/**
* 转换图片成圆形
* @param bitmap 传入Bitmap对象
* @return
*/
public Bitmap toRoundBitmap(Bitmap bitmap) {
    int width = bitmap.getWidth();
    int height = bitmap.getHeight();
    float roundPx;
    float left, top, right, bottom, dst_left, dst_top, dst_right, dst_bottom;
    if (width <= height) {
        roundPx = width / 2;
        top = 0;
        bottom = width;
        left = 0;
        right = width;
        height = width;
        dst_left = 0;
        dst_top = 0;
        dst_right = width;
        dst_bottom = width;
    } else {
        roundPx = height / 2;
        float clip = (width - height) / 2;
        left = clip;
        right = width - clip;
        top = 0;
        bottom = height;
        width = height;
        dst_left = 0;
        dst_top = 0;
        dst_right = height;
        dst_bottom = height;
    }
    Bitmap output = Bitmap.createBitmap(width,
                                        height, Config.ARGB_8888);
    Canvas canvas = new Canvas(output);
    final int color = 0xff424242;
    final Paint paint = new Paint();
    final Rect src = new Rect((int)left, (int)top, (int)right, (int)bottom);
    final Rect dst = new Rect((int)dst_left, (int)dst_top, (int)dst_right, (int)dst_bottom);
    final RectF rectF = new RectF(dst);
    paint.setAntiAlias(true);
    canvas.drawARGB(0, 0, 0, 0);
    paint.setColor(color);
    canvas.drawRoundRect(rectF, roundPx, roundPx, paint);
    paint.setXfermode(new PorterDuffXfermode(Mode.SRC_IN));
    canvas.drawBitmap(bitmap, src, dst, paint);
    return output;
}