Surface discontinuity modelling by LSM through patch adaptation and use of edges

Abstract

This paper handles the aspect of least squares matching (LSM) for DSM generation and focuses mainly on the exploitation of edge information in the least squares approach. The mathematical model of LSM employs an affine transformation to model geometrical distortions between the template and the search images. However, the model, when used for single points that lie close or on edges does not suffice to model discontinuities, especially when patch size is large. To alleviate the above-mentioned problem, LSM is firstly extended to edgels using an approximation of the rotation between search image and template, computed from a signal analysis. The signal analysis is performed by computing the signal ellipses from the template and patch grey levels. Furthermore, the template patch can be pre-rotated and its dimensions can be adjusted, so that the patch becomes a thin ribbon along the edge. Secondly, the model is extended to edge features with topological attributes (edges and vertices with link information). The patch size is adapted to include a whole edge and a non-uniform weighting scheme is utilised in the normal equations of LSM, giving a very low weight to all pixels outside the given edge. After matching the edge middle points, the edge is divided in two segments and the procedure is repeated until the edge segments fall below a certain length threshold. The final stage is the matching of the edge end points, where other edges having the same end point are also taken into account. The proposed techniques are compared to each other and to standard LSM. An evaluation of the methods is done with ADS40 imagery. As it is shown, they lead to less discontinuity smoothing and increase of the success rate and accuracy. * Corresponding author. 1