android 画图之setXfermode 处理两图相交叉的情况

Canvas canvas = new Canvas(bitmap1);

paint.setXfermode(new PorterDuffXfermode(Mode.SRC_IN));

canvas.drawBitmap(mask, 0f, 0f, paint);  

package com.weidingqiang.testviewb;

import android.content.Context;
import android.graphics.Bitmap;
import android.graphics.BitmapFactory;
import android.graphics.Canvas;
import android.graphics.Color;
import android.graphics.Paint;
import android.graphics.Path;
import android.graphics.PorterDuff;
import android.graphics.PorterDuffXfermode;
import android.util.AttributeSet;
import android.view.MotionEvent;
import android.view.View;

/**
 * Created by weidingqiang on 15/11/20.
 */
public class XfermodeView extends View {

    private Bitmap mBgBitmap,mFgBitmap;
    private Paint mPaint;
    private Canvas mCanvas;
    private Path mPath;

    public XfermodeView(Context context) {
        super(context);
    }

    public XfermodeView(Context context, AttributeSet attrs) {
        super(context, attrs);
        init();
    }

    private void init(){
        mPaint = new Paint();
        mPaint.setAlpha(0);
        mPaint.setXfermode(new PorterDuffXfermode(PorterDuff.Mode.DST_IN));
        mPaint.setStyle(Paint.Style.STROKE);
        mPaint.setStrokeJoin(Paint.Join.ROUND);
        mPaint.setStrokeWidth(50);
        mPaint.setStrokeCap(Paint.Cap.ROUND);

        mPath = new Path();
        mBgBitmap = BitmapFactory.decodeResource(getResources(),R.mipmap.image_bg);
        mFgBitmap = Bitmap.createBitmap(mBgBitmap.getWidth(), mBgBitmap.getHeight(), Bitmap.Config.ARGB_8888);

        mCanvas = new Canvas(mFgBitmap);
        mCanvas.drawColor(Color.GRAY);
    }

    @Override
    public boolean onTouchEvent(MotionEvent event) {

        switch (event.getAction()){
            case MotionEvent.ACTION_DOWN:
                mPath.reset();
                mPath.moveTo(event.getX(),event.getY());
                break;
            case MotionEvent.ACTION_MOVE:
                mPath.lineTo(event.getX(),event.getY());
                break;
        }

        mCanvas.drawPath(mPath,mPaint);
        invalidate();
        return true;
    }

    @Override
    protected void onDraw(Canvas canvas) {
        canvas.drawBitmap(mBgBitmap,0,0,null);
        canvas.drawBitmap(mFgBitmap,0,0,null);
    }
}