1. An apparatus comprising: an imager adapted to generate a first image comprising a two-dimensional projection of an object, and to subsequently generate a second image comprising a further two-dimensional projection of the object, while the object and the imager are moving relative to each other; a signal processor adapted to generate a two-dimensional representation of an object portion arbitrarily arranged within the object, and to receive information about a position and a shape of the object portion within the object and its relative motion so as to combine, on the basis of the information received, dots within image portions within the first image and within the second image which are associated with the object portion; and a determiner adapted to determine the relative motion of the object on the basis of information from the first and second images, the determiner being configured to determine the relative motion of the object on the basis of distinctive details of the object or a sample incorporated in the object.