Reference:Aoi Pattern

The aoi pattern can be used with pigment, normal and texture statements. The syntax is as follows:

pigment {
  aoi
  pigment_map {
    [0.0 MyPigmentA]
    ...
    [1.0 MyPigmentZ]
    }
  }

normal {
  aoi
  normal_map {
    [0.0 MyNormalA]
    ...
    [1.0 MyNormalZ]
    }
  }

texture {
  aoi
  texture_map {
    [0.0 MyTextureA]
    ...
    [1.0 MyTextureZ]
    }
  }

It gives a value proportional to the angle between the ray and the surface; for consistency with the slope pattern, values range from 0.5 where ray is tangent to the surface, to 1.0 where perpendicular; in practice, values below 0.5 may occur in conjunction with smooth triangles or meshes.

Note: This differs from the current MegaPOV implementation, where the values range from 0.5 down to 0.0 instead. If compatibility with MegaPOV is desired, it is recommended to mirror the gradient at 0.5, e.g.:

pigment {
  aoi
  pigment_map {
    [0.0 MyPigment3]
    [0.2 MyPigment2]
    [0.5 MyPigment1]
    [0.8 MyPigment2]
    [1.0 MyPigment3]
    }
  }