When working in a PBR workflow, an artist must note the base reflectivity or minimum amount of colour and light reflected.
“Specular reflection” refers to light that has been reflected off the surface. The light ray is reflected off the surface and travels in a different direction. It follows the law of reflection, which states that on a perfectly planar surface, the angle of reflection is equal to the angle of incidence.
However, most surfaces are irregular and the reflected direction will vary based on the roughness of the surface. This changes light direction, but the light intensity remains constant.
Rougher surfaces will have highlights that are larger and appear dimmer. Smoother surfaces will keep specular reflections focused and they will appear to look brighter or more intense when viewed from the proper angle.
The terms diffusion, diffuse light or subsurface scattering all describe the effect of light that has been absorbed or scattered internally. When light is scattered, the ray direction changes randomly and the amount of deviation depends on the surface roughness of the material, as rough surfaces scatter light. Scattering randomises the direction of light but doesn’t change its intensity. At times the scattered light may reappear on the surface, making itself visible once more.
Materials that have both high scattering and low absorption are sometimes referred to as participating media or translucent materials. Examples of these are smoke, milk, skin, jade and marble.
When passing through translucent material, light can be absorbed or scattered. When light is absorbed, the light intensity decreases as it transforms into another form of energy, like heat. These colour changes depend on the wavelength, but the ray’s direction won’t change.
If there is no scattering and absorption is low, rays can pass directly through the surface, which is true of glass. Imagine swimming in a clean pool. You could open your eyes and see through the clear water. However, if that same pool was fairly dirty, the dirt particles would scatter the light, lowering the water’s clarity and how far you could see.
The further light travels in such a material, the more it is absorbed and/or scattered. That’s why object thickness plays a key role in how much the light is absorbed or scattered.