Shape ellipticity based on the first Hu moment invariant

•A new ellipticity measure, for planar shapes, is derived.•A fast computation is enabled by using the moments up to the second order.•New ellipticity measure is invariant with respect to similarity transformations.•The highest ellipticity value is obtained for the ellipses only.

In this paper we use the first Hu moment invariant to define a new ellipticity measure. The new ellipticity measure ranges over the interval (0,1](0,1] and picks the value 1 if and only if the measured shape is an ellipse. The measure is invariant with respect to translation, rotation and scaling transformations. It is straightforward and fast to compute.