Physically Based Rendering Continued
The Fresnel Effect
This refers to the change of reflectivity based on the angle at which a surface is viewed. This means that there will be a higher reflection from a light source that hits a surface at a grazing angle rather than a perpendicular one. This phenomenon can be observed in real life such as a lake that produces higher reflectivity of the opposite shore when observed from a lower angle. This also means that the Fresnel gives brighter reflection near the edges of objects in real life, as well as PBR graphics.
An interesting component of the Fresnel Effect is that any smooth surface viewed at the right grazing angle acts as a perfect mirror. In a PBR workflow, the artist specifies the base reflectivity of an object which means the minimum amount of color of light reflected. Using the Fresnel Effect adds reflectivity over that value by calculating the required reflection at different angles.
Unlike other materials, metals have almost no diffuse reflection, with their reflection consisting almost exclusively of specular reflection. Due to this high reflectivity, light does not penetrated the surface so no scattering occurs and the result is a very shiny look. Metals also produce a colored tint on the reflected light, for example bronze, gold and silver appear in different colors.
Microsurface is the tiny detail and imperfections on a surface such as grooves, scratches, cracks and lumps. Although invisible to the naked eye, these imperfections have a considerable effect on reflection. In PBR, instead of computing microsurface detail directly which would be very taxing, a measure of roughness called “Gloss”, “Smoothness” or “Roughness” is specified as a constant for any material.
Games, W. (2015) Physically based rering. Available at: http://blog.wolfire.com/2015/10/Physically-based-rendering (Accessed: 22 November 2016).end
LLC, M. (2014) PBR theory
. Available at: https://www.marmoset.co/toolbag/learn/pbr-theory
(Accessed: 22 November 2016).