The Advanced Art of Stop-Motion Animation: Visual Effects - Part 3
Removing a rig from a stop-motion scene is now a built-in tool in some newer versions of certain frame-grabbing software programs, which helps avoid doing it in another program like Photoshop. Alternatively, a rig can also be removed using masks and alpha channels in After Effects. Another alternative to using one still frame as a background plate for rig removal is to shoot a series of frames of the clean background plate and place them into another layer under the animation with the rig in it. The rig can be masked out, and underneath will be an actual movie sequence of the clean background plate. The advantage to this approach is that if there are any lighting changes or pixel fluctuations in the scene, there won’t be any noticeable difference between the animation frames and the frozen background image under them. The background plate has a danger of standing out as a still image because of the lack of noise that would be present in the sequential animation frames.
Being able to erase or mask out parts of an image in stop-motion also comes in handy for fixing mistakes that occur on set while shooting. One mistake that can occur in the middle of a stop-motion shoot is the animator’s shadow flashing into the frame. Ideally, you should be standing in exactly the same spot each time you capture a frame, with your shadow completely free of the camera frame. In the heat of the moment while animating, though, it is common to forget this and have certain frames where your shadow creeps into the shot. Unfortunately, this happened to me a few times while shooting my two-character dialogue scene. However, using After Effects, these problem frames were identified and noted as to how much of the frame had a shadow flash into it. A mask was created from some held frames that didn’t have a shadow flashing into them (Figure 9.59), and this mask could be composited over the problem frames (Figure 9.60). The edges of the mask were feathered slightly to help blend them into the scene, and then all of the shadow flashes were gone. (Compositing and screen grabs for the masks in Figures 9.59 and 9.60 courtesy of Gautam Modkar.)
Motion blur is a favored technique of stop-motion animators for replicating the smooth movement of live action in their work. Part of the reason that older stop-motion films always had a jerky quality to the movement was that every frame was always in focus. An even bigger part of the jerkiness, however, was the distance between frames and poor registration of the positions in relation to the speed of the movement. If the distance between two positions on a fast movement (a sword swooping through the air, for example) was too far apart, a strobing effect would occur because the eye was not able to fill in the gap between those two very clear images. If that same fast motion occurred over just a few frames captured in live action, it is likely that some of the frames would be blurred if studied frame by frame. In Chapter 4: Digital Cinematography, I went over a few techniques for achieving motion blur on the actual stop-motion set. In this chapter, I will present a few examples of ways to get motion blur into your animation in post-production.
One really interesting method for creating an illusion of motion blur was relayed to me by Ron Cole. I noticed his work on In the Fall of Gravity had a very smooth, ethereal quality to it, so I asked him if he used any particular motion blur technique. He told me about a relatively simple method he used that was actually borrowed from an old film technique. The effect is one of blending the frames to suggest a kind of look that isn’t really there as a blur but makes the animation feel much smoother. Ron created at least three copies of each animation scene; he then removed the first frame from the first copy, the first two frames from the second copy, and left the third as-is. These copies were layered together in QuickTime Pro, and the opacity was altered in each of the layers. That way, each frame showed three images overlapped, with the one in the center the most visible and the before-and-after frames very transparent (Figure 9.61). This gives the illusion that one frame at a time is fading in and out, and the various degrees of opacity could be adjusted depending on the speed and quality of the animation. The multiple exposures of images typically shows up more on a fast movement, but for slow movements, it can create a much more subtle motion blur effect. This same effect can be done easily in After Effects, TVPaint, or any other package that allows you to layer copies of the same sequence over each other and adjust the opacity.