Introduction

The advent of photography and computer graphics has not replaced artists. Imagery generated by artists provides information about objects that may not be readily apparent in photographs or real life. The same goal should apply to computer-generated images. This is the driving force behind non-photorealistic rendering. The term non-photorealistic rendering (NPR) is applied to imagery that looks as though it was made by artists, such as pen-and-ink or watercolor. Many computer graphics researchers are exploring NPR techniques as an alternative to photorealistic rendering. More importantly, non-photorealistic rendering is now being acknowledged for its ability to communicate the shape and structure of complex models. Techniques which have long been used by artists can emphasize specific features, expose subtle shape attributes, omit extraneous information, and convey material properties. These artistic techniques are the result of an evolutionary process, refined over centuries. Therefore, imitating some of these artistic methods and exploring the perceptual psychology behind the techniques of artists are good first steps in going beyond photorealistic rendering.

In these notes, we have gathered the ideas and methods from our previous work [11,12,13] to demonstrate how non-photorealistic rendering methods can be used to convey a more accurate representation of the shape and material properties of objects than traditional computer graphics methods. In order to demonstrate how non-photorealistic rendering can be used to communicate shape, we have explored computer-generated technical illustrations.