android 把图片状态由彩色变成灰色

 public static final Bitmap grey(Bitmap bitmap) {
        int width = bitmap.getWidth();
        int height = bitmap.getHeight();

        Bitmap faceIconGreyBitmap = Bitmap
                .createBitmap(width, height, Bitmap.Config.ARGB_8888);

        Canvas canvas = new Canvas(faceIconGreyBitmap);
        Paint paint = new Paint();
        ColorMatrix colorMatrix = new ColorMatrix();
        colorMatrix.setSaturation(0);
        ColorMatrixColorFilter colorMatrixFilter = new ColorMatrixColorFilter(
                colorMatrix);
        paint.setColorFilter(colorMatrixFilter);
        canvas.drawBitmap(bitmap, 0, 0, paint);
        return faceIconGreyBitmap;
    }