By MIT.
In the researchers’ experiments, the robot helicopter was equipped with a continuous-light source, a photographic flash, and a laser rangefinder.
Courtesy researchers
Based only on the specification of the “rim width” — the desired width, from the camera’s perspective, of the subject’s illuminated border — the helicopter not only assumes the right initial position but readjusts in real time as the subject moves, enabling delicate rim lighting of action shots.
According to Manohar Srikanth, who worked on the system as a graduate student and postdoc at MIT and is now a senior researcher at Nokia, he and his coauthors —MIT professor of computer science and engineering Frédo Durand and Cornell’s Kavita Bala, who also did her PhD at MIT — chose rim lighting for their initial experiments precisely because it’s a difficult effect.
“It’s very sensitive to the position of the light,” Srikanth says. “If you move the light, say, by a foot, your appearance changes dramatically.”
Intuitive control
With the new system, the photographer indicates the direction from which the rim light should come, and the miniature helicopter flies to that side of the subject. The photographer then specifies the width of the rim as a percentage of its initial value, repeating that process until the desired effect is achieved.
The helicopter automatically adjusts its position to maintain the same lighting effect as the subject moves.
Courtesy of the researchers
Quick study
As Srikanth explains, that required some algorithmic streamlining. “When we first started looking at it, we thought we’d come up with a very fancy algorithm that looks at the whole silhouette of the subject and tries to figure out the morphological properties, the curve of the edge, and so on and so forth, but it turns out that those calculations are really time-consuming,” Srikanth says.
Although the experiments took place in a motion-capture studio, the only measurement provided by the motion-captures system was the helicopter’s horizontal position, which onboard sensors should be able to approximate adequately.
Courtesy of the researchers
Instead, the algorithm simply looks for the most dramatic gradations in light intensity across the whole image and measures their width. With a rim-lit subject, most of those measurements will congregate around the same value, which the algorithm takes to be the width of the rim.
In experiments, this quick approximation was able to keep up with the motions of both the subject and the photographer while maintaining a consistent rim width.
The researchers tested their prototype in a motion-capture studio, which uses a bank of high-speed cameras to measure the position of specially designed light-reflecting tags with millimeter accuracy; several such tags were affixed to the helicopter.
But, Srikanth explains, the purpose of the tests was to evaluate the control algorithm, which performed well. Algorithms that gauge robots’ location based only on measurements from onboard sensors are a major area of research in robotics, and the new system could work with any of them. Even rim lighting, Srikanth says, doesn’t require the millimeter accuracy of the motion-capture studio. “We only need a resolution of 2 or 3 centimeters,” he says.