When I run OpenCL-accelerated cv::HoughLines for horizontal lines and provide min_theta/max_theta parameters, it does not find lines. But without min_theta/max_theta (i.e. default 0/CV_PI) it finds ...

OpenCV 4.0 comes with an experimental Graph API module (see opencv/modules/gapi). This is a new API which allows to enable offload and optimizations for image processing / CV algorithms on pipeline ...