Chapter 4: Procedural Modeling and Animation of Gases and Fluids

This chapter describes modeling and animating procedural gases, liquids, and textures. 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, 5, 4, 3]. Hypertextures [16], metaballs [20](also called implicit surfaces and soft objects), and Inakage’s flames [8] are other examples of the use of volume density functions. Gases such as fog, steam, smoke, and clouds are a part of our everyday environment. Therefore, in order to create realistic images representing this 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. In indoor scenes, realism can also be enhanced by the inclusion of steam rising from a cup of coffee or smoke from a fireplace. There have been several previous approaches to modeling gases in computer graphics. Kajiya, [9], has used a simple physical approximation for the formation and animation of clouds. Gardner, [6], has use solid textured hollow ellipsoids in modeling clouds and more recently produced animations of smoke rising from a forest fire [7]. Other approaches include the use of height fields [12], constant density media [11, 13], and fractals [19]. The author has developed several approaches for modeling and controlling the animation of gases [2, 5, 4, 3, 1]. Recently, Stam has used ‘‘fuzzy blobbies’’ as a three-dimensional model for animating gases with good results [18]. The purpose of these notes is to describe the design approach taken by the author for modeling, rendering, and animating gases. These notes will help explain how this approach developed and also show the development of several example procedures. In the discussion that follows, an overview of gas rendering issues are discussed. Next, a brief description of the development of my approach to modeling and animating gases, called solid spaces is presented, followed by an in-depth description of the modeling of the gases and fluids. Finally, animation techniques for solid textures, hypertextures, and gases are thoroughly discussed, including detailed descriptions of several example procedures.