Random crashes when OpenCV is trying to use OpenCL

[phlash]

I've recently discovered that as of v3.x OpenCV will silently attempt to use OpenCL (and thus the GPU) if it finds the required libraries on the system (https://opencv.org/opencl/). Unfortunately operating multiple threads in OpenCV (eg: 2 x video capture streams + processing, in particular cv::cvtColor() calls) appears to randomly cause errors or segmentation faults in the OpenCL backend

Workaround: set the following environment variable to disable this silent use of OpenCL

% OPENCV_OPENCL_RUNTIME=disabled backscrub ...

NB: this wasted a good hour of my time today, as I thought the problem was in my code :angry:

[floe]

Oh, good to know, should we put this in a Troubleshooting section in the README maybe?

[phlash]

Retesting needed on this after a year I think. We could force the environment variable setting in the app if it's still an issue?