Issue link: http://digital.copcomm.com/i/6179
January 2010 12 n n n n Science•Engineering grackles, woodpeckers, and red-winged black- birds," Holt says. "e little birds are easy to catch, but they fly fast. e big birds flap their wings more slowly, so we could get better data, but they're really smart. e grackles eventu- ally figured out where the mist net was, and we never caught one again." Bostwick released the birds in a flight tun- nel, a wooden structure a little more than 13 meters long with ports along the length. A viewing pyramid juts out from one side, and another port sits on the top for looking down. When released, the wild birds see the light at the end of the tunnel and fly through to es- cape. In general, the usable flight data lasted less than one second. "We thought woodpeckers would be a good choice because of their patterned wings, but they flew into the tunnel, landed on the wood- en ceiling, and stayed there," Holt says. Instead, they settled on red-winged black- birds. To capture the bird's motion, Holt ap- plied markers hole-punched from 3M retro- reflective tape that they could easily remove after the test. "Spheres would have been visible from more directions, but they would have interfered with the flight," he says. Bostwick's knowledge of bird anatomy was invaluable as Holt determined where to apply the markers. Because his goal was to capture bone ori- entation and feather deformation, he attached markers at the shoulder, elbow, and wrist joints, at the tip of the hand, and on the feath- ers. "Bird wings have basically the same bone structure as mammal arms," he says. "ey have a humerus, radius, and ulna for the fore- arm, and a simplified hand with kind of a thumb bone and two digits that are a little bit flexible in some birds." Although acquiring the best estimate of joint center rotation might have meant adding markers midway on the bones, they decided to minimize the number of markers for this pilot study. "We weren't sure how well this would work," Holt says. "And it's difficult and con- fusing to digitize lots of markers, so we kept it simple. Perhaps later, people will decide to continue the study using more markers." Because the cameras would see both sides of the feathers during flight, the researchers applied markers to the tops and bottoms of a few feather tips, as well as midway along the length. "We figured that the most interesting deformation occurs near the distal part of the wing, near the hand," Holt explains. "e first nine feathers from the wing tip come off the hand bone. e feathers bend, rotate around the axis of the rachis, which is the quill, and splay like a fan opening and closing. We couldn't capture feather twisting with our lim- ited set of markers, but we could get bending and a little bit of the splay." Holt and Bostwick positioned two Fastec Imaging TroubleShooter cameras inside the tunnel—one behind the bird and off to the side, and one mostly to the side and low to the ground. Kaplan built a custom light mount for the cameras to provide the bright light neces- sary for the high frame rate: e digital video cameras shoot 1280x1024-resolution images at 500 frames per second. So that they could locate points in 3D space later, the researchers placed a cube of known size in the region to calibrate the cameras. To track the markers on the birds in flight captured by the video cameras and triangulate them, Holt used a MathWorks' MatLab script written by Tyson Hedrick. "at gave us dots moving in space," Holt says. "We fit splines to their trajectories." And that gave Holt paths in space and time. "It didn't let us see the wing, though," he says. "Initially, we treated each marker as a vertex of a mesh and connected the vertices to make a face. e resulting wing mesh gave a crude impression of a flapping motion, but it did not portray the underlying anatomy. So we built a more informative visualization using rods to represent feathers and bones." To do that, Holt created a simple polygo- nal model in Maya and rigged it with inverse kinematics (IK) to drive the animation. en, he applied the data from the motion capture to the joints. "e model has something like 18 feathers, but we had markers on only a few feathers," he says. "So we created a NURBS curve from the outermost feather tip to the shoulder using the other feather tips as control points. en we evenly subdivided that curve by 18, the number of feathers." As the wing bones fold and open during flight, driven by the motion-captured data, the curve changes shape and size. e wing flaps. "at's how we fit the feathers to the data," Holt says. "e feathers fan in and out and make a smooth surface. So, we have a model of a wing that we're driving with real motion data. It isn't an accurate simulation. e results are somewhat interpreted in the way we fit the feathers and bones to the data with IK. We had only two cameras, so we had missing data. And, it's hard to validate our work because there is very little research showing joint rota- tions of bones. But, we showed that it's pos- sible to get free flight data from a bird without relying on a wind tunnel." Flapping Wings Kaplan, who had studied mechanical engi- neering as an undergraduate and moved into computer graphics as a graduate student, took the work done by Wang and Holt to the next scientific level. First, using a CUMV speci- men, he scanned individual feathers of a red- winged blackbird, the same type of bird that Holt had motion-captured in the tunnel, with a 3D laser scanner. en, working in Maya, he changed Wang's 3D model to match. "Jeff had made a very, very accurate 3D model," Kaplan With help from Cornell ornithologist Kim Bostwick, CG grad student Brendan Holt attached retro- reflective motion-capture markers to the joints and feathers of a red-winged blackbird.