Models for raytracing applications are usually supplied by 3D modeling tools, which predominantly work with NURBS surfaces. Therefore, a model has to be tessellated into triangles before being raytraced. During the tessellation process some shape information is lost. If the initial model is represented by trimmed NURBS surfaces, the tessellation process becomes more complicated and often results in artifacts along trimming contours [Guthe et al. 2002]. Moreover, the tessellation of complex shape models results in a large number of triangles, which require large amount of memory and preprocessing time for building acceleration data structures.