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I am trying to calculate HOG descriptors for blocks only. Here is the code.

void cv::gpu::HOGDescriptor::getDescriptors(const GpuMat& img, Size win_stride, GpuMat& descriptors, int descr_format)
{
    CV_Assert(win_stride.width % block_stride.width == 0 && win_stride.height % block_stride.height == 0);

    computeBlockHistograms(img);

    const size_t block_hist_size = getBlockHistogramSize();
    Size blocks_per_win = numPartsWithin(win_size, block_size, block_stride);
    Size wins_per_img   = numPartsWithin(img.size(), win_size, win_stride);

    Size blocks_per_img = numPartsWithin(img.size(), block_size, block_stride);

    descriptors.create(wins_per_img.area(), static_cast<int>(blocks_per_win.area() * block_hist_size), CV_32F);

    switch (descr_format)
    {
    case DESCR_FORMAT_ROW_BY_ROW:
        hog::extract_descrs_by_rows(win_size.height, win_size.width, block_stride.height, block_stride.width,
                                    win_stride.height, win_stride.width, img.rows, img.cols, block_hists.ptr<float>(), descriptors);
        break;
    case DESCR_FORMAT_COL_BY_COL:
        hog::extract_descrs_by_cols(win_size.height, win_size.width, block_stride.height, block_stride.width,
                                    win_stride.height, win_stride.width, img.rows, img.cols, block_hists.ptr<float>(), descriptors);
        break;
    default:
        CV_Error(CV_StsBadArg, "Unknown descriptor format");
    }
}

I want to comment the following part of the code.

switch (descr_format)
{
case DESCR_FORMAT_ROW_BY_ROW:
    hog::extract_descrs_by_rows(win_size.height, win_size.width, block_stride.height, block_stride.width,
                                win_stride.height, win_stride.width, img.rows, img.cols, block_hists.ptr<float>(), descriptors);
    break;
case DESCR_FORMAT_COL_BY_COL:
    hog::extract_descrs_by_cols(win_size.height, win_size.width, block_stride.height, block_stride.width,
                                win_stride.height, win_stride.width, img.rows, img.cols, block_hists.ptr<float>(), descriptors);
    break;
default:
    CV_Error(CV_StsBadArg, "Unknown descriptor format");
}

Can some one help me to download the data in block_hists.ptr<float>() to CPU. I know I have to transfer block_hists.ptr<float>() from the device to the host using cudaMemcpyDeviceToHost