This is done by adding an optional normal statement. Note that attaching a normal pattern does not really modify the surface. It only affects the way light reflects or refracts at the surface so that it looks bumpy. If no identifier is specified then the items modify the normal values in the current default texture. Typical values range from 0.0 to 1.0 but any value may be used.
Negative values invert the pattern.
The default value if none is specified is 0.5. They are block pattern normals, continuous pattern normals, specialized normals and bump maps. They differ in the types of modifiers you may use with them. The pattern type is optionally followed by one or more normal modifiers. Normal modifiers of any kind apply only to the normal and not to other parts of the texture.