A method and apparatus are provided for determining a descriptor for a region of interest in an image. The method includes defining a grid of points for the region of interest. A respective zone of influence is associated with each point of the grid of points. A representative value is determined for each point of the grid of points as a function of its respective zone of influence. The representative value obtained for each point of the grid of points is compared with at least one reference value. The result of the comparison serves to define a descriptor for the region of interest.