On the integral formulas of Crofton and Hurwitz relative to the visual angle of a convex set

We provide a unified approach that encompasses some integral formulas for functions of the visual angle of a compact convex set due to Crofton, Hurwitz and Masotti. The basic tool is an integral formula that also allows us to integrate new functions of the visual angle. As well we establish some upper and lower bounds for the considered integrals, generalizing in particular those obtained by Santal\'o for Masotti's integral.