Chapter 3: Mathematics of Lighting and Shading

Chapter 3: Mathematics of Lighting and Shading

OVERVIEW

In mathematics you don't understand things. You just get used to them.

--Johann von Neumann

Nothing is really work unless you would rather be doing something else.

--James M. Barrie

One of the nice things about shaders is that you can create your own for whatever special effects you are looking for In fact, one of the reasons that shaders have finally made it into mainstream 3D computer graphics is the flexibility that they provide, which can finally be realized in real time on consumer grade hardware. Unfortunately, with power comes responsibility—the responsibility to understand how lighting and shading in computer graphics is traditionally done and how you can do it yourself (or do it differently) in a shader. But first, you'll need an understanding of the mathematics behind lighting and shading.

LIGHTS AND MATERIALS

In order to understand how an object's color is determined, you'll need to understand the parts that come into play to create the final color. First, you need a source of illumination, typically in the form of a light source in your scene. A light has the properties of color (an rgb value) and intensity. Typically, these are multiplied to give scaled rgb values. Lights can also have attenuation, which means that their intensity is a function of the distance from the light to the surface. Lights can additionally be given other properties such as a shape (e.g., spotlights) and position (local or directional), but that's more in the implementation rather than the math of lighting effects.

Given a source of illumination, we'll need a surface on which the light will shine. Here's where we get interesting effects. Two types of phenomena are important lighting calculations. The first is the interaction of light with the surface boundary, and the second is the effect of light as it gets absorbed, transmitted, and scattered by interacting with the actual material itself. Since we really only have tools for describing surfaces of objects and not the internal material properties, light—surface boundary interactions are the most common type of calculation you'll see used, though we can do some interesting simulations of the interaction of light with material internals.

Materials are typically richer in their descriptions in an effort to mimic the effects seen in real light—material surface interactions. Materials are typically described using two to four separate colors in an effort to catch the nuances of real-world light—material surface interactions. These colors are the ambient, diffuse, specular, and emissive colors, with ambient and specular frequently grouped together, and emissive specified only for objects that generate light themselves. The reason there are different colors is to give different effects arising from different environmental causes. The most common lights are as follows:

  • Ambient lighting: The overall color of the object due to the global ambient light level. This is the color of the object when there's no particular light, just the general environmental illumination. That is, the ambient light is an approximation for the global illumination in the environment, and relies upon no light in the scene. It's usually a global value that's added to every object in a scene.

  • Diffuse lighting: The color of the object due to the effect of a particular light. The diffuse light is the light of the surface if the surface were perfectly matte. The diffuse light is reflected in all directions from the surface and depends only on the angle of the light to the surface normal.

  • Specular lighting: The color of the highlights on the surface. The specular light mimics the shininess of a surface, and its intensity is a function of the light's reflection angle off the surface.

  • Emissive lighting: When you need an object to "glow" in a scene, you can do this with an emissive light. This is just an additional color source added to the final light of the object. Don't be confused just because we're simulating an object giving off its own light; you'd still have to add a real "light" to get an effect on objects in a scene.

Before we get into exactly what these types of lighting are, let's put it in perspective for our purpose of writing shader code. Shading is simply calculating the color reflected off a surface (which is pretty much what shaders do). When a light reflects off a surface, the light colors are modulated by the surface color (typically, the diffuse or ambient surface color). Modulation means multiplication, and for colors, since we are using rgb values, this means component-by-component multiplication. So for light source l with color (r1,g1,b1 shining on surface s with color (rs,gs,bs, the resulting color r would be:

or, multiplying it out, we get

where the resulting rgb values of the light and surface are multiplied out to get the final color's rgb values.

The final step after calculating all the lighting contributions is to add together all the lights to get the final color. So a shader might typically do the following:

  1. Calculate the overall ambient light on a surface.

  2. For each light in a scene, calculate the diffuse and specular contribution for each light.

  3. Calculate any emissive light for a surface.

  4. Add all these lights together to calculate the final color value.

This is pretty much what the FFP does, and it's fairly simple to do as long as you don't let the number of lights get too large. Of course, since you're reading this, you're probably interested not only in what the traditional pipeline does, but also in ways of achieving your own unique effects. So let's take a look at how light interacts with surfaces of various types.

In the real world, we get some sort of interaction (reflection, etc.) when a photon interacts with a surface boundary. Thus we see the effects not only when we have a transparent—opaque boundary (like air-plastic), but also a transparent—transparent boundary (like air-water). The key feature here is that we get some visual effect when a photon interacts with some boundary between two different materials. The conductivity of the materials directly affects how the photon is reflected. At the surface of a conductor (metals, etc.), the light is mostly reflected. For dielectrics (nonconductors), there is usually more penetration and transmittance of the light. For both kinds of materials, the dispersion of the light is a function of the roughness of the surface (Figures 3.1 and 3.2).

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Figure 3.1: Light reflecting from a rough and smooth surface of a conductor.

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Figure 3.2: Light reflecting from a rough and smooth surface of a dielectric showing some penetration.

The simplest model assumes that the roughness of the surface is so fine that light is dispersed equally in all directions as shown in Figure 3.1, though later we'll look at fixing this assumption.

A generalization is that conductors are opaque and dielectrics are transparent. This gets confusing since most of the dielectric surfaces that we are interested in modeling are mixtures and don't fall into the simple models we've described so far. Consider a thick colored lacquer surface. The lacquer itself is transparent, but suspended in the lacquer are reflective pigment off of which light gets reflected, bounced, split, shifted or altered before perhaps reemerging from the surface. This can be seen in Figure 3.3, where the light rays are not just reflected but bounced around a bit inside the medium before getting retransmitted to the outside.

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Figure 3.3: Subsurface scattering typical of pigment-saturated translucent coatings.

Metallic paint, brushed metal, velvet, etc. are all materials for which we need to examine better models to try to represent these surfaces. But with a little creativity in the modeling, it's possible to mimic the effect. Figure 3.4 shows what you get when you use multiple broad specular terms for multiple base colors combined with a more traditional shiny specular term. There's also a high-frequency normal perturbation that simulates the sparkle from a metallic flake pigment. As you can see, you can get something that looks particularly striking with a fairly simple model.

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Figure 3.4: A simple shader to simulate metallic paint: (a) shows the two-tone paint shading pass; (b) shows the specular sparkle shading pass; (c) shows the environment mapping pass; (d) shows the final composite image

The traditional model gives us a specular term and a diffuse term. We have been able to add in texture maps to give our scenes some uniqueness, but the lighting effects have been very simple. Shaders allow us to be much more creative with lighting effects. As Figure 3.4 shows, with just a few additional specular terms, you can bring forth a very interesting look. But before we go off writing shaders, we'll need to take a look at how it all fits together in the graphics pipeline. And a good place to start is by examining the traditional lighting model that has been around for the last two decades.