Model mammals: procedural textures help animals move in sync with their coats. (techwatch).(Unisinos' Marcelo Walter)

Where the tiger goes, so go its stripes. Unfortunately, this simple law of nature has no equally simple counterpart in computer graphics. In the digital realm, the tiger and its stripes (as with the giraffe and its patches and the leopard and its spots) are fundamentally independent of each other. A geometric model of a tiger will be mapped with a striped texture, but there is nothing natural about the union. "It is a typical divide-and-conquer idea, where a complex problem is divided into 2 simpler problems, but later we have to somehow integrate the two solutions," says Marcelo Walter, a computer graphics professor and researcher at Unisinos, a private university in Brazil.

"Texture mapping is a great idea, but in some cases it is hard to get the right result. The main problems are the distortions that can occur and the fact that we often do not have a good texture map to start with," says Walter. Moreover, he adds, "some care must be taken when animating texture-mapped deformable objects."

While effective for many applications, the two-step process breaks down when the geometry of the modeled object is highly complex, in which case it becomes difficult to map visual characteristics to every point on the surface. Also, implicit in the two-step approach is the assumption that there's no interplay between the processes that define shape and visual attributes. In fact, often, the visual appearance is the direct result of interaction between these processes.

A case in point are patterned animals such as zebras, giraffes, and tigers. "The pattern visible on the fur of an adult animal is the result of a much earlier process that took place while the animal was in the womb," says Walter. In such cases, he notes, "it is important to model not only the individual processes themselves, but also the interplay between the embryo growing and the pattern-formation process [in order to achieve a believable dynamic pattern]."

In an effort to achieve integration between such pattern-formation and geometry, Walter, with the help of the late Alain Fournier at the University of British Colombia and Daniel Menevaux of the Laboratoire SIC in France, has devised a method through which the visual attributes of an object are defined directly on the surface of that object while at the same time taking into account any dynamic change of shape due to growth or other reasons. The proof-of-concept application shows how a mammalian coat pattern can be generated by a biologically plausible model simulated on the surface of a changing geometry.

At the heart of the system is a procedural texturing model targeted to animal coat patterns that generates the texture on the surface of the object, avoiding the mapping step. The idea dates back to the introduction of reaction/diffusion textures in 1991. Unique to this implementation, however, is that it does not only synthesize the texture on the surface of the object, but also takes into account the interplay between geometry and texture. "In other words, we can morph the shape while the pattern is being formed," says Walter. "This introduces a new paradigm where changes in ...