public static Bitmap toRoundImage(Bitmap bitmap) {

    float roundPx = 0;
    int rectStartX=0;
    int rectStartY=0;
    Bitmap output = Bitmap.createBitmap(bitmap.getWidth(),
            bitmap.getHeight(), Bitmap.Config.ARGB_8888);

    Rect rect=null;

/**

擷取寬高較小的圖片，就是擷取正中間的正方形

*/
    if (bitmap.getWidth()>bitmap.getHeight()){
        roundPx=bitmap.getHeight()/2
 
;//半徑
        rectStartX=(bitmap.getWidth()-bitmap.getHeight())/2; //矩形x開始位置
        rect = new Rect(rectStartX, 0, bitmap.getWidth()-rectStartX, bitmap.getHeight());//取中間部分
    }else{
        roundPx=bitmap.getWidth()/2;//半徑
        rectStartY=(bitmap.getHeight()-bitmap.getWidth())/2; //矩形Y開始位置
        rect = new 
 
Rect(0, rectStartY, bitmap.getWidth(), bitmap.getHeight()-rectStartY);//取中間部分
    }

    Canvas canvas = new Canvas(output);
    int color = 0xff424242;
    Paint paint = new Paint();

    RectF rectF = new RectF(rect);

    paint.setAntiAlias(true);
    canvas.drawARGB(0, 0, 0, 0);
    paint.setColor(color);
    canvas.drawRoundRect(rectF, roundPx, roundPx, paint);  //畫拐角，拐角的半徑為正方形的邊長一半
    paint.setXfermode(new 
 
PorterDuffXfermode(PorterDuff.Mode.SRC_IN));
    canvas.drawBitmap(bitmap, rect, rect, paint);
    return output;
}

//下面上圖來解釋，就一目瞭然