How OpenMP multi-threading can be utilized to parallelize an OpenCV based sequential program? Any example is not found.

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Suppose I have to find the negative of a large dimension image, say 1000x1000. If I simply perform pixel value subtraction from 255, it needs to process 10,00,000 numbers of pixels one by one in sequential. If I want to allow OpenMP multi-thread for performing the whole operation partially in several threads which are run on different cores simultaneously. This concept has to be implemented in OpenCV using OpenMP specifications. Need a startup program structure to work on parallel image processing. Your advices will be valuable for me. Thanks.