The Advanced Art of Stop-Motion Animation: Digital Cinematography: Part 2
Around this same time, production was going forward on Coraline, which would take the third dimension to another level of artistry and technology. The main difference was that Coraline was actually shot in stereoscopic 3D, in addition to being projected in 3D for certain screenings. Shooting in stereoscopic 3D means that instead of having one camera view that takes a flat picture of a three-dimensional scene, there are two images taken from different views of the same scene to mimic the different perspectives of our left and right eyes (Figure 4.28).
The distance between our eyes is referred to as IO (interocular distance). This same sense of distance is mimicked by stereo photography—the greater the distance is, the greater the sense of three-dimensions and depth that will be created. However, having the IO too far apart will also create a ghosting effect, where the double image shows up even with the glasses on, and the focus distracts the eye towards the edges, rather than the illusion of one solid object on screen. Also, if the IO is too great, the effect would be much too intense when projected in 3D, which would cause major headaches for the audience. However, once the comfortable parameters for the IO have been set, since the IO creates the depth to your shot, animating that distance (having the IO incrementally change throughout the shot) will cause your scene to visibly stretch away from the audience.
Putting this principle into context of stop-motion, in terms of how the viewer receives the image that is projected, the distance between our eyes is very slight, not much more than a half-inch or so. The eyes of a typical miniature stop-motion puppet are even closer together, only a few centimeters apart. Therefore, when shooting at miniature scale for stop-motion, taking the two separate images at this very small distance apart from each other is necessary to achieve a stereo effect that our brains can actually handle. On a miniature scale like a stop-motion set, two camera lenses cannot easily get as close together as mere millimeters or centimeters, so the solution is to use one camera on a slider that moves the camera back and forth (Figure 4.29). The animator positions their puppet, the camera takes a left-eye view of it, and the camera slides over slightly to take a right-eye view of the same puppet. The camera then slides back to the left-eye view in preparation for the next image, the animator moves the puppet, and the process repeats, with the camera taking two separate images for each frame of the animation. Stereoscopic films shot on a larger scale may possibly combine two lenses in one camera, but with stop-motion being on a miniature scale, if you want to shoot in stereo, it is better with the slider option.
Once you set your IO, the other step in figuring out how deep your scene will go in or pop out of the screen has to do with alignment of your shot. If you treat the screen as the middle ground of your shot, the trick is to have your left- and right-eye images line up with each other wherever you want the middle ground (also called a zero plane or zero parallax). In many cases, you may want to focus this middle ground on your puppet character or another object on screen. This way the background will appear to be deep behind the character, and if they stretch out their arm or throw something forward, for instance, this will seem to pop out at the audience.