Fast Opacity Control in Rendering of Volumetric CT Data

Balázs Csébfalvi, László Szirmay-Kalos, Gábor Márton
Department of Process Control, Technical University of Budapest,
Budapest, Muegyetem rkp. 11, H-1111, HUNGARY


In 3D rendering of volumetric CT data the alpha-blending technique can be used for modeling semi-transparent tissues. This method assigns different opacity values to different tissue types. In practice, only four materials can be separated according to their CT numbers (Hounsfield density): air, fat, soft tissue and bone. Application of alpha-blending rendering enhances the more important tissues, assigning high opacity value for important tissues and low opacity for less important ones. For each pixel of the image, a ray is traced and the colour of the appropriate pixel is computed as a sum of intersected voxel colours weighted by opacity values. However, this computation is very slow if the size of 3D array is big. Furthermore, if the opacity value of one of the tissues needs to be changed, then the complete time-consuming rendering must be repeated in order to recompute the new weighted sums. In order to allow fast control of opacity values, a new algorithm is presented that provides fast opacity and colour modification after preprocessing the volume with fixed point of view and light source locations.


Volume Rendering, Alpha blending, Ray Tracing, Medical Imaging, CT.