IBL Basics
Global illumination is a term to describe light originating from every reflective and transmissive surface in a scene as well as light sources. As a technique, its most common implementations are radiosity and a variety of image-based lighting (IBL) techniques. Image-based lighting uses imagery in some way to illuminate the scene. In the past several years, much research has gone into implementing many IBL techniques to produce spectacular images. Many of these also incorporate very complex computer vision or image-based rendering techniques. The term global illumination is thrown around by a number of artists, supervisors and directors, but it is often actually describing a shortcut or a fake of true global illumination.

[Figure 1] IBL technique creating, coloring and positioning lights according to the background image.

[Figure 2] IBL technique using the background image as an environment map.

[Figure 3] IBL technique using a filtered background image as an environment map.

Multiple Lights
One basic IBL technique involves creating lights colored and positioned by the scene in the image. Suppose we have an image of a day at the park in which a CG character is to be placed. The background image is sampled and lights of the sampled color and intensity are placed in their apparent position (Figure 1). It could be assumed that the park is similar on the right, left and behind the character, and the lights can be copied to each side. Additionally, only the brightest lights may be allowed to produce a specular reflection. Higher sampling creates more lights and more accurate results, but more lights are typically computationally expensive. This technique has been used to create automated lighting for entire sequences of shots. Once set up, a lighting TD need not touch another shot if all goes well. A 360-degree photo of the environment would generate a more accurate set of lights. This could be created from many stills tiled together to capture a panorama (which appears later in this chapter) or just a couple of fisheye lens shots.

Environment Map
Why do we even need lights at all? Another IBL technique simply uses the image like an environment map and multiplies the CG object’s diffuse color by the corresponding color in the environment map. This technique uses no lights at all, so it can render very quickly. The shortcomings of this technique are that specular highlights are missing, and the object looks like the environment was shrink-wrapped onto it (Figure 2). The lighting all comes from the same distance away and each piece of the environment map strikes an equal area on the object. To make this technique more useful (or useful at all), lights are added for bright locations of the image, and the image is filtered before it is used as an environment map (Figure 3). Suddenly this technique is more efficient than the first one mentioned, and it can provide almost the same quality and automation capability.

Of course, there are still many more tricks to getting these techniques to produce beautiful images, but that discussion can and has filled volumes.

If you do not have the ability to implement an IBL or radiosity technique, there are a few ways to simulate their qualities. Appropriately colored and positioned bounce lights mimic reflective illumination. It is essentially an implementation of the above technique, by hand with only a select set of samples and lights. Using a texture map that already has the environment’s lighting built in is also a very common way to simulate global illumination. Unless camera and character motion is limited, however, this “cheat” is easily noticed.

Shadows in Lights
In the real world, there is no such animal as a light that does not cast a shadow. In the CG world, however, shadows are optional. This level of control is beneficial for optimization and saving render time, but it is horrible for creating believable imagery. Computer graphics lights also enable the lighter to choose from a variety of shadow methods, including ray-traced shadows, depth map shadows, and shadows rendered as separate passes. Chapter 6 illustrates some of the problems with using non-shadowed lights (shown specifically in Figures 6.6 and 6.7). one being that light passes through objects if shadows are turned off. In many cases this is not an issue, but it can often cause problems by lighting up interior surfaces of a model that would otherwise be dark. The inside of a creature or character’s mouth is a good example (Figure 4).