We focus in this paper on the problem of adding computer-generated objects in video sequences. A two-stage robust statistical method is used for computing the pose from model-image correspondences of tracked curves. This method is able to give a correct estimate of the pose even when tracking errors occur. However, if we want to add virtual objects in a scene area which does not contain (or contains few) model features, the reprojection error in this area is likely to be large. In order to improve the accuracy of the viewpoint, we use 2D keypoints that can be easily matched in two consecutive images. As the relationship between two matched points is a function of the camera motion, the viewpoint can be improved by minimizing a cost function which encompasses the reprojection error as well as the matching error between two frames. The reliability of the system is shown on the encrustation of a virtual car in a sequence of the Stanislas square.