As per the documentation, the algorithm is described in papers [190] and [191].
[190]
Zoran Zivkovic and Ferdinand van der Heijden. Efficient adaptive density estimation per image pixel for the task of background subtraction. Pattern recognition letters, 27(7):773–780, 2006.
[191]
Zoran Zivkovic. Improved adaptive gaussian mixture model for background subtraction. In Pattern Recognition, 2004. ICPR 2004. Proceedings of the 17th International Conference on, volume 2, pages 28–31. IEEE, 2004.
