public Bitmap toRoundBitmap(Bitmap bitmap) {

             //圆形图片宽高

             int width = bitmap.getWidth();

             int height = bitmap.getHeight();

             //正方形的边长

             int r = 0;

             //取最短边做边长

             if(width > height) {

                 r = height;

             } else {

                 r = width;

             }

             //构建一个bitmap

             Bitmap backgroundBmp = Bitmap.createBitmap(width,

                      height, Config.ARGB_8888);

             //new一个Canvas，在backgroundBmp上画图

             Canvas canvas = new Canvas(backgroundBmp);

             Paint paint = new Paint();

             //设置边缘光滑，去掉锯齿

             paint.setAntiAlias(true);

             //宽高相等，即正方形

             RectF rect = new RectF(0, 0, r, r);

             //通过制定的rect画一个圆角矩形，当圆角X轴方向的半径等于Y轴方向的半径时，

             //且都等于r/2时，画出来的圆角矩形就是圆形

             canvas.drawRoundRect(rect, r/2, r/2, paint);

             //设置当两个图形相交时的模式，SRC_IN为取SRC图形相交的部分，多余的将被去掉

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

             //canvas将bitmap画在backgroundBmp上

             canvas.drawBitmap(bitmap, null, rect, paint);

             //返回已经绘画好的backgroundBmp

             return backgroundBmp;

         }