One of the most important issues to address when trying to create illustrations is how to calculate the edge lines. Edge lines for polygonal objects can be generated interactively using the techniques of Markosian et al. [24]. In order to calculate edge lines for higher-order geometric models, like NURBS, the surfaces would have to be tessellated to apply Markosian's technique. On high-end systems, image-processing techniques [29] could be made interactive. In Section 4.1, I will discuss how silhouettes, surface boundaries, and discontinuities can be calculated for NURBS surface, as well as suggest some other techniques for calculating edge lines. After calculating edge lines, the illustrations are completed by considering a new shading model and material properties presented by Gooch et al. [15], summarized in Section 4.2. In Section 4.3, I will discuss the considerations that need to be made to create 3D technical illustrations.