In this approach the model is stored as a 3-D volume of probabilities. There are three techniques that can be used to obtain a reprojected image. (i) The most correct approach is to integrate along each ray marginalising the probabilities using Bayes Rule. (ii) The next best approach is to use a maximum likelihood (ML) approach. In this case the single best voxel along each ray is selected. (iii) The final technique generates a conventional binary voxel array by classifying each voxel as being solid or empty.