Rendering lead Jessica McMackin spoke about the use of ray tracing on the film, saying: "In addition to creating accurate reflections, we used ray tracing to achieve other effects. We were able to use this approach to create accurate shadows, like when there are multiple light sources and you want to get a feathering of shadows at the edges. Or occlusion, which is the absence of ambient light between two surfaces, like a crease in a shirt. A fourth use is irradiance. An example of this would be if you had a piece of red paper and held it up to a white wall, the light would be colored by the paper and cast a red glow on the wall." Character supervisor Tim Milliron explained that the film uses a ground–locking system that kept the cars firmly planted on the road, saying: "The ground-locking system is one of the things I’m most proud of on this film. In the past, characters have never known about their environment in any way. A simulation pass was required if you wanted to make something like that happen. On "Cars", this system is built into the models themselves, and as you move the car around, the vehicle sticks to the ground. It was one of those things that we do at Pixar where we knew going in that it had to be done, but we had no idea how to do it."