android 图片去色返回灰度图片

public static Bitmap toGrayScale(Bitmap bm) {
        int width, height;
        height = bmp.getHeight();//图片高度
        width = bmp.getWidth();
        Bitmap bmpGrayscale = Bitmap.createBitmap(width, height, Bitmap.Config.RGB_565);
        Canvas c = new Canvas(bmpGrayscale);
        Paint paint = new Paint();
        ColorMatrix cm = new ColorMatrix();
        cm.setSaturation(0);
        ColorMatrixColorFilter f = new ColorMatrixColorFilter(cm);
        paint.setColorFilter(f);
        c.drawBitmap(bmp, 0, 0, paint);
        return bmpGrayscale;
    }