Calculate Gunnar Farnebäck flow on a masked image

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Hello,

I'm wondering if it's possible to use masked images for flow calculation?

E.g. I'm assuming a non-rectangular (although connected and convex shaped without holes) image. There are some portions that should be ignored for flow calculation, as they lead to wrong matches.

What would be the best approach to calculate the flow, preferably a Gunnar Farnebäck flow here?

Removing all flow vectors that point "outside" of the image does not work.