  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.
