opencv CV:mat 转为NCHW

一、很多时候模型输入都是NCHW，opecv加载图片的时候需要转换一下，代码如下

    cv::Mat frame = cv::imread(filePath);
    cv::resize(frame, frame, cv::Size(640, 640));
    cv::Mat rgb_image;
    cv::cvtColor(frame, rgb_image, cv::COLOR_BGR2RGB);

    // 转换为 NCHW 格式
    cv::Mat nchw_image;
    {
        int height = rgb_image.rows;
        int width = rgb_image.cols;
        int channels = rgb_image.channels();

        // 将输入图像转换为 CHW 格式
        cv::Mat chw(height * width * channels, 1, CV_8UC1);
        int index = 0;

        for (int c = 0; c < channels; ++c) {
            for (int h = 0; h < height; ++h) {
                for (int w = 0; w < width; ++w) {
                    chw.at<uchar>(index++) = rgb_image.at<cv::Vec3b>(h, w)[c];
                }
            }
        }

        // 创建 NCHW 格式的输出图像
        nchw_image.create(1, channels * height * width, CV_8UC1);
        std::memcpy(nchw_image.data, chw.data, chw.total() * chw.elemSize());
    }

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