/**
	 * 轉換圖片成圓形
	 * 
	 * @param bitmap
	 *            傳入Bitmap物件
	 * @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;
			left = 0;
			top = 0;
			right = width;
			bottom = 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);// 設定畫筆無鋸齒
 
		canvas.drawARGB(0, 0, 0, 0); // 填充整個Canvas
		paint.setColor(color);
 
		// 以下有兩種方法畫圓,drawRounRect和drawCircle
		// canvas.drawRoundRect(rectF, roundPx, roundPx, paint);// 畫圓角矩形，第一個引數為圖形顯示區域，第二個引數和第三個引數分別是水平圓角半徑和垂直圓角半徑。
		canvas.drawCircle(roundPx, roundPx, roundPx, paint);
 
		paint.setXfermode(new PorterDuffXfermode(Mode.SRC_IN));// 設定兩張圖片相交時的模式,參考http://trylovecatch.iteye.com/blog/1189452
		canvas.drawBitmap(bitmap, src, dst, paint); //以Mode.SRC_IN模式合併bitmap和已經draw了的Circle
		
		return output;
	}
 

原理圖：