The Advanced Art of Stop-Motion Animation: Visual Effects - Part 2
Front Light/Back Light
Another compositing method for stop-motion that harkens back to an old film technique is a checkerboard matte (or front light/back light compositing). The general idea behind it is to take a frame of the puppet against a black or neutral gray background, referred to as a “beauty shot.” In this frame, the puppet is lit from the front only, with the light shielded from reflecting onto the background (Figure 9.36). Using a black card or curtain may help shield or absorb the lighting, or using “barn doors” on the light fixture. Next, the same frame is shot with a wash of light reflected onto a white card behind the puppet to create a silhouette image of it (Figure 9.37). The puppet can then be moved into the next position and the process repeated. This creates an extra step of taking each frame twice—once as a front-lit beauty shot and once as a back-lit silhouette, which serves as a transparent matte for compositing with a background. In the compositing process of layering these images together for each frame of the animation, the background is on the bottom layer. On top of the background is the back-lit silhouette matte, which provides an opaque silhouette image of the puppet over the background, and the negative space around it is transparent so the background shows through. On top of this matte, the beauty shot image is placed over the exact silhouette, which its negative space also made transparent, resulting in a clean composite of all elements (Figure 9.38).
Although it needs twice as many frames and an extra repetitive step to execute during shooting, this method is essentially an alternative to a green screen. It avoids the issues of the green color reflecting onto the puppet or appearing as a rim around it, and in many cases it provides a cleaner, softer edge around the puppet for compositing. Obviously, during the animation, the lighting set-up for both sets of frames, as well as the camera, must be locked down tightly so that all of the images line up exactly. It is common for the back-lit silhouette images of the puppet to still have some highlights spilling into the edges and to have visible features within the silhouette. In many cases, these frames will need to have the brightness and contrast cranked up to create a crisper matte that is completely black and white. In some cases, there may also need to be the option of rig removal in the frames if your puppet is defying gravity in some way.
When this method is used on film, the single strip of film consists of alternating black-and-white images—hence, the term “checkerboard matte.” It also means that the animation, when played back at speed, is twice as slow and flashes quickly with black-and-white frames. The alternate frames are separated and put back together in the optical printer. Shooting digitally with frame-grabbing software, the alternate silhouette frames can be hidden during the animation process and exported separately into another folder. Some software programs, like Dragon, can also separate the alternate frames into subfolders while you shoot the animation. Either way, at the end of the shoot, you want your beauty-shot and silhouette frames separated and organized into different directories.