Capture that actor!

One of the most common ways to create animated characters is with motion capture. The movements of real actors are applied to animated characters, making it possible, for example, for Andy Serkis to play the tiny, thin Gollum in Lord of the Rings, followed by the enormous King Kong in Peter Jackson’s remake.

In order to capture the data from Andy’s movement, he wore a suit made from Lycra and decorated with markers at important spots on his body. Computers track the movement of the markers and translate that into an animated character. The next step in research, though, is how to capture people’s movement without needing to put them in special suits. The suits are expensive and time-consuming, not to mention revealing. Not everyone wants to wear a Lycra uniform to work.

Fortunately, a group at Dundee University is helping to relegate the form-fitting motion capture suit to the past. They’ve developed a system for getting the actor’s motion data straight from a video of their performance. The key to grabbing the data is a search technique called ‘particle swarm optimisation’. Imagine you’ve got a frame of video with your actor in it. To find the body position of your actor, you release a swarm of mathematical particles into the image. Within their programming is a way of finding the actor, and rules on how to follow what their neighbour is doing as well. What this means is that, over time, the particles swarm towards the image of your actor in the frame of video, like bees to a flower.

One of the biggest challenges with this method of motion capture is something you might not think about when you’re using software. It’s very important for the Dundee team to make their system easy enough for the film crew to use. After all, not many directors are computer scientists too. All of the deep knowledge of algorithms and swarms has to be embedded in the software so it can fine-tune itself to spot any actor, even when they might move in lots of different ways. Whatever weird creature Andy Serkis plays next, the computer has to be sure to capture him.