  The surface of metal, glass and plastic objects is often characterized by
microscopic scratches caused by manufacturing and/or wear. A closer look onto
such scratches reveals iridescent colors with a complex dependency on viewing
and lighting conditions. The physics behind this phenomenon is well understood;
it is caused by diffraction of the incident light by surface features on the
order of the optical wavelength. Existing analytic models are able to reproduce
spatially unresolved microstructure such as the iridescent appearance of
compact disks and similar materials. Spatially resolved scratches, on the other
hand, have proven elusive due to the highly complex wave-optical light
transport simulations needed to account for their appearance. In this paper, we
propose a wave-optical shading model based on non-paraxial scalar diffraction
theory to render this class of effects. Our model expresses surface roughness
as a collection of line segments. To shade a point on the surface, the
individual diffraction patterns for contributing scratch segments are computed
analytically and superimposed coherently. This provides natural transitions
from localized glint-like iridescence to smooth BRDFs representing the
superposition of many reflections at large viewing distances. We demonstrate
that our model is capable of recreating the overall appearance as well as
characteristic detail effects observed on real-world examples.
