A method of registering a digital image with a polygonal mesh comprising the steps of providing a digital image, providing a polygonal mesh comprising a plurality of vertices that approximates an object in the digital image, propagating a ray from a vertex of the polygonal mesh into the digital image and calculating a line integral of opacity values wherein each opacity value corresponds to the intensity of a pixel of the image through which said ray passes, terminating the ray at a point in the image when an opacity threshold is reached, and adding the termination point to a set of closest points. A registration is computed between the vertices of the polygonal mesh and the set of closest points, and the registration is applied to the polygonal mesh to obtain a new set of vertices.