1. A computer implemented method of determining a rigid transformation for matching a position of an object with a position of a target object represented by a target model comprising data identifying 3D positions of a set of vertices of a triangulated 3D wire mesh and connectivity indicative of connections between vertices, the method comprising: obtaining stereoscopic images of an object; processing the stereoscopic images utilizing a computer to identify 3D positions of a plurality of points on a surface of the imaged object; generating an array identifying, for a regular grid of points in 3D space, the vertices of the target model closest to those grid points by: for each vertex in the target model, using 3D co-ordinates associated with the vertex of the target model to identify grid points of the regular 3D grid corresponding to corners of a cube that contains the location of the vertex, determining a distance between the vertex and the positions corresponding to the identified grid points, and determining whether said identified grid points are associated with data indicative of the grid points being closer to another vertex in the target model and if not associating said grid points with data identifying the vertex and the distance between the vertex and said grid points; and traversing the regular 3D grid in each of the directions corresponding to the axes of the regular 3D grid and determining when traversing the regular 3D grid, whether a vertex associated with a neighboring grid point on the regular 3D grid in a direction of traversal is closer to the position associated with a current grid point than any vertex previously associated with that grid point and if so associating the grid point with data identifying the vertex associated with the neighboring grid point; identifying, for each point on the surface of the imaged object, grid points of the regular 3D grid corresponding to the corners of a cube that contains the point on the surface of the imaged object; utilizing the vertices associated with the identified grid points by the array to identify triangles in the target model surface closest to each of said points on the surface of the imaged object; utilizing the computer to select a set of the identified 3D positions on the surface of the imaged object as points to be utilized to determine a rigid transformation for matching the position of the object with the position of the target object on the basis of the determined distances between the identified 3D positions and vertices of the target model identified as being closest to said positions; and utilizing the computer to calculate a rigid transformation which minimizes point to plane distances between the identified set of 3D positions and the planes containing the triangles of the target model surface identified as being closest to said positions.