Christophe Hery Talks Global Illumination at Pixar
With the upcoming release of Pixar’s latest feature, Monsters University, much of the discussion on the studio’s recent technological innovations has focused on their new lighting system, Global Illumination. GI, as it’s known, represents a completely new approach to lighting for the studio. Instead of the time consuming and difficult application of potentially hundreds of pinpoint lights, artists can use a few “area” lights that provide a much more accurate physically-based and realistic effect. In addition to the visual benefits, GI provides filmmakers a fast sense of what the final lighting will look like much earlier in the production.
At FMX this past April, I had a chance to sit with Christophe Hery, a senior scientist and lighting expert, who came to Pixar three years ago from ILM to join the studio’s lighting and rendering technology development efforts. He shared his thoughts on the new GI tools as well as how the next generation of lighting advances needs to happen in collaboration with advances across the entire animation production process.
Dan Sarto: Tell us about some of the latest lighting advancements that you've helped engineer at Pixar?
Christophe Hery: I was hired in order to simplify the lighting setups. There was a DP on Monsters University who had decided to change the lighting approach. There were sometimes hundreds of lights in a shot, placed artistically. It was like painting, what the artists were doing. The setups were pretty heavy, pretty complex, hard to understand from one artist to another. So the idea was, “Can we use more modern technologies like Global Illumination and bounce the light around? Could we express lights that are more physical to the point where we don't need potentially hundreds of lights for a character? We may need only two or three. [Can we] let the computer bring some of the complex light transports into the image?” So that's basically what I brought.
I come from visual effects so I had been exposed to those techniques and I was able to bring them into the land of animation at Pixar. Once we had them in place on Monsters we used them on the short film The Blue Umbrella and pretty much every feature film afterwards. I can't completely predict the future but at this point we're using those lights now everywhere.
DS: Can you explain a bit more about Global Illumination and more physically-based lighting?
CH: Yeah, it's really fascinating. At Pixar, as I said, the lighting was very complex. For instance, working with early benchmarking shots on Toy Story 3, we were trying to understand what the lighters had done in their shots and how we could simplify that, what we should do nowadays with more modern techniques. In some of the scenes there were hundreds of lights. But sometimes the Sun was in the shot 10 times because the characters reacted differently to that light. So part of the idea of moving towards more physically-based techniques is also to unify what the material response is. You need materials that are energy conserving. You need shaders that are more physical, than can import and sample, that the notion of reflection versus specular is the same thing in nature even though in computer graphics historically we tune them separately. So, it was like trying to bring this back together in a more meaningful manner.
In the end, we had a new set of tools and so I had to re-train people. The language becomes more one of a live action DP in some ways. Like where you're talking in terms of exposures, you're talking in terms of really high dynamic ranges. Now, an intensity on the light is not one or two, it may be in the thousands, or 50,000. The sun would be at 64,000 or something like that in these new units. So I had to work really closely with the DPs and with the lighters to make the transition happen. In the end it's a new pallet in a way. They're moving a little bit from painting or a painterly approach to a more photographic approach. We're not removing tools. You can express the same thing in a way but then it's slightly displaced.
Let's take an example, if you don't mind. When you shoot indoors, let's say you shoot in a room where there's a window and the Sun comes in. There's a little bit of light coming from the outside into the room. In your camera, most of the frame is going to be dark. There's usually not enough light coming from outdoor into the room to illuminate, to properly expose the shot. A live action DP will usually enhance that light coming from the window by hiding a light on the ceiling out of the frustum of the camera, or behind the camera. So that's the type of cheat we needed to re-learn. We spent quite a bit of time understanding the setups from live action DPs, what they were doing, and re-learning that process, shooting the lighting in those ways. But what it brings at the end is a much richer image because the behavior of the light is more complex. It comes from different directions, its picking up the colors of the wall, picking up the colors of all the objects, bouncing around. So it's more complex in terms of richness of the image while at the same time the setups are easier. So you get probably 90% of the shot done very quickly. You don't have to struggle with secondary lights and things like that. And so you spend a lot of your time being artistic again.
Now, when I move a light, it’s because I really want this effect on the character, not because the material is not responding properly, not because what used to be a specular highlight that was working well in one environment doesn't work well in this new lighting environment. Now it's going to work in any environment. It's going to react naturally. There's this notion now of softness. Softness of the highlights, softness of the shadows. All this is controlled by the size of the light, by the emission of the light. You can put barn doors, you can put all those extra physical properties on the light to shape the behavior. So it's another richness and language and tool box for the users.