Digital Video - July 2008 - (Page 27) DIGITAL FILM TOOLS POWER MATTE MAY SAVE YOU HOURS OF TEDIUM. ROTOSCOPING The term “rotoscoping” originally referred to the technique developed by the Fleischer Brothers in the 1930s for filming live subjects and tracing them into animation cel frames as a foundation for hand-drawn animated human motion. This technique was used to animate the original Superman cartoons and a number of other classics from the Fleischer studio. masks should be set to Mode/None. Now, the masks must be animated to roughly match the motion of the foreground subject. To animate masks so that they move over time, you need to turn on the Timevary Stop Watch icon for the Mask Shape parameter in the Timeline. You want to create the fewest possible keyframes and let After Effects interpolate the rest. To do this, scroll next to the out point in the clip and adjust the masks for that frame. Now scroll to a midpoint and adjust the mattes, creating keyframes for that point. Scroll through the footage and find where other keyframes must be added to keep the motion of the masks consistent with the foreground subject. Segments with sudden changes in trajectory or speed will need more keyframes than other areas. Once the dual masks are animated to your satisfaction, apply Power Mask to the footage layer. In the control for Foreground, select the mask that is inside the foreground subject (in this case, Mask 1). In the control for Background, select the mask that is outside the foreground, defining the background (in this case, Mask 2). The area outside of Mask 2 will be transparent and the area inside of Mask 1 will be opaque; the area between them is where Power Matte will work to define edge and transparency. Other controls are available as well, though the classic edge controls usually found in keying plug-ins are absent. You can link multiple masks together using either a common color or a common name. Sill control is available for blue- and greenscreen footage; and additional masks can be linked as Holes in the principal foreground. The output (opposite) shows that Power Matte works best where there is a clean, non-noisy background that is clearly different from the foreground. It does a great job on clear, welldefined edges and in dealing with wispy hair edges or semitransparent foregrounds such as smoke or water. But let’s face it, no matter how fancy the algorithm, software can’t read your mind. If there is little visible difference between the foreground and background, Power Matte is going to have trouble finding the edge. You can see this on the actor’s left arm, where the busy weave of the curtain together with a marginal difference in the www.dv.com luminance values combine to create a messy edge. When necessary, these areas can be dealt with through more detailed, careful animation of the masks so that they closely follow the desired edge. The closer the masks are together, the easier it is for Power Matte to find a clean edge. This is a lot of work, but it’s still a lot less work than manually animating the entire complex mask that would be needed for this shot! Once a clean matte is created, any number of effects can be applied to this shot. The color balance of the wall can be changed without changing the subject; the wall rack can be eliminated or swapped out for a CGI unit; a painting can be hung on the wall. The matted actor can still walk in front of these features. Power Matte makes the tedious chore of rotoscoping significantly easier for many shots. As always, it’s best to know you will be using roto and plan shots to accommodate the need for relatively clean backgrounds in production. Avoid background colors that are too close to colors in the edge of the foreground subject. Often this can be done with a minor lighting tweak or angle change; but lack of planning will mean it’s back to hand-animating every bit of a complex mask! DV The Rev. John Jackman has been creating special effects of one sort or another for over 30 years. He is the author of Bluescreen Compositing (Focal Press, 2007). He can often be found in the DV.com forums. >FYI http://DV.com http://www.prompterpeople.com http://www.dv.com http://www.prompterpeople.com
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