold;
+
+            fThresholdUniHandle = builder->addUniform(GrGLShaderBuilder::kFragment_Visibility,
+                                                      kFloat_GrSLType,
+                                                      "threshold",
+                                                      &threshold);
+            builder->fsCodeAppendf("\t\tfloat thresh = %s;\n", threshold);
+
+            builder->fsCodeAppend("\t\tif (mask < 0.5) {\n"
+                                    "\t\t\tif (color.a > thresh) {\n"
+                                    "\t\t\t\tfloat scale = thresh / color.a;\n"
+                                    "\t\t\t\tcolor.rgb *= scale;\n"
+                                    "\t\t\t\tcolor.a = thresh;\n"
+                                    "\t\t\t}\n"
+                                    "\t\t} else if (color.a < thresh) {\n"
+                                    "\t\t\tfloat scale = thresh / color.a;\n"
+                                    "\t\t\tcolor.rgb *= scale;\n"
+                                    "\t\t\tcolor.a = thresh;\n"
+                                    "\t\t}\n");
+
+            builder->fsCodeAppend("color = ");
+            SkString outStr;
+            outStr.appendf("\t\t%s = ", outputColor);
+            GrGLSLModulatef<4>(&outStr, inputColor, "color");
+            outStr.append(";\n");
+            builder->fsCodeAppend(outStr.c_str());
+        }
+
+        virtual void setData(const GrGLUniformManager& uman, const GrDrawEffect& e) SK_OVERRIDE {
+            const ThresholdEffect& effect = e.castEffect<ThresholdEffect>();
+            fBmpMatrix.setData(uman, effect.fBmpMatrix, e, effect.fBmpAccess.getTexture());
+            fMaskMatrix.setData(uman, effect.fMaskMatrix, e, effect.fMaskAccess.getTexture());
+            if (fPrevThreshold != effect.fThreshold) {
+                uman.set1f(fThresholdUniHandle, effect.fThreshold);
+            }
+        }
+
+        static inline EffectKey GenKey(const GrDrawEffect& e, const GrGLCaps&) {
+            const ThresholdEffect& effect = e.castEffect<ThresholdEffect>();
+
+            EffectKey bmpMKey = GrGLEffectMatrix::GenKey(effect.fBmpMatrix,
+                                                         e,
+                                                         GrEffect::kLocal_CoordsType,
+                                                         effect.fBmpAccess.getTexture());
+            EffectKey maskMKey = GrGLEffectMatrix::GenKey(effect.fMaskMatrix,
+                                                          e,
+                                                          GrEffect::kLocal_CoordsType,
+                                                          effect.fMaskAccess.getTexture());
+            return bmpMKey | (maskMKey << GrGLEffectMatrix::kKeyBits);
+        }
+
+    private:
+        GrGLEffectMatrix fBmpMatrix;
+        GrGLEffectMatrix fMaskMatrix;
+
+        GrGLUniformManager::UniformHandle fThresholdUniHandle;
+        SkScalar                          fPrevThreshold;
+    };
+
+    GR_DECLARE_EFFECT_TEST;
+
+private:
+    ThresholdEffect(GrTexture* bmpTexture, const SkMatrix& bmpMatrix,
+                    GrTexture* maskTexture, const SkMatrix& maskMatrix,
+                    SkScalar threshold)
+    : fBmpAccess(bmpTexture, GrTextureParams())
+    , fMaskAccess(maskTexture, GrTextureParams())
+    , fBmpMatrix(bmpMatrix)
+    , fMaskMatrix(maskMatrix)
+    , fThreshold(threshold) {
+        this->addTextureAccess(&fBmpAccess);
+        this->addTextureAccess(&fMaskAccess);
+    }
+
+    virtual bool onIsEqual(const GrEffect& other) const SK_OVERRIDE {
+        const ThresholdEffect& e = CastEffect<ThresholdEffect>(other);
+        return e.fBmpAccess.getTexture() == fBmpAccess.getTexture() &&
+               e.fMaskAccess.getTexture() == fMaskAccess.getTexture() &&
+               e.fBmpMatrix == fBmpMatrix &&
+               e.fMaskMatrix == fMaskMatrix &&
+               e.fThreshold == fThreshold;
+    }
+
+    GrTextureAccess fBmpAccess;
+    GrTextureAccess fMaskAccess;
+
+    SkMatrix fBmpMatrix;
+    SkMatrix fMaskMatrix;
+
+    SkScalar fThreshold;
+};
+
+GR_DEFINE_EFFECT_TEST(ThresholdEffect);
+
+GrEffectRef* ThresholdEffect::TestCreate(SkMWCRandom* rand,
+                                         GrContext*,
+                                         const GrDrawTargetCaps&,
+                                         GrTexture* textures[]) {
+    GrTexture* bmpTex = textures[GrEffectUnitTest::kSkiaPMTextureIdx];
+    GrTexture* maskTex = textures[GrEffectUnitTest::kAlphaTextureIdx];
+    U8CPU thresh = rand->nextU() % 0xff;
+    return ThresholdEffect::Create(bmpTex, SkMatrix::I(), maskTex, SkMatrix::I(), thresh);
+}
+
+GrEffectRef* BATShader::asNewEffect(GrContext* context, const SkPaint& paint) const {
+    SkMatrix localInverse;
+    if (!this->getLocalMatrix().invert(&localInverse)) {
+        return NULL;
+    }
+
+    GrTextureDesc maskDesc;
+    if (context->isConfigRenderable(kAlpha_8_GrPixelConfig)) {
+        maskDesc.fConfig = kAlpha_8_GrPixelConfig;
+    } else {
+        maskDesc.fConfig = kRGBA_8888_GrPixelConfig;
+    }
+    maskDesc.fFlags = kRenderTarget_GrTextureFlagBit | kNoStencil_GrTextureFlagBit;
+    const SkIRect& bounds = fRegion.getBounds();
+    // Add one pixel of border to ensure that clamp mode will be all zeros
+    // the outside.
+    maskDesc.fWidth = bounds.width() + 2;
+    maskDesc.fHeight = bounds.height() + 2;
+    GrAutoScratchTexture ast(context, maskDesc, GrContext::kApprox_ScratchTexMatch);
+    GrTexture*  maskTexture = ast.texture();
+    if (NULL == maskTexture) {
+        return NULL;
+    }
+
+    GrPaint grPaint;
+    grPaint.setBlendFunc(kOne_GrBlendCoeff, kZero_GrBlendCoeff);
+    SkRegion::Iterator iter(fRegion);
+    context->setRenderTarget(maskTexture->asRenderTarget());
+    context->clear(NULL, 0x0);
+
+    // offset to ensure border is zero on top/left
+    SkMatrix matrix;
+    matrix.setTranslate(SK_Scalar1, SK_Scalar1);
+    context->setMatrix(matrix);
+
+    while (!iter.done()) {
+        SkRect rect = SkRect::MakeFromIRect(iter.rect());
+        context->drawRect(grPaint, rect);
+        iter.next();
+    }
+
+    GrTexture* bmpTexture = GrLockAndRefCachedBitmapTexture(context, fBitmap, NULL);
+    if (NULL == bmpTexture) {
+        return NULL;
+    }
+
+    SkMatrix bmpMatrix = localInverse;
+    bmpMatrix.postIDiv(bmpTexture->width(), bmpTexture->height());
+
+    SkMatrix maskMatrix = localInverse;
+    // compensate for the border
+    maskMatrix.postTranslate(SK_Scalar1, SK_Scalar1);
+    maskMatrix.postIDiv(maskTexture->width(), maskTexture->height());
+
+    GrEffectRef* effect = ThresholdEffect::Create(bmpTexture, bmpMatrix,
+                                                  maskTexture, maskMatrix,
+                                                  fThreshold);
+
+    GrUnlockAndUnrefCachedBitmapTexture(bmpTexture);
+
+    return effect;
+}
+
+#endif
diff --git a/src/gpu/SkGpuDevice.cpp b/src/gpu/SkGpuDevice.cpp
index 571b8a0a7d..ea8a06e632 100644
--- a/src/gpu/SkGpuDevice.cpp
+++ b/src/gpu/SkGpuDevice.cpp
@@ -535,6 +535,11 @@ inline bool skPaint2GrPaintShader(SkGpuDevice* dev,
         return skPaint2GrPaintNoShader(dev, skPaint, false, constantColor, grPaint);
     }
 
+    // SkShader::asNewEffect() may do offscreen rendering. Setup default drawing state
+    // Also require shader to set the render target .
+    GrContext::AutoWideOpenIdentityDraw awo(dev->context(), NULL);
+    GrContext::AutoRenderTarget(dev->context(), NULL);
+
     // setup the shader as the first color effect on the paint
     SkAutoTUnref<GrEffectRef> effect(shader->asNewEffect(dev->context(), skPaint));
     if (NULL != effect.get()) {
diff --git a/tests/GLProgramsTest.cpp b/tests/GLProgramsTest.cpp
index c9f69f0ef3..316465840e 100644
--- a/tests/GLProgramsTest.cpp
+++ b/tests/GLProgramsTest.cpp
@@ -228,6 +228,7 @@ DEFINE_GPUTESTCLASS("GLPrograms", GLProgramsTestClass, GLProgramsTest)
 #include "SkLightingImageFilter.h"
 #include "SkMagnifierImageFilter.h"
 #include "SkColorMatrixFilter.h"
+#include "SkBitmapAlphaThresholdShader.h"
 
 void forceLinking();
 
@@ -240,6 +241,7 @@ void forceLinking() {
                                      SkMatrix::I());
     SkScalar matrix[20];
     SkColorMatrixFilter cmf(matrix);
+    SkBitmapAlphaThresholdShader::Create(SkBitmap(), SkRegion(), 0x80);
 }
 
 #endif