The Performance of the Generalised Hough Transform: Concavities, Ambiguities and Positional Accuracy

This paper studies the use of the generalised Hough transform to locate objects possessing concavities. Hhen locating objects with internal symmetries, ambiguities can arise which nay not be resolvable in the presence of occlusions. On the other hand concavities can help to lake object location more accurate. The boundary orientation distribution can be used to analyse the situation: it shows that enhanced accuracy of location along one axis may result in reduced accuracy in a perpendicular direction. Finally, accuracy of location is independent of the position of the object localisation point. This paper starts with the observation that a problem arises when the GHT is used to detect objects possessing concavities. In principle this can lead to ambiguities in the location of such objects, and it was felt necessary to investigate the problem closely in order to find how its effects could be minimised. In section II we briefly describe the operation of the GHT. Then in section III we consider the problem posed by concavities, following this in section IV by a discussion of the effects of symmetries. In sections V and VI we investigate how the accuracy with which an object can be located is affected by its shape and by errors in the estimation of edge orientation.