Posted by This is a guest post by David Kerns, one of Greta’s top student writers for Spring 2007.
As movie special effects technology improves, more and more live-action shots are being replaced with computer animation. Harry Potter flies across the Quidditch field; Spider-Man swings from web to web through the cityscape of New York City, and miniaturized Hobbits fight the overpowering Orcs of Middle-earth. All of these are examples of human movements that have been reconstructed with computer animation. But sometimes this type of animation fails to come across as real. When Harry falls from his broom, and his computer animated body contorts in a way that appears not humanly possible, we are reminded that it is just a computer generated figure. So what is it about physical movements that make them appear human or artificial?
Body language is a critical form of communication for human beings. We can pick up a lot of meaning from physical movements, even when we only see a very limited amount of information about that movement. For example, most special effects animation is created by putting sensors on several parts of the human body to determine how body parts interact when the body is in motion. A human figure made up of just ten dots located in the different major body regions is enough to convey a wide range of emotions and complex physical movements. How does this work? And what is it about this movement that allows us to distinguish natural movements from artificial ones? A research group lead by Jan Jastorff sought to answer these questions by testing if people could learn to distinguish between complex physical motions of artificial movements as well as they could distinguish between the complex physical motions of natural movements.
So what makes natural movements different from other types of movements? As Jastorff’s team explains, biological movements have three distinct characteristics:
- Natural movements are pretty smooth and fluid.
- Natural movements are connected by an underlying structure. In the case of humans, this is a skeleton.
- Natural movements represent a physical form or familiar motion pattern that is familiar to us. That is to say, we can easily tell that the shape we see represents a human body in motion.
These three factors might be important in how we recognize natural movements and distinguish them from other types of movement. Jastorff’s team tested how these factors play a role in how we contribute to our perception and understanding of physical movement.
The team conducted four experiments testing the roles of these factors. Participants in the study had to watch videos of natural or artificial movements. Jastorff’s team then measured how easy it was for the people in the experiment to distinguish between different artificial movements versus how easy it was for them to distinguish between different natural movements. The movements could be anything from running or kicking to jumping or punching. All of the figures performing these movements were point-light displays — nothing more than ten black dots displayed on a gray background. The natural moving forms looked like normal human figures, but the artificial moving forms either looked like contorted skeletons or completely random dot configurations.
One type of movement simulated the actions of an actor (QuickTime required).
A second type mimicked the movements of contorted artificial skeletons.
Why are these movies all wobbly? The researchers found that viewers focused too much on the individual points when they moved normally, so a bit of random motion was added to the display.
By comparing how fast people learned to distinguish the different actions within each movement group, the researchers could determine what factors are important in conveying information about a moving form. For example, is it easier to distinguish kicking from punching when the moving form is a natural human figure?
For this last question, the answer is not always yes. When comparing ability to learn about human-like figures and artificial ones, the results were clear-cut:
As you can see, after learning how to differentiate the motions in a pretest, viewers were equally accurate learning to distinguish different motions in both human-like and artifical figures.
But what about when the forms do not have an underlying structure – that is, when the dots aren’t connected by a skeleton? Jastorff’s team created a new sort of movie to test this. The motions of each dot were identical to the human-like movie above, but the position of each dot was scrambled, so that the motion couldn’t easily be interpreted as being connected by an internal skeleton:
This graph compares how well viewers were able to differentiate between different motions, for human-like and scrambled figures.
By posttest 2, viewers were significantly better at distinguishing between human-like figures compared to scrambled figures.
Jastorff’s team argues that a key factor in learning physical movements is having a connected unifying structure, but having a familiar shape or familiar motion pattern is not a key factor. This study also brings up further questions about what else is important in conveying physical information. For example, we have yet to see if changing the fluidity and smoothness of a given motion will make it harder to distinguish between actions. Only future studies will answer this question and help complete our understanding of how physical communication works. For movie-animation gurus, clearly having an internally connected structure is a key to generating realistic images. But if their goal is to create an otherworldly, completely imaginary figure, perhaps removing the internal skeleton could have a dramatic impact.
Jastorff, J., Kourtzi, Z., & Giese, M.A. (2006). Learning to discriminate complex movements: biological versus artificial trajectories. Journal of Vision, 6, 791-804.
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