samplecode/SampleLayerMask.cpp

#include "SampleCode.h"
#include "SkCanvas.h"
#include "SkPaint.h"
#include "SkPorterDuff.h"
#include "SkView.h"

///////////////////////////////////////////////////////////////////////////////

class LayerMaskView : public SkView {
public:
	LayerMaskView() {}
    
protected:
    // overrides from SkEventSink
    virtual bool onQuery(SkEvent* evt) {
        if (SampleCode::TitleQ(*evt)) {
            SampleCode::TitleR(evt, "LayerMask");
            return true;
        }
        return this->INHERITED::onQuery(evt);
    }
    
    void drawMask(SkCanvas* canvas, const SkRect& r) {
        SkPaint paint;
        paint.setAntiAlias(true);

        if (true) {
            SkBitmap mask;
            int w = SkScalarRound(r.width());
            int h = SkScalarRound(r.height());
            mask.setConfig(SkBitmap::kARGB_8888_Config, w, h);
            mask.allocPixels();
            mask.eraseColor(0);
            SkCanvas c(mask);
            SkRect bounds = r;
            bounds.offset(-bounds.fLeft, -bounds.fTop);
            c.drawOval(bounds, paint);
            
            paint.setPorterDuffXfermode(SkPorterDuff::kDstIn_Mode);
            canvas->drawBitmap(mask, r.fLeft, r.fTop, &paint);
        } else {
            SkPath p;
            p.addOval(r);
            p.setFillType(SkPath::kInverseWinding_FillType);
            paint.setPorterDuffXfermode(SkPorterDuff::kDstOut_Mode);
            canvas->drawPath(p, paint);
        }
    }

    void drawBG(SkCanvas* canvas) {
        canvas->drawColor(0xFFDDDDDD);
    }
    
    virtual void onDraw(SkCanvas* canvas) {
        this->drawBG(canvas);
        
        SkRect  r;
        r.set(SkIntToScalar(20), SkIntToScalar(20), SkIntToScalar(120), SkIntToScalar(120));
        canvas->saveLayer(&r, NULL, SkCanvas::kARGB_ClipLayer_SaveFlag);
        canvas->drawColor(SK_ColorRED);
        drawMask(canvas, r);
        canvas->restore();
    }
    
private:
    typedef SkView INHERITED;
};

///////////////////////////////////////////////////////////////////////////////

static SkView* MyFactory() { return new LayerMaskView; }
static SkViewRegister reg(MyFactory);