Apparatus for determining model parameters, the apparatus comprising an object model transformer, a region comparator, and a model parameter determiner. The object model transformer is configured to receive an object model of a known object and to transform the object model based on a set of model parameters from a first frame of reference to a second frame of reference, and is further configured to determine as result of this transformation a transformed object model comprising at least one region that is associated to an object region of the object. The region comparator is 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. The model parameter determiner is configured to determine an updated set of model parameters on the basis of the region-related similarity measure and an optimization scheme.