Camera view is blocked, uniform image

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Hi, how would you approach a problem of detecting if camera view is blocked?

Simplest solution is to calculate standard deviation and look for really low values, but it is good when whole view is blocked. What to do if for example 3/4 of the view is obstructed?

Other solution is to calculate percentage of pixels that are below some threshold. I am not convinced that is good enough, because of two reasons:

1. it is not easy to determine threshold for RGB image
2. pixels should lie in the same region