Teaching Texture Mapping Visually

Rosalee Wolfe
DePaul University
wolfe@cs.depaul.edu

Visually demonstrating the behavior of texture mapping is beneficial to both computer science and art students. For computer science students it can serve as prelude to delving into the mathematical underpinnings of the topic, while in an art class it can be the main vehicle of explanation for students learning to use a rendering package as a medium of creative expression. The SIGGRAPH 97 Education Slide set covers not only the techniques of two- and three-dimensional texture mapping, but procedural textures, bump mapping, and environment mapping as well. In addition it discusses aliasing in texture mapping, and what can be done to counteract it.

The following is a reprint of an article that appeared in the November 1997 issue of Computer Grapics and contains thumbnails and accompanying narrative for the slide set. To order the full color 35mm slide set, call ACM Member Services at 800 342 6626 in the U.S. and Canada, and +1 212 626 0500 for the greater New York area and all other countries. The ACM order number of the education slide set is 434975 (ISBN 0 89791 956-4) and the price is $35.00 for ACM/SIGGRAPH members and $45 for non-members.

1. Mapping techniques add realism and interest to computer graphics images. Texture mapping applies a pattern of color to an object. Bump mapping alters the surface of an object so that it appears rough, dented or pitted. In this example, the umbrella, background, beachball and beach blanket have texture maps. The sand has been bump mapped. These and other mapping techniques are the subject of this slide set.
2: When creating image detail, it is cheaper to employ mapping techniques that it is to use myriads of tiny polygons. The image on the right portrays a brick wall, a lawn and the sky. In actuality the wall was modeled as a rectangular solid, and the lawn and the sky were created from rectangles. The entire image contains eight polygons.Imagine the number of polygon it would require to model the blades of grass in the lawn! Texture mapping creates the appearance of grass without the cost of rendering thousands of polygons.
3: Knowing the difference between world coordinates and object coordinates is important when using mapping techniques. In object coordinates the origin and coordinate axes remain fixed relative to an object no matter how the object’s position and orientation change. Most mapping techniques use object coordinates. Normally, if a teapot’s spout is painted yellow, the spout should remain yellow as the teapot flies and tumbles through space. When using world coordinates, the pattern shifts on the object as the object moves through space.
4: Depending on the mapping situation, we may need to bound an object with a box, a cylinder, or a sphere. It’s often useful to transform the bounding geometry so its coordinates range between zero and one. Transformed bounding boxes have coordinates that range from (0,0,0) to (1,1,1). For a bounding cylinder, we set the circumference to one and the height to one. For a sphere, we scale the latitude and the longitude so that they both range between zero and one.
5: Texture mapping can be divided into two-dimensional and three-dimensional techniques. Two-dimensional techniques place a two-dimensional (flat) image onto an object using methods similar to pasting wallpaper onto an object. Three-dimensional techniques are analogous to carving the object from a block of marble.
6: Two-dimensional mappings use pre-existing images. This slide shows some images that might be used for texture mapping. The images on the left are either scanned photographs or images created in a paint or drawing package. POVRay, a raytracer, created the images on the right.

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