Plausible and realtime rendering of scratched metal by deforming MDF of normal mapped anisotropic surface

Abstract

An effective method to render realistic metallic surface in realtime application is proposed. The proposed method perturbs the normal vectors on the metallic surface to represent small scratches. General approach to the normal vector perturbation is to use bump map or normal map. However, the bumps generated with those methods do not show plausible reflectance when the surface is modeled with a microfacet-based anisotropic BRDF. Because the microfacet-based anisotropic BRDFs are generally employed in order to express metallic surface, the limitation of the simple normal mapping or other normal vector perturbation techniques make it difficult to render realistic metallic object with various scratches. The proposed method employs not only normal perturbation but also deformation of the microfacet distribution function (MDF) that determines the reflectance properties on the surface. The MDF deformation enables more realistic rendering of metallic surface. The proposed method can be easily implemented with GPU programs, and works well in realtime environments.