Monday, February 4, 2008

Techniques of rotoscoping

Analog Rotoscoping for Visual Effects

While the technique is useful for animation, rotoscoping eventually became an important tool for visual effects in general. From the 1940s through the 1960s, U.B. Iwerks, a well-known animator, turned to effects work, where he pioneered the use of the rotoscope on films such as Alfred Hitchcock's “The Birds” (1963).

Rotoscoping in visual effects was used primarily to make holdout mattes. "You frequently want to composite different elements into the same shot to create that shot," explained Tom Bertino, who was head of Industrial Light & Magic’s rotoscoping department from 1987-93. "By using the tracing to create black mattes, you can hold out certain elements."

For example, Bertino imagines a scene of an explosion behind two people on-screen, where the explosion is added after the fact. "You could print the explosion over the frame. But you'd also cover up the people," he said. "You'd need to isolate them with the rotoscope." To make a traditional holdout matte, a rotoscope artist would trace the figures that had to be isolated onto an animation cel. The outline traced onto the cel then would be filled in with black paint, so that it would block the appropriate section of the frame. "You create a solid black matte," Bertino said. This black matte then could "hold out" the part of the explosion image where the two people would appear, so that when the two images were printed together, the people would appear to be in front of the explosion.

Rotoscoping also could be used to stabilize a shaky film image. To do stabilization, each film frame was rotoscoped onto an alignment chart. A comparison of the charts allowed changes in position to be tracked from frame to frame. Using this information, an optical copy of the film could be made, with the printer offsetting the shifts in each frame's movement.

Bertino said people underestimate the difficulty of rotoscoping during the photochemical era: "It was a painstaking process. There were so many moving parts to the rotoscope camera, and so many places for things to get out of hand." Rather than being a refuge for the unskilled artist, he added, rotoscoping was a demanding craft. "The rotoscoper had to be a skilled animator to make the line follow through. That's actually something that plagued some early uses of the rotoscope as a special effects tool -- without actual animators to handle it, it could get jittery."

Good rotoscope artists were very precise about their work. "It was so exacting," Bertino said. "It's almost like -- I don't know if you’ve ever seen those incredibly detailed Chinese tapestries that they made in the monasteries generations ago. They finally stopped making them because the artisans would go blind. I'm surprised that more rotoscopers didn't go that route."

Jack Mongovan, a paint and rotoscope supervisor at ILM, began his career in traditional rotoscoping and has been working in the field for 19 years. He remembers working in rooms that were completely dark except for the light coming out of the projector. The rotoscope artists were at the mercy of the painters who would later fill in their outlines, and who could with a few stray brushstrokes outside the outline make the image suddenly jittery. "I would never go back to traditional for anything," Mongovan said.

Digital rotoscoping for Visual Effects
Today, rotoscoping is done in the computer, using programs such as Shake, FFI and Pinnacle Commotion. The shift to computer-based rotoscoping began in the early 1990s with a software called Colorburst, an image editing tool like Photoshop, that later evolved into Matador. "When computers became prodigiously viable around here, right after the 'Terminator 2'/'Jurassic Park' era, we realized that the computer had great capabilities for this," Bertino said. "It obviously became much simpler."

Mongovan said that today, one rotoscope artist can do the same amount of work that eight used to do, and in one quarter of the time. This is often because in traditional rotoscoping, each frame had to be drawn individually. The computer, on the other hand, can use the previous frame as a basis, which means most of the drawing may already be done.

Rotoscoping software works using splines, which are a series of points connected by a line or curve. These splines are adjusted from frame to frame, so that they continue to conform to whatever shape the artist is tracing. Because rotoscoping software includes the tools to paint an image, rotoscope artists now find themselves doing a lot of paint work as well. "Rotoscoping is becoming the lesser part of what we do," Mongovan said. "We do so much more painting." Painting might mean taking someone out of a shot, or replacing a sky, or painting out the tennis balls used as visual effects tracking markers.

Some skills remain necessary, including a sense of what is important. "One of the hardest things for people to do in our department is to realize that they're looking at a very zoomed-up plate," Mongovan said. Also, he pointed out, a movie audience will see an image for only 1/24th of a second, too short a time to register flaws that may torture the artists. More important is consistency. "I tell people, 'You can paint that first frame wrong, just keep it wrong it all the way through.'"
That kind of understanding is key, Bertino agreed. "The secret to good rotoscoping has always been -- regardless of what it's used for -- an educated eye and good judgment as to what to include and what to leave out," he said. "Most people think the rotoscope is very literal -- you trace what's there, and that's it. It's possible to put too much detail and confuse matters. You need to have that sense for judicious editing. That hasn't changed at all. And not everybody's got that."
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