The Advanced Art of Stop-Motion Animation: Visual Effects - Part 2
The technique of shooting stop-motion puppets against a neutral blue or green screen is pretty straightforward. If your intention is to combine stop-motion with a live-action shot, first shoot your live action separately (Figure 9.24), and then animate your stop-motion puppet on a miniature set with the screen behind it (Figure 9.25). In the computer, you can place the live action into another layer under the stop-motion sequence, and remove (or key out) the blue or green background to reveal the live action underneath. The result will be both images composited together in the same frame (Figure 9.26).
Whether you use a green or blue screen depends on several different factors. Traditionally, blue was the best color for optical matte shots on film and green became the preferred choice for video because of the nature of the media themselves. Today, with most films being shot with video or digital cameras, there are subjective and artistic in addition to the technical considerations. One factor is the colors present in the stop-motion puppet you are shooting. If your character is designed with many shades of green, a blue screen may be a better choice (and vice versa—using a green screen for a blue character). This separation of colors helps in the compositing process and avoids any color from the actual puppet being keyed out along with the background. The lighting may also have an impact on which screen to use; a blue screen may separate from the puppet better in warm light situations, and cold lighting may be better for a green screen. The screen itself can be purchased as a pre-colored posterboard (Figure 9.27), a fabric sheet, or a flat screen material. You can also buy paint (Rosco or a similar brand) in the specific key color and apply it to a flat sheet of board or foam core. With either method, the background should be lit evenly or illuminated from behind to allow for a clean wash of color and no shadows, which makes it easier to key out the color later.