Function calculateHistogramUsingCL

example/opencv_histogram.cpp:127–181

Get the device context Create GPU array/vector Copy the image & set up the kernel Execute the kernel Copy GPU data back to CPU cv::Mat data pointer OpenCV conversion for convienient display

//Copy GPU data back to CPU cv::Mat data pointer
//OpenCV conversion for convienient display
void calculateHistogramUsingCL(cv::Mat src, compute::command_queue &queue)
{
    compute::context context = queue.get_context();

    // Convert image to BGRA (OpenCL requires 16-byte aligned data)
    cv::cvtColor(src, src, CV_BGR2BGRA);

    //3 channels & 256 bins : alpha channel is ignored
    compute::vector<int> gpu_b_hist(histSize, context);
    compute::vector<int> gpu_g_hist(histSize, context);
    compute::vector<int> gpu_r_hist(histSize, context);

    // Transfer image to gpu
    compute::image2d gpu_src =
            compute::opencv_create_image2d_with_mat(
                src, compute::image2d::read_only,
                queue
                );

    compute::program histogram_program =
            compute::program::create_with_source(source, context);
    histogram_program.build();

    // create histogram kernel and set arguments
    compute::kernel histogram_kernel(histogram_program, "histogram");
    histogram_kernel.set_arg(0, gpu_src);
    histogram_kernel.set_arg(1, gpu_b_hist.get_buffer());
    histogram_kernel.set_arg(2, gpu_g_hist.get_buffer());
    histogram_kernel.set_arg(3, gpu_r_hist.get_buffer());

    // run histogram kernel
    // each kernel thread updating red, green & blue bins
    size_t origin[2] = { 0, 0 };
    size_t region[2] = { gpu_src.width(),
                         gpu_src.height() };

    queue.enqueue_nd_range_kernel(histogram_kernel, 2, origin, region, 0);

    //Make sure kernel get executed and data copied back
    queue.finish();

    //create Mat and copy GPU bins to CPU memory
    cv::Mat b_hist(256, 1, CV_32SC1);
    compute::copy(gpu_b_hist.begin(), gpu_b_hist.end(), b_hist.data, queue);
    cv::Mat g_hist(256, 1, CV_32SC1);
    compute::copy(gpu_g_hist.begin(), gpu_g_hist.end(), g_hist.data, queue);
    cv::Mat r_hist(256, 1, CV_32SC1);
    compute::copy(gpu_r_hist.begin(), gpu_r_hist.end(), r_hist.data, queue);

    b_hist.convertTo(b_hist, CV_32FC1); //converted for displaying
    g_hist.convertTo(g_hist, CV_32FC1);
    r_hist.convertTo(r_hist, CV_32FC1);

    showHistogramWindow(b_hist, g_hist, r_hist, "Histogram");
}

int main( int argc, char** argv )
{