1. Apparatus for use with an image that contains an object, comprising: an output device; and at least one computer processor configured to: automatically identify whether a given pixel within the image corresponds to a portion of the object by: sampling, from the image, a set of concentric circles that are disposed around the pixel; applying a first function to each of the circles such that the circles are defined by a first set of rotationally invariant descriptors; applying a second function to the set of circles such as to generate a second set of descriptors, each of the second set of descriptors representing a difference between respective pairs of the circles; applying a third function to the second set of descriptors, such that the second set of descriptors forms a second set of rotationally invariant descriptors, by becoming rotationally invariant; and identifying whether the given pixel corresponds to the portion of the object, based upon the first and second sets of rotationally invariant descriptors; and generate an output on the output device, at least partially in response to identifying that the given pixel corresponds to the portion of the object.