add SkEmptyShader, and return it from CreateBitmapShader if the bitmap is empty

(i.e. has no pixels at all)



git-svn-id: http://skia.googlecode.com/svn/trunk@1792 2bbb7eff-a529-9590-31e7-b0007b416f81

include/core/SkEmptyShader.h

@@ -0,0 +1,48 @@
+/*
+    Copyright 2011 Google Inc.
+
+    Licensed under the Apache License, Version 2.0 (the "License");
+    you may not use this file except in compliance with the License.
+    You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+    Unless required by applicable law or agreed to in writing, software
+    distributed under the License is distributed on an "AS IS" BASIS,
+    WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+    See the License for the specific language governing permissions and
+    limitations under the License.
+ */
+
+
+#ifndef SkEmptyShader_DEFINED
+#define SkEmptyShader_DEFINED
+
+#include "SkShader.h"
+
+/**
+ *  \class SkEmptyShader
+ *  A Shader that always draws nothing.
+ */
+class SK_API SkEmptyShader : public SkShader {
+public:
+    SkEmptyShader();
+
+    virtual uint32_t getFlags();
+    virtual uint8_t getSpan16Alpha() const;
+    virtual bool setContext(const SkBitmap& device, const SkPaint& paint,
+                            const SkMatrix& matrix);
+    virtual void shadeSpan(int x, int y, SkPMColor span[], int count);
+    virtual void shadeSpan16(int x, int y, uint16_t span[], int count);
+    virtual void shadeSpanAlpha(int x, int y, uint8_t alpha[], int count);
+
+protected:
+    SkEmptyShader(SkFlattenableReadBuffer&);
+    virtual Factory getFactory();
+    virtual void flatten(SkFlattenableWriteBuffer&);
+
+private:
+    typedef SkShader INHERITED;
+};
+
+#endif

src/core/SkBitmapProcShader.cpp

@@ -228,6 +228,7 @@ void SkBitmapProcShader::shadeSpan16(int x, int y, uint16_t dstC[], int count) {
 
 #include "SkUnPreMultiply.h"
 #include "SkColorShader.h"
+#include "SkEmptyShader.h"
 
 // returns true and set color if the bitmap can be drawn as a single color
 // (for efficiency)
@@ -264,7 +265,10 @@ SkShader* SkShader::CreateBitmapShader(const SkBitmap& src,
                                        void* storage, size_t storageSize) {
     SkShader* shader;
     SkColor color;
-    if (canUseColorShader(src, &color)) {
+    if (src.isNull()) {
+        SK_PLACEMENT_NEW(shader, SkEmptyShader, storage, storageSize);
+    }
+    else if (canUseColorShader(src, &color)) {
         SK_PLACEMENT_NEW_ARGS(shader, SkColorShader, storage, storageSize,
                               (color));
     } else {

src/core/SkShader.cpp

@@ -344,3 +344,26 @@ SkShader::GradientType SkColorShader::asAGradient(GradientInfo* info) const {
     }
     return kColor_GradientType;
 }
+
+///////////////////////////////////////////////////////////////////////////////
+
+#include "SkEmptyShader.h"
+
+SkEmptyShader::SkEmptyShader() {}
+SkEmptyShader::SkEmptyShader(SkFlattenableReadBuffer& b) : INHERITED(b) {}
+
+uint32_t SkEmptyShader::getFlags() { return 0; }
+uint8_t SkEmptyShader::getSpan16Alpha() const { return 0; }
+bool SkEmptyShader::setContext(const SkBitmap& device, const SkPaint& paint,
+                               const SkMatrix& matrix) {
+    return false;
+}
+void SkEmptyShader::shadeSpan(int x, int y, SkPMColor span[], int count) {}
+void SkEmptyShader::shadeSpan16(int x, int y, uint16_t span[], int count) {}
+void SkEmptyShader::shadeSpanAlpha(int x, int y, uint8_t alpha[], int count) {}
+
+SkFlattenable::Factory SkEmptyShader::getFactory() { return NULL; }
+void SkEmptyShader::flatten(SkFlattenableWriteBuffer& buffer) {
+    this->INHERITED::flatten(buffer);
+}
+