Assisting a User: Modeling and Graftals
Non-photorealistic rendering isn't only about the images. NPR has also had an effect on geometric modeling. In this chapter we review several systems that use a two-dimensional sketching paradigm to create three-dimensional geometric models. Robert C. Zeleznik, Kenneth P. Herndon, and John F. Hughes [Zeleznik et al. 96] introduce the SKETCH system, which combines NPR with gesture recognition to create three-dimensional geometric models. Takeo Igarashi, Satoshi Matsuoka, and Hidehiko Tanaka's Teddy system allows a user to draw the silhouette of an object, which the system uses to create a three-dimensional polygon mesh [Igarashi et al. 99]. Taking motivation from the children's book "Harold and the Purple Crayon," [Johnson 77] Jonathan M. Cohen, John F. Hughes, and Robert C. Zeleznik have built an immersive system that allows users to draw their own world and move around in it [Cohen et al. 00]. Lee Markosian, Jonathan M. Cohen, Thomas Crulli, and John F. Hughes [Markosian, et al. 99] present a particle-based system for modeling smooth polygonal surfaces. A user of their "Skin" system interactively guides a particles system to grow over a given collection of polyhedral elements yielding smooth surfaces. Sidney W. Wang and Arie E. Kaufman [Wang, Kaufman 95] present a volumemodeling technique based on a sculpting metaphor. Sculpting is accomplished with their system using three-dimensional tools in a volumetric voxel-based model.