Calculating entropy with octomap representation?

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Hello,

I am trying to implement concepts such as entropy of a map and information gain through 3D sensing, where I am storing my map data as an Octomap. I am a beginner when it comes to concepts like this, so I have been reading up recently on information theory and how the concepts of entropy, mutual information etc. work, but I am failing to grasp how to implement something like that practically. Given a certain scene and associated Octomap data, is the whole set of cells treated as a probability distribution which then can be used to determine entropy? Or is there a pre programmed straightforward way of determining the entropy of the map at a given instant? Any suggestions/inputs would be very welcome.

Thanks, Sai