Understanding Global Illumination

When creating high quality renders, understanding global illumination is a vital step in that process. It's important that you are familiar with global illumination and how it works. In this article you will get a crash course in global illumination, how it works, why it's needed as well as some helpful tips when setting it up. While much of the content in this article is primarily focused at mental ray, which is the primary renderer built into Maya, 3ds Max and Softimage, we'll also take a look at some of the differences between global illumination in mental ray and other renderers. What Is Global Illumination? Global illumination is the name of a process that simulates indirect lighting, like light bouncing and color bleeding. Global illumination is an important part of 3D animation and design that helps give it a more realistic feel. Without global illumination, objects simply would look right in certain cases, which would lead to pulling the audience out of the moment. For example, if you look at the image below closely you'll notice that the green color of the wall is being cast onto the sphere on the right side of the image. That effect is referred to as indirect lighting because the green light isn't being cast directly from a light but rather is the result of a white light being cast in onto the green wall which is then bleeding onto the nearby sphere. Global illumination plays a big role in getting this distinctive and often necessary look. To achieve its effect, global illumination relies on the use of photons in mental ray. From a scientific standpoint, a photon is basically the particle that is responsible for light energy. Think of the photons as a tiny particle in your render that holds a certain amount of energy and when you put together a lot of photons in your scene, the end result is the light that you see in the render. These photons are emitted from the direct light source in your scene. When a photon is emitted and comes in contact with a surface in your scene it inherits that surfaces color and energy value. Once it bounces off that first surface it carries those values to the next surface it contacts, and continues bouncing until it is absorbed, creating an indirect illumination effect as shown in the sample image below. So global illumination essentially imitates the same effect that you would see in the real world. So when would you want to use global illumination? Global illumination certainly isn't going to be the "one size fits all" way to achieving photorealistic renders in every project. For example, you may not want it for something like a toon-style render where you'd very specifically want to avoid indirect lighting effects. It is; however, great for architectural visualization, interior renders, scenes with direct sunlight and photorealistic renders. Basically, you would want to use global illumination whenever light needs to interreflect (or be cast back) and bounce multiple times over a large area in your scene. This is especially vital when trying to make things look as realistic as possible. Using global illumination gives you the ability to capture indirect illumination, the real-world phenomenon where light bounces off anything in its path until it is completely absorbed. For example, a crack at the bottom of a door can cause light to come into a room or red walls reflecting light from the light source can cause the wood floor to have a red hue. By using global illumination to achieve these types of effects will create a much higher level of realism and believability in your renders. Take a moment out of your day to simply look around your home or office and notice the ways light interacts with the environment. Make a particular mental note about the ways light reflects on objects as well as the source of the lighting. By becoming more observant about simple everyday occurrences like this, you’ll become much more adept at capturing that look in your renders. Tips for Setting Up Global Illumination in mental ray When rendering with mental ray you will need to turn photon emission on for both the light source and in the render settings. If you don't do this, there will be no indirect lighting effects applied to your render. By default, all objects will cast and receive photons but typically not all objects really have to both cast and receive photons for you to achieve the look that you want. To reduce render times or fine-tune the look of global illumination you can specify exactly which objects should cast and/or receive photons. For example, you can set only specific objects within your scene to receive global illumination. Sometimes, global illumination can run into a few problems, but you shouldn’t fret over them. A common occurrence with global illumination is the render may come out splotchy and not create a smooth effect. Luckily this is a relatively easy fix. By increasing the bounces or the level of photons emitted from the light source, you will create a smoother cleaner indirect lighting effect. Take note that by increasing the amount of photons emitted, it will noticeably increase the render time. That’s where other approaches can prove effective. In order to achieve the best possible look for your render, you can combine global illumination with another rendering technique called final gather. Using final gather with global illumination will allow you to get smoother cleaner results without having to use a higher amount of photons. mental ray's Global Illumination vs. Other Applications Although global illumination is in most 3D applications, not all of them use mental ray's same photon approach. In fact, global illumination in other rendering packages is often times closer to mental ray's final gather feature, which we talk about in the article linked above. If you are rendering with your software's built-in render engine check to make sure what is need for enabling global illumination. It's important to remember whether using a photon approach or not, the results of global illumination and the intended use are often the same in any render engine. Now that you are familiar with global illumination's basic concepts, try it out in your next render! Here are some resources that you can check out to dig deeper into global illumination in your 3D application of choice: Mental ray Workflows in Maya: Global Illumination Introduction to mental ray in 3ds Max Introduction to mental ray in Softimage Introduction to Lighting in CINEMA 4D