10 Interactive Non - Photorealistic Rendering

Most of the work in NPR has been static 2D images or image sequences generated by a batch process. In this part of the course notes we explore interactive NPR through the example of interactive technical illustration [4]. Work that has been done on computer generated technical illustrations has focused on static images and has not included all of the techniques used to hand draw technical illustrations. We present a paradigm for the display of technical illustrations in a dynamic environment. This display environment includes all of the benefits of computer generated technical illustrations such as a clearer picture of shape, structure, and material composition than traditional computer graphics methods. It also takes advantage of the threedimensional interactive strength of modern display systems. This is accomplished by using new algorithms for real time drawing of silhouette curves, algorithms which solve a number of the problems inherent in previous methods. We incorporate current non-photorealistic lighting methods, and augment them with new shadowing algorithms based on accepted techniques used by artists and studies carried out in human perception. An interactive NPR system needs the capability to interactively display a custom shading model and edge lines. In addition, this interaction must be possible for complex geometric models. In this part of the course notes we describe a variety of techniques for achieving these goals, and describe the tradeoffs involved in choosing a particular technique.