Modeling , Animation , and Rendering of Gases , Fluids , and Textures

This chapter describes modeling and animating procedural textures, gases, and liquids. The main thrust of the chapter is on procedurally modeling and animating volume density functions for creating realistic images and animations of gases and fluids. A volume density function is a three-dimensional function (vfd(x; y; z)) that defines the density of a continuous three-dimensional space. By using volume rendering techniques[5], volume density functions can create volumetric gases and fluids. Volume density functions are the natural extension of solid texturing to describing the actual geometry of the object. Volume density functions are being used extensively in computer graphics for modeling and animating gases, fire, fur, liquids, and other ‘‘soft’’ objects. I have used them extensively for modeling and animating gases such as steam and fog [2, 7, 5, 3]. Hypertextures [18], metaballs [22](also called implicit surfaces and soft objects), and Inakage’s flames [10] are other examples of the use of volume density functions. As mentioned above, this chapter described how to model and animate volumetric gases. Therefore, I should first explain why we want to model and animate gases in computer graphics. There are two reasons. First, we need gases for visual realism. Gases such as fog, steam, smoke, and clouds are a part of our everyday environment. In order to create realistic images of our environment, these gases must be included. Both indoor and outdoor scenes benefit from the addition of gases. The realism and mood of outdoor scenes, such as a dark, dreary forest can be increased greatly by the addition of elements such as fog. Realism of indoor scenes can also be enhanced by the inclusion of steam rising from a cup of coffee or smoke from a fireplace. Second, gases can be used for artistic effects. As the movie director uses fog machines and similar devices to set the stage of his drama, so can the computer animator use computer generated fog (and other gases) for dramatic effects. There have been several previous approaches to modeling gases in computer graphics. Kajiya, [11], has used a simple physical approximation for the formation and animation of clouds. Gardner, [8], has use solid textured hollow ellipsoids in modeling clouds and more recently produced animations of smoke rising from a forest fire [9]. Other approaches include the use of height fields [14], constant density media [13, 15], and fractals [21]. The author has developed several approaches for modeling and controlling the animation of