void filter2(const cv::Mat &image, cv::Mat &out)
{
    CV_Assert(!image.empty() && (image.type() == CV_8UC3));
    out.create(image.rows / 2, image.cols / 2, image.type());
    for (int i = 0; i < out.rows; i++) {
        for (int j = 0; j < out.cols; j++) {
            out.at<cv::Vec3b>(i, j) = image.at<cv::Vec3b>(i * 2, j * 2);
        }
    }
}