  A simple and efficient algorithm to numerically compute the genus of surfaces
of three-dimensional objects using the Euler characteristic formula is
presented. The algorithm applies to objects obtained by thresholding a scalar
field in a structured-collocated grid, and does not require any triangulation
of the data. This makes the algorithm fast, memory-efficient and suitable for
large datasets. Applications to the characterization of complex surfaces in
turbulent flows are presented to illustrate the method.
