图像合成Xfermode和任意形状ImageView的示例代码 http://www.jianshu.com/p/96c2bfc4f33a

RectImageView.java

import android.content.Context;
import android.graphics.Bitmap;
import android.graphics.Canvas;
import android.graphics.Color;
import android.graphics.Paint;
import android.graphics.PorterDuff;
import android.graphics.PorterDuffXfermode;
import android.graphics.Xfermode;
import android.util.AttributeSet;
import android.widget.ImageView;

/**
 * 图像合成Xfermode和任意形状ImageView的示例代码
 * http://www.jianshu.com/p/96c2bfc4f33a
 * 注: 在边长计算中假设了`ImageView`本身是一个正方形
 */

public class RectImageView extends ImageView {
    private Paint mPaint;
    private Xfermode mXfermode;
    private Bitmap mRectMask;

    public RectImageView(Context context) {
        this(context, null);
    }

    public RectImageView(Context context, AttributeSet attrs) {
        this(context, attrs, 0);
    }

    public RectImageView(Context context, AttributeSet attrs, int defStyleAttr) {
        super(context, attrs, defStyleAttr);
        mPaint = new Paint(Paint.ANTI_ALIAS_FLAG);
        mPaint.setColor(Color.RED);
        mXfermode = new PorterDuffXfermode(PorterDuff.Mode.DST_IN);
    }

    @Override
    protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
        super.onMeasure(widthMeasureSpec, heightMeasureSpec);
        createMask();
    }

    private void createMask() {
        if (mRectMask == null) {
            int maskWidth = getMeasuredWidth();
            int maskHeight = getMeasuredHeight();
            mRectMask = Bitmap.createBitmap(maskWidth, maskHeight, Bitmap.Config.ALPHA_8);
            Canvas canvas = new Canvas(mRectMask);
            canvas.translate(maskWidth / 2, 0);
            canvas.rotate(45);
            int rectSize = (int) (maskWidth / 2 / Math.sin(Math.toRadians(45)));
            canvas.drawRect(0, 0, rectSize, rectSize, mPaint);
        }
    }


    @Override
    protected void onDraw(Canvas canvas) {
        int id = canvas.saveLayer(0, 0, canvas.getWidth(), canvas.getHeight(), null, Canvas.ALL_SAVE_FLAG);
        super.onDraw(canvas);
        mPaint.setXfermode(mXfermode);
        canvas.drawBitmap(mRectMask, 0, 0, mPaint);
        mPaint.setXfermode(null);
        canvas.restoreToCount(id);
    }
}