i have an application where the the object of interest is rotating, i have to find the speed of the object and rotation angle
here below are few images, full video can be found here 
i have an idea to proceed,
as my object of interest is moving i can just select my area of interest or create a sub image
after sub image is created i shall calculate centroid of white object
i shall keep on tracking all points until it completes its revolution
once done i can find radius
but how to calculate the rotation angle
i have done basic open cv course of using functions, but i havent come across any functions which calculate angles bewteen three points, i need some help in solving my problem