I have OpenCV built with with_CUDA on so I don't know what the problem is, but I get the error below when I run the program:

start_vid:1gpuID:0flow method: 1 frameSkip: 1
OpenCV Error: No GPU support (The library is compiled without CUDA support) in setDevice, file /build/opencv-SviWsf/opencv-2.4.9.1+dfsg/modules/dynamicuda/include/opencv2/dynamicuda/dynamicuda.hpp, line 84
terminate called after throwing an instance of 'cv::Exception'
what(): /build/opencv-SviWsf/opencv-2.4.9.1+dfsg/modules/dynamicuda/include/opencv2/dynamicuda/dynamicuda.hpp:84: error: (-216) The library is compiled without CUDA support in function setDevice

Aborted (core dumped)