Anisotropic Specularity

You may notice that a lot of metals have a "brushed" appearance to them, especially aluminum and stainless steel. This is caused by tiny scratches that soften the reflection of the metal. This may be done as a side effect of manufacturing, or for its visual design.

Brushed metal like this has a very distinctive visual behavior. Specular highlights are broadened, but only in the direction perpendicular to the brushing. This is known as anisotropic reflection. It's more common than you may expect, especially on fixtures like hand rails.

You can also see anisotropy in materials that are made of hair or threads; a horse's hide has a special sheen to it, and so does long straight hair. A popular Christmas ornament consists of a sphere with threads covering the surface neatly from pole to pole, which has a very distinctive look. A silk top hat is similar.

LightWave (and nearly all other 3D programs) do not attempt to shade anisotropic materials at all, but we have included the ability in Gaffer. It's not a common option to use, but when you do have a surface that needs it, there's simply no substitute.

You can define the direction and strength of the anisotropy, including linear, cylindrical, and radial "scratch" directions.

The images to the left show a wheel rim with and without anisotropic highlights. Notice in the bottom image how the shape of the specularity is dramatically different. This effect is especially evident in animations when your viewpoint is changing.

Note: The above images also show before and after examples of Gaffer's Area Light Shadows and Advanced Specularity Control. See related sections for more details.