1. Apparatus for determining model parameters, the apparatus comprising: an object model transformer configured to receive an object model of a known object and to perform a transformation of the object model based on a set of model parameters from a first frame of reference to a second frame of reference, to determine as result of this transformation a transformed object model comprising at least one region, the at least one region being associated to an object region of the object; a region comparator configured to receive the transformed object model and an image depicting the object, to determine for a selected region of the transformed object model a region-related similarity measure representative of a similarity between the selected region and an image section of the image associated to the selected region via a transformation-dependent mapping, wherein the similarity measure comprises a geometric similarity component and an image value similarity component; and a model parameter determiner configured to determine an updated set of model parameters on the basis of the region-related similarity measure and an optimization scheme.