Important: The following tutorial was written back in 2010.
A lot of it still makes sense, but some parts should (and will eventually) be revised to include the concept of PBR materials.
Brief Considerations About Materials
I originally made this tutorial for a lecture I gave at CCAA, a school in Brazil where I help running the Post-Graduation Course on Game Art.
It came from the necessity to explain some elements of the materials that are commonly misunderstood. Especially about Specular Maps that, sometimes, seem to be a bit more obscure for most people.
Most artists, in general, have a pretty good understanding of diffuse, normals, alpha, but it’s often on the specular map that they will get lost and some times even ruin a pretty good asset.