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Science•Engineering
The CG feathers in the digital red-winged blackbird (at left), which have hundreds of polygons and are topologically accurate, help researchers look at flapping flight. Motion captured from a blackbird in free flight (above) can validate the digital animation. says. “I changed the length and parameters to make it the size and proportions of a blackbird, removed the ivorybill feathers, and attached blackbird feathers.” The feathers have hundreds of polygons and are topologically accurate. “Jeff’s model was so morphologically accurate that I wanted to stay true to that attention to detail and keep as high a degree of accuracy on the feathers as I could,” Kaplan says. Kaplan had “printed” Wang’s model of the woodpecker wings using a rapid-prototyping machine, and put two wings in a wind tunnel. “I measured the force data at a couple different air speeds and angles of attack. I also used a strobe light that illuminated helium bubbles to visualize the flow,” he says. The wings were in a quasi-steady state, however. So, to look at flapping wings, he decided to use the digital blackbird feathers and computer simulations. “I’m doing fairly rudimentary aero dynamics using Brendan’s [Holt] wing beat of a red-winged blackbird and applying blade element analysis to each feather as the bird flaps,” Kaplan says. “No one has done this before with an emphasis on geometry of this detail. Zoran Popovic wrote a paper a few years ago [“Realistic Modeling of Bird Flight Animations,” with Jia-Chi Wu, for SIGGRAPH 2003], but he modeled his feathers as two triangles with a hinge. And, he didn’t validate the animation against motion-captured data like that Brendan [Holt] came up with.” What Kaplan hopes to achieve is a predictive model for flapping flight. “I don’t know if we’ll ever get there,” he says. “We’ve studied nonflapping flight quite thoroughly, but as soon as something starts flapping, it’s hard to predict the forces on that flapping body. There’s an important interaction between the feathers and the air. Air pushes on the feathers, they bend and push back. It’s like a spring-mass relationship.” “Brendan had markers along the length of the feathers so we can see the way they 14
January 2010
bend,” Kaplan continues. “But, a feather twists around its axis to some degree, and that affects the aerodynamics enormously. A small feather twist can create a different angle of attack. Even though Brendan’s data was accurate and great to work with, he didn’t capture a number of degrees of freedom. It might not be possible. So, I’m trying to tweak the feathers by hand to see how much they change the lift and drag. I need to look at air speed and angle of attack for each polygon.” Someday, Kaplan’s research might help aerospace engineers design small aircraft with flapping wings. “This is where it’s all headed,” he says. “We want to learn how to take advantage of the loopholes in the laws of aerodynamics that birds take advantage of by design.”
Into the Wild Bostwick had proposed a field study using motion capture long before meeting the CG team, so the collaboration was timely and fruitful; it helped her develop a protocol for the grant she received for the manakin study. Recently she led a team that took the system designed with the CG students into the mountains of Ecuador to capture the motion of manakins in free flight. “I would have had no idea where to begin [designing the motion-capture system] without Don [Greenberg] and his students,” Bostwick says. “The lenses, the sensors, the need to calibrate, the different perspectives, and then the whole world of taking data and putting points on a screen through time . . . that whole process of motion capture. I had no idea how to do it. These are incredible tools. And, most biologists don’t know about them.” Bostwick and her crew packed 350 pounds of equipment—two TroubleShooter cameras, which can run on batteries, audio equipment, syncing devices, calibration cubes, and more— to study birds that make sounds with their wings. “The very instant they produce a sound
is important,” Bostwick says. “So we have a customized device to sync the audio and the high-speed video to one millisecond.” The students will capture the manakins and, as did Holt with the red-winged blackbird, apply markers, then release the birds from a perch onto which they’ve attached the two cameras. To calibrate the cameras, the computer graphics students helped invent a special device. The Tinkertoy calibration “cube” looks something like a model of an atom—plastic balls with stems attached to form a 3D lattice that is firm, transportable, and easy to disassemble. “We put the bird on the perch, and after it flies away, we place the calibration cube on the perch and record it with our cameras using the same focus and zoom,” Bostwick explains. “By pointing a laser from the cameras to the display perch (often many feet above the ground), we can measure the distance. Then, we can calculate backward to find points on the wings to create a volume.” With Bostwick on the field trip is one of Greenberg’s students. “It’s been a really fruitful collaboration,” Bostwick says. “In my world, when people study bird anatomy and how it functions, they’re pretty much limited to pigeons trained to fly in wind tunnels under specific circumstances. I wanted to become independent from the lab. I wanted to find creative methods to get information from wild birds doing their thing.” Thanks to Don Greenberg, a computer graphics pioneer who has long advocated collaboration between computer graphics and many departments, and the hard-working students he inspires, Bostwick’s dream has become a reality. n Barbara Robertson is an award-winning writer and a contributing editor for Computer Graphics World. She can be reached at BarbaraRR@comcast.net.