Understanding Texture Maps
Materials in modern CG art and games are not defined by only one texture anymore. Today’s game engines and render engines support material setups like Physically Based Rendering (PBR). These setups use a full set of texture maps to calculate a surface’s color, height details and lighting effects. The most common texture files are described below.
Diffuse maps contain mainly color information. These textures are created from photographs or scanned images. Shadows from directional light sources have to be removed, so that the texture can be freely rotated without causing inconsistent lighting. A diffuse texture looks good on it’s own when there is a uniform lighting across the scene and the texture is not viewed from a shallow angle.
A normal, in a mathematical context, is “a vector perpendicular to a surface” (Wikipedia). A normal map defines this vector for every pixel in a texture. This means that every pixel is given a direction (relative to the actual geometry) that is used for calculating which parts of a material are illuminated by a directional light.
The normal vectors are stored in 3 dimensions that are represented by the RGB values in an image. Because most pixels are relatively flat on the surface, normal maps usually look very bluish.
Unfortunately, the illusion of depth decreases when looking at the surface from a shallow angle, because the normal map does not change the geometry of an object.
Like the name suggests, displacement maps affect the geometry and the silhouette of 3D models. Displacement Textures need a relatively high resolution model to create a desirable effect. The amount of polygons needed is usually too high for use in games. Modern game engines implemented a technique called “Parallax Mapping” (Unity 5) that imitates the effect of displacement maps to a certain degree at lower costs.
The amount of displacement is stored in a gray-scale texture using the brightness as height information.
A Specular map is another gray-scale texture where white indicates highly reflective areas and black means only diffuse.
Ambient Occlusion Map
The shadows in the corners and deeper areas of the texture are called “Occlusion Shadows”. This texture indicates where those shadows are. Often areas that receive only little light are also dirty and less reflective.
All textures combined result in the following image: