1. A computer-implemented method of calculating pose or shape of an articulated or deformable entity comprising: receiving at least one image of the entity; accessing a model of a class of articulated or deformable entities of which the imaged entity is a member, the model comprising a plurality of parameters specifying the pose or shape of the model; using the image to access a plurality of candidate correspondences between image elements of the received image and model points which are locations on or in the model; carrying out an optimization process to find values of the parameters specifying the pose or shape of the model which agrees with the received image; and where the optimization is influenced by at least some of the candidate correspondences; and at least one of accessing the candidate correspondences from a random decision forest arranged to take image elements of the received image and, for a plurality of image elements of the received image, calculate a probability distribution over candidate correspondences using information associated with its leaves, or carrying out the optimization process by optimizing an energy function, the energy function being over agreement between the model and the received image, where the energy function omits model points which are not visible from a view point of an image capture device which captured the received image by taking into account the direction of surface normals of the model.