Issue link: https://digital.copcomm.com/i/6179
January 2010 10 n n n n Science•Engineering plotted rotation angles for the joints of a Euro- pean starling. To animate the joints in his CG wing, Holt used the angles provided by Dial as rotations for forward kinematic controls. In addition, by incorporating information from a Science paper by Farish Jenkins that described the furcula, or the wishbone, as a spring, he added secondary motion. "At that point, we had an ivory-billed woodpecker that flew like a starling," Wang says, "the same cycle over and over." In addition, the team shot a high-speed video of a flying pileated woodpecker, the common but very large woodpecker that most resembles an ivorybill. By rotoscoping this bird in flight, the animators could match the rotations of the wing bones during each wing beat. e high-speed video of the pile- ated woodpecker and still photos also showed that in the transition between a downstroke and upstroke, the feathers oriented themselves in a nearly parabolic, continuous surface, and bent in response to aerodynamic loads during a wing beat—all of which affected the amount of black and white visible to the camera. e Cornell team mimicked these effects using mathematical orientation constraints. e next steps were tracking Luneau's cam- era in the video to discover the path it took, and then matching it to a digital camera. Wang did the match using 2d3's Boujou. Once he knew the camera's path, he could calculate the path of the bird. "e ornithologists at Cornell who searched for the bird in Arkansas knew about how far away the bird was when it first took off from the tree," Wang says. "So we asked them to go back to the same place and mea- sure how high they thought the bird was off the ground. ey went back to Arkansas and measured the tree." Knowing the height of the tree and the dis- tance from the camera, Wang could calculate the position of the bird in 3D space when it took off. "For the rest of the flight path, we drew a ray from the camera, starting with the first position of the bird," Wang explains. Known data provided flight speed for the birds, and from that, they could calculate how far the bird flew within a frame. "Because we knew the 3D location, how fast the bird flies, and how much time passes between each frame, we could calculate a dis- tance within one frame," Wang says. "at distance became the radius of a sphere. We knew the bird was somewhere on that sphere, but not exactly where. So to figure that out, we lined up the camera, an image plane with the video, and the sphere in 3D space. We then drew a line starting at the camera through the center of the bird in the video. e intersec- tion of that ray and the sphere was the location of the bird. We did that for the entire video, which was only 100 frames or so to get a three- dimensional flight path for the bird." en, Wang animated the ivory-billed woodpecker model on that flight path, scaled the bird down to the size of the smaller pileated woodpecker, changed the colors on the pileated woodpecker's wings, and repeated the flight. "at gave us three sets of images," Wang says, "the original video, our animation of the ivory-billed woodpecker, and the animation of the pileated woodpecker." Did either animation match the video? "is is where it gets tricky," Wang says. "We made so many assumptions, if we were to take what we did to the scientific press, it wouldn't hold water." Greenberg agrees. "It's inconclusive," he says. "I find that with the data we have, it's impossible to make a conclusive statement. I could take a subset of frames and say, 'Un- questionably, it's an ivorybill.' And then take another subset of frames and it's not clear. I wish I could come up with a different answer. My buddies at the Cornell Ornithology Lab were praying that I could come up with a dif- ferent answer. But the bird in the video was too far away, and it was a bird escaping. At its closest point it was 400 pixels. If you put your thumb on a normal-size TV screen, that was the image of the bird." More important, except perhaps to the ornithologists who were hoping for proof, the project has opened new lines of research at Cornell. "None of us want to get into the debate about whether it's ivory-billed or pileated," Wang says. "at was the launching point, but it isn't the be-all, end-all of our project. It is the v arious disciplines coming together." Motion-Capturing a Wild Bird If you were to create an animation of a bird, of a woodpecker or any bird, for that matter, how would you know whether that animation was valid? Researchers have studied birds in wind tunnels, but the unnatural airflow in the confined space might have affected the birds' flight. Holt wanted to capture data from wild birds flying naturally and use that data to drive a wing. Wang, Kaplan, and Bostwick helped make that possible. "We couldn't just capture wild songbirds," Holt says. "You have to have a licensed orni- thologist work with you." Bostwick strung specially designed fine nets that the birds couldn't see between trees, put birdseed on the ground, and captured, over time, a variety of birds. "We tried robins, chickadees, swallows, The digital wing, with red rods representing individual feathers, flaps using IK and data captured from a red-winged blackbird in free flight. Overlaying the flapping wing on a video of the bird showed how closely the digital wing matched the bird's motion. A CT scan of an ivory-billed woodpecker specimen from the Smithsonian helped the Cornell grad students create an accurate skeleton.