Incremental generation of projections of CT-volumes

In order to achieve the illusion of rotation of an object in a 3-D image (density volume), a sequence of projections of the image must be computed. A method that alleviates the need to perform a full-fledged ray-casting operation for each projection in the sequence when the projections are depth coded (range images) is described. The method proceeds incrementally exploiting the coherence between successive projections. The idea behind the incremental method (INC) is that speed can be gained if costly ray-casting methods are as far as possible replaced by 2-D geometric transformations of the currently visible surface points. As is shown, INC significantly reduces the number of ray-casting operations needed. Experiments indicate that a speed up factor of about 10 is achievable. Increasing the angle increment φ diminishes the coherence between successive projections, and thus more rays are required. As φ is further increased, INC gradually degenerates into full ray casting