I originally made this tutorial for a lecture I gave at CCAA, a school in Brazil where I help to run the Post-Graduation course in Game Art. It came from the need to explain some elements of the materials that are commonly misunderstood. Especially about Specular maps which, sometimes, seem to be a bit confusing for some people.
Most artists, in general, have a pretty good understanding of Diffuse, Normal and Alpha maps, but it's often on the Specular map that they will get lost and sometimes even ruin a pretty good asset.
I hope you enjoy this tutorial.
Textures & Shaders
If the shape of an object, in 3D, is the result of its geometry, the material from which this object is made of is the result of its shader and textures. Shaders are instructions that tell the render how an object should be displayed. Textures are images that provide visual information to the shader. They can be extracted from real life photographs, they can be digital paintings, or even procedurally generated (Fig.01).
In this way, we use textures as inputs of a shader, defining attributes such as color, reflectivity, transparency, roughness, brightness and other characteristics of a possible material. Out of these textures, the most common are:
- Specular (and Specular Power - aka Glossiness)
Normals Maps will change the normals of a given surface. They will make the light bounce off the surface as if it had hit something other than just the polygons.
Alpha Maps define transparency. It can be on materials such as water, glass, fire, smoke... Or it can also be used for creating different silhouettes, gaps and rips on simple surfaces (Fig.02).
So, in some ways, if we disregard the truly transparent or translucent materials, like glass, where the transparency is at the core nature of the material, it would be ok to say that Alpha and Normal maps alter how we see the shape of an object more than the material itself.
Self-Illumination or Glow only defines parts of the object that emits light; that glow in the dark (Fig.03). Thus, transparency, embossing and brightness are material characteristics conferred by the respective use of the following maps: Alpha, Normals and Self-Illumination.
But what about all the other aspects? Those that really define the nature of a material. Is it light or dark? Smooth or rough? Clean or dirty? Reflective or matte? It is made of metal, clay, wood or plastic? Is it wet or dry? Rusty? Stained? Scratched? Old and worn, or brand new? All these variables, and many more, are controlled by two maps: Diffuse and Specular (well, maybe three maps, since there's still the Specular Power. But because this one only affects the specularity, I'll consider it as part of the Specular slot).
But what are they? Everyone who works with 3D has at least a basic understanding that Diffuse is the color of an object and Specular is the light reflected on its surface... the highlights. But this definition is somewhat simplistic. We must remember that everything we see around us in the world is only visible because they are reflecting light. In essence, everything we see, if it doesn't emit light, reflects light (Fig.04).
As in this example, the light-bulb will emit light in all directions and some of these rays will hit the teapot. The teapot will absorb part of the rays, in this case mostly the blues and greens, and bounce off the reds. Some of these bounced rays will pass through our pupils hitting our retinas, and this information will be sent to our brain so we can finally "see" the teapot.
The thing is, there are two ways this reflection may happen:
Diffuse reflection is a type of reflection where the light is reflected from the surface at multiple different angles (Fig.05).