How do we find which function belongs to which opencv library?

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Okay steps are simple

• Go to http://docs.opencv.org/
• Hit the OpenCV version you use, but the top one should be your choice by default
• Add calcOpticalFlowFarneback inside the search box at the top right of the page
• Hit enter and click on one of the search results
• Scroll to the top of the page, which states which module should contain this
• Now go to the corresponding module, in this case, it is the Object Tracking part of the video analysis module which can be found here.
• So include should be #include "opencv2/video.hpp"

Or if you really only want the tracking part, you can go for #include "opencv2/video/tracking.hpp" which will allow you to use those functions but not the background foreground segmentation functionality.