Just use OpenCV, it will take care of parallelism for you. Most of the code in OpenCV already makes a good use of OpenMP so if your machine supports these features, they will be utilized heavily by the framework itself.
Read these threads for answer,
Does OpenCV functions really use OpenMP - OpenCV Q&A Forum[
^],
How opencv use openmp thread to get performance? - OpenCV Q&A Forum[
^],
Where is OpenMP used in OpenCV? - OpenCV Q&A Forum[
^]. If you search for (like I did), "opencv and openmp", Google will produce even more results that tell you that the framework itself is using these features for you. But if you want to add more, you are always welcome.
Otherwise, you can embed the OpenCV statements in multiple parallel threads in OpenMP; where you think it can be added.
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