How can I change the device on wich OpenCL-code will be executed with Umat in OpenCV?

Question

As known, OpenCV 3.0 supports new class cv::Umat which provides Transparent API (TAPI) to use OpenCL automaticaly if it can: http://code.opencv.org/projects/opencv/wiki/Opencv3#tapi

There are two indtroductions to the cv::Umat and TAPI:

• Intel: https://software.intel.com/en-us/articles/opencv-30-architecture-guide-for-intel-inde-opencv
• AMD: http://developer.amd.com/community/blog/2014/10/15/opencv-3-0-transparent-api-opencl-acceleration/

But if I have:

1. Intel CPU Core i5 (Haswell) 4xCores (OpenCL Intel CPUs with SSE 4.1, SSE 4.2 or AVX support)
2. Intel Integrated HD Graphics which supports OpenCL 1.2
3. 1st nVidia GPU GeForce GTX 970 (Maxwell) which supports OpenCL 1.2 and CUDA
4. 2nd nVidia GPU GeForce GTX 970 ...

If I turn on OpenCL in OpenCV, then how can I change the device on wich OpenCL-code will be executed: on 8 Cores of CPU, on Integrated HD Graphics, on 1st nVidia GPU or 2nd nVidia GPU?

How can I select one of each of these 4 devices to use OpenCL for parallel execution algorithms with cv::Umat?

For example, how can I use OpenCL acceleration on 4xCores of CPU Core-i5 with cv::Umat?

Answer 1

I use something like this to check versions and hardware being used for OpenCL support.

ocl::setUseOpenCL(true);
if (!ocl::haveOpenCL())
{
    cout << "OpenCL is not available..." << endl;
    //return;
}

cv::ocl::Context context;
if (!context.create(cv::ocl::Device::TYPE_GPU))
{
    cout << "Failed creating the context..." << endl;
    //return;
}

cout << context.ndevices() << " GPU devices are detected." << endl; //This bit provides an overview of the OpenCL devices you have in your computer
for (int i = 0; i < context.ndevices(); i++)
{
    cv::ocl::Device device = context.device(i);
    cout << "name:              " << device.name() << endl;
    cout << "available:         " << device.available() << endl;
    cout << "imageSupport:      " << device.imageSupport() << endl;
    cout << "OpenCL_C_Version:  " << device.OpenCL_C_Version() << endl;
    cout << endl;
}

Then you can set your preferred hardware to use, using this

cv::ocl::Device(context.device(1));

Hope this helps you.

Answer 2

You can also set a desired OpenCL device from within your code using environment variable method as follows (example is first GPU device):

if (putenv("OPENCV_OPENCL_DEVICE=:GPU:0") != 0 || !cv::ocl::useOpenCL())
{
    std::cerr << "Failed to set a desired OpenCL device" << std::endl;
    std::cerr << "Press any key to exit..." << std::endl;
    getchar();
    return 1;
}

Call to cv::ocl::useOpenCL() will force OpenCV to setup a default OpenCL device to the one specified in the environment variable OPENCV_OPENCL_DEVICE which is setup prior to that call.

I checked that this actually happens by setting a break-point at opencv_core310d.dll!cv::ocl::selectOpenCLDevice() Line 2256 (opencv\source\modules\core\src\ocl.cpp):

static cl_device_id selectOpenCLDevice()
{
    std::string platform, deviceName;
    std::vector<std::string> deviceTypes;

    const char* configuration = getenv("OPENCV_OPENCL_DEVICE");
    if (configuration &&
            (strcmp(configuration, "disabled") == 0 ||
             !parseOpenCLDeviceConfiguration(std::string(configuration), platform, deviceTypes, deviceName)
            ))
        return NULL;