  The number of holes in a connected component in 2D images is a basic
invariant. In this note, a simple formula was proven using our previous results
in digital topology (Chen 2004, Chen and Rong (2010). The new is: $h =1+
(|C_4|-|C_2|)/4$, where h is the number of holes, and $C_i$ indicate the set of
corner points having $i$ direct adjacent points in the component.
