// pattern for use in texture/pigment/normal
// new in 3.7:
/* Implemented AOI pattern (thanks to Grimbert Jerome for most of the code); 
      the syntax is as follows:
	
        pigment { aoi pigment_map { ... } }
        normal { aoi normal_map { ... } }
        texture { aoi texture_map { ... } }
	
      The pattern 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 suftace, to 1.0 where perpendicular; in practice, 
      values below 0.5 may occur in conjunction with smooth triangles or meshes.
	
      (Note that this differs from the current MegaPOV implementation, where the values
      range from 0.5 down to 0.0 instead.)
*/