Ray tracing NPR-style feature lines

Though the goal of ray tracing and other physically based rendering techniques is ultimately to produce photorealistic images, it is often helpful to use non-photorealistic rendering techniques to illustrate or highlight certain features in a rendering. We present a method for ray tracing constant screen-width NPR-style feature lines on top of regularly rendered scenes, demonstrating how a variant of line rasterization can be included in a ray tracer, thus allowing for the inclusion of NPR-style enhancements. We are able to render silhouette edges, marking the boundary of an object in screen space against the background (or against farther parts of the same object), intersection lines, marking the curves along which two primitives intersect, and crease edges, indicating curves along which a primitivepsilas normal field is dicontinuous. Including these lines gives the viewer an additional cue to relative positions of objects within the scene, and also enhances particular features within objects, such as sharp corners. The method in this paper was developed in particular for enhancing glyph-based scientific visualization; however, the basic technique can be adapted for many illustrative purposes in different settings.