Reading: Eran Dinur, The Complete Guide to Photorealism for Visual Effects, Visualization and Games (Focal Press, 2022), Chapter 9, pages 94-97. This page is a guide to that chapter, not a substitute for it.
The premise
Chapter 5 treated absorption, specular reflection, diffuse reflection and transmission as separate outcomes. A real material is a combination of some or all of them. Tinted glass absorbs, transmits and reflects both diffusely and specularly. The particular balance of interactions is what defines the look of any given material.
Materials vary enormously, but some properties apply to all of them, and some to a whole category. This short chapter covers the two categories that matter most.
Dielectric materials
Dielectric means non-metal - also called insulators. The group is huge: rock, leaves, wood, skin, clay, water, plastic, paper, fabric, cement, glass, diamond, flesh, milk. They differ wildly in appearance and still share properties that separate them from metals.
Diffuse and specular balance
Every dielectric shows a combination of diffuse and specular reflection. The diffuse reflection defines the base color. On top of that, depending on how glossy the material is, specular reflection adds the colors of the environment - and here is the key property:
In a dielectric, the specular reflection is not tinted by the material. Clear dielectrics like water and glass do not tint their specular reflections; the reflected environment keeps its own colors. Most shiny dielectrics get their gloss from a thin transparent coating - the resin shell on some plastics, the lacquer on polished wood, the wax layer on an apple, the thin oily layer on skin. In all of those the specularity happens at the outer transparent layer, so it is untinted.
CG shading treats diffuse and specular as separate parameters. In the real world they are two facets of the same thing: a highly irregular surface scatters almost all the incident light at random angles, while a more organized microscopic structure returns at least some of it at predictable angles. That mixed balance characterises every dielectric from the dullest to the glossiest - and it is strongly view-dependent, which is what the next section is about.
The Fresnel effect
Augustin-Jean Fresnel - the physicist largely responsible for the wave theory of light being accepted - derived a set of equations describing the ratios between reflection, transmission and refraction. CG artists call the practical consequence the Fresnel effect. You do not need the maths; you do need the principle.
The prominence of specular reflection depends on the angle between the surface and your line of sight.
- At a shallow, grazing angle - near parallel to the surface - reflection is strongest.
- At a steep angle - perpendicular, looking straight in - reflection is weakest.
The classic demonstration: stand at the edge of a calm lake and look down at the water at your feet. It is transparent and you can see the bottom. Shift your gaze toward the opposite shore and the water gradually becomes opaque and reflective, most strongly further away. Your viewing angle is steep for the water near you and gets progressively shallower toward the horizon.
That example also shows the inverse relationship between specular reflection and transmission: as the angle of incidence gets shallower, the surface becomes less transparent and more reflective. Look at your phone's glass from the side and it does the same thing - more reflective, less transparent.
On a curved surface
Apply it to a shiny plastic sphere. The part facing the camera, seen perpendicular, has the least specular reflection. The sides of the sphere, seen at a grazing angle, have the most. So reflection in dielectrics is highly dependent on the viewing angle and therefore on the shape of the object relative to the viewer.
The Fresnel equations calculate how much attenuation occurs from the material's index of refraction. A lower IOR means less specular reflection at a straight angle; at a higher IOR the reflections expand toward the straight-angle zone, which diminishes the Fresnel effect. So viewing angle has a much stronger influence on water at IOR 1.33 than on diamond at IOR 2.4.
Metals
Metals behave completely differently, and the reason is structural. The nuclei do not hold the outer shell of electrons strongly enough, so those electrons float and interact freely. That sea of electrons acts like an impenetrable force shield around the molecule, either bouncing photons straight back or killing them at the surface.
Because photons never penetrate and never scatter, metals reflect all light specularly, with no diffuse component at all. Some energy is never re-emitted, but a strong entirely specular albedo is what gives polished, freshly prepared metal its look.
Color works differently too. Silver, chrome, aluminium and platinum re-emit across a wide range of wavelengths, which gives them their neutral silvery appearance. Gold, bronze and copper do not re-emit all wavelengths equally, so they reflect with a specific tint - that is where their color comes from.
The consequence to remember: the diffuse color of every metal is black - zero diffuse albedo - and its visible color comes only from the tinting of its specular reflection. That is the opposite of a dielectric, where the diffuse gives the color and the specular is untinted.
Fresnel behaves differently in metals
Because of that molecular structure, the simple refractive index calculation does not apply. Metals are described by a complex index of refraction, and the practical result is that metals show little reduction in specularity at straight viewing angles. Reflections stay much more consistent across the whole surface. A mirror reflects equally well from any viewing angle thanks to its metallic backing.
And since specularity in a metal is not mixed with any diffuse component, the reflected image is far less attenuated than it would be in a dielectric, so it shows much more distinct detail.
Anisotropy - the pronounced stretching of specular reflections - is usually caused by elongated or circular grooves in processed metal surfaces such as brushed aluminium.
Dinur's photographs make the practical point that clean theory rarely survives contact with a real object: a stainless steel bowl shows a mirror reflection punctuated by scratches, most visible around the bright window reflections; decay, wear and dirt occlude most of a bronze piece's original specularity; circular grooves on a pot lid produce anisotropic reflections; and a CD shows anisotropic specularity plus a rainbow effect from thin film interference in its plastic coating.
Terms from this chapter
- Dielectric - a non-metal or insulator. Has both diffuse and specular reflection; the specular is untinted.
- Fresnel effect - specular reflection strengthens at grazing viewing angles and weakens at perpendicular ones, with transmission doing the reverse.
- Metal - purely specular, no diffuse component. Diffuse albedo is black; color comes from tinted specular.
- Anisotropic reflection - directional specular produced by a grooved or brushed surface.
Check yourself
- In a dielectric, which reflection carries the color? In a metal?
- Why is the specular reflection on an apple or a polished wooden table not tinted?
- Where on a lake is the water most reflective, and why?
- Where on a shiny sphere is the specular reflection strongest?
- Why is the diffuse albedo of gold black?
- Why is metal a bad surface to project onto? Give two reasons.
- Which position in your venue puts the audience at the worst Fresnel angle?