DirectX® 9.0 and SmartShader™ 2.0 White Paper

Bear

This demonstration showcases some of the advancements in real-time fur rendering made possible with DirectX 9.0. The bear's fur is generated using vertex and pixel shaders to implement an improved version of the established "shell and fin" fur rendering method. Besides looking considerably more realistic, these new shaders provide a much greater degree of control over the fur's appearance and behavior than was possible before.

Real-time fur rendering is accomplished by first using a vertex shader program to "dilate" a 3D model. This involves creating new shells or layers of polygons that completely encase the original model. The locations of the vertices in a new shell are determined by taking the co-ordinates of the vertices in the layer below and extending them outward in the direction of their normal vectors.

By applying this technique recursively, any number of shells can be applied to control the length and quality of the fur. The more shells, the longer the fur, but each additional shell significantly increases the polygon count of the scene, which can degrade performance if the number of shells gets too large. It is possible to increase the spacing between the shells to get longer fur with fewer polygons, but if the spacing becomes too large, the individual hairs will cease to appear solid, resulting in a much less realistic image.

Once the shells have been generated, a pre-generated fur texture is applied to each layer. These textures are largely transparent, except for a series of spots that correspond to individual hair strands. When these textures are closely stacked one on top of another, they produce the appearance of solid strands of hair. The effect can look quite realistic, as long as the shells are not viewed edge-on. Because the fur textures are only two-dimensional, they become invisible around the edges of a 3D model, where they are viewed edge-on.

To address this issue, the vertex shader must generate additional polygons called "fins" that radiate out from the polygon boundaries in the original model. The length of the fins corresponds to the distance of the farthest fur shell from the base model. The fins are then textured to appear partially transparent with individual strands of fur. The combination of the fins and shells ensure that the fur stays visible regardless of the viewing angle.

Fur Parameters

Fur can have a limitless variety of appearances. The fur textures used in the Bear demo were generated procedurally using a tool called FurGen, developed by ATI. This tool makes it possible to control a wide range of fur parameters, including:

• Number of shells
• Maximum height of fur
• Resolution (texture size)
• Density (hairs per unit area)
• Hair thickness at base and tip
• Hair length
• Direction of hair growth
• Hair curliness

Each of these parameters can be fixed for the entire surface, varied according to a random distribution, or sampled from other textures. The fur textures are all filtered and mip-mapped in order to avoid artifacts that can otherwise appear when the fur is viewed from very short or very long distances.

Anisotropic Per-pixel Lighting

Common materials like metal and plastic tend to reflect the same amount of light regardless of the viewing angle or the direction of the light. Fur, hair, and other materials made up of many small strands behave differently. These materials can reflect varying amounts of light (and thus appear shiny or dull) depending on the viewing angle and the location of the light source.

To faithfully reproduce this effect, an anisotropic lighting model is required. Given the incident light angle and viewing angle for each pixel, the pixel shaders used in the Bear demo look up a special texture that determines how reflective that pixel will be. This texture is generated at the same time as the other fur textures, and controls how lustrous or dull the fur will appear. Anisotropic lighting is essential to giving the fur its characteristic sheen as the bear moves its head.