Normal Mapping

We've been using textures as a source of detailed albedo, but if we look closely at a textured surface, we can tell its only illusion. The color changes, but the lighting behaves as it would on a flat surface. We can achieve more realistic lighting by storing not just albedo in a texture, but also lighting data like normals and shininess.

Consider this albedo texture of the diamond plate pattern that is often found on steel floors, ramps, and running boards on trucks:

The albedo map alone may be enough to convince the viewer that the floor is made of steel. But the shading reveals that the floor is just a perfectly flat quadrilateral. The indentations in the pattern can be made to behave like real, physical indentations if the albedo map is accompanied by a normal map:

On a very flat surface like a floor, the fragment will grab its normal from the texture and not from the vertex shader. On more complex surfaces, the normal from the normal map will be added to the incoming normal from the model itself, resulting in slight perturbations of the model's surface. Such bump mapping is often used to put wrinkles, scales, pores, and other irregularities on skin and walls.

Normal maps encode 3D vectors in the red, green, and blue channels of the texture. When we perform a texture lookup, we get back a vector whose components are in [0, 1]. Normals can have negative components, but colors can't. How then can normals be stored in textures? Before being written into the normal map, the normal components are scaled and offset from [-1, 1] to [0, 1].

When we read the normal from the texture, we must undo this scaling and offset to get back the original.

The normal is then used to compute the diffuse and specular terms. Additionally, the shininess used to compute the specular term may be read from a specular map like this one:

This renderer uses all three maps to apply Blinn-Phong illumination to a quadrilateral:

Use the controls to switch between unshaded albedo, diffuse-only shading using the albedo and normals, and full diffuse and specular shading that uses albedo, normals, and shininess. With all three maps in use, there's a fair amount of grittiness between just four vertices.