How can I track humans in a video stream?

System.Drawing.Rectangle[] regions;

if (GpuInvoke.HasCuda)
{
    GpuHOGDescriptor des = new GpuHOGDescriptor();

    des.SetSVMDetector(GpuHOGDescriptor.GetDefaultPeopleDetector());

    using (GpuImage<Bgr, Byte> gpuImg = new GpuImage<Bgr, byte>(frame))
        using (GpuImage<Bgra, Byte> gpuBgra = gpuImg.Convert<Bgra, Byte>())
        {

            regions = des.DetectMultiScale(gpuBgra, 0, new Size(8, 8), new Size(0, 0), 1.04, 8);

            foreach (var item in regions)
            {
                frame.Draw(new Rectangle(item), new Bgr(Color.Red), 1);
            }
        }
        des.Dispose();
    }
    else
    {  //this is the CPU version
        using (HOGDescriptor des = new HOGDescriptor())
        {
            des.SetSVMDetector(HOGDescriptor.GetDefaultPeopleDetector());

            regions = des.DetectMultiScale(frame);
        }
    }
}