Computing filters on sub images: border handling

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I have the following use case.

I preprocess an image so that I know for which parts that I want to do the complete (expensive) processing. I therefore create several cv::Mat from my main cv::Mat and do filtering only on those. However, on boundary pixels I know the filter could very nicely go across the border of the sub-cv::Mat I have created, and compute the exact value for my filtering also for the border pixels. So my question is: Is there among the border handling modes a way to say to simply peak a pixel outside the cv::Mat? It seems only thing like constant, mirror or wrap are available...