In the latest excerpt from The Advanced Art of Stop-Motion Animation, Ken A. Priebe discusses the use of bluescreen and advanced compositing.
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.
One of the challenges of shooting with a blue or green screen is the complete removal of the screen’s color from around the animated subject. Often, there may be issues with the green reflecting onto the puppet or remaining as a thin layer around the edges. With today’s digital tools, though, there are ways to deal with this. Removing the color from most of the frame around the subject is the easiest part. Depending on the software, this is usually done by selecting the color (typically with an eye-dropper tool) and hitting a button or adjusting a tolerance slider to wipe it out of the frame. In After Effects, a plug-in called Keylight is commonly used, and Combustion uses a function called the Diamond Keyer for the general removal of the green (Figures 9.28 and 9.29). Once most of the green is keyed out, there will often still be a thin rim of green around the edges of the subject (Figure 9.30). Some additional keying tools can be used for fine tuning the keying out of this remaining color. In Combustion, for example, these tools include the Discreet Keyer and suppressing the green color on the color map (Figure 9.31). To create further atmosphere once the background is comped in behind the puppet, a blur can be added to help create the illusion of a shallow depth of field (Figures 9.32 and 9.33). (Compositing and screen captures for Figures 9.28 to 9.33 courtesy of Shawn Tilling.)
Shooting any object or stop-motion puppet with a color behind it for keying also allows for simple re-sizing and moving of that object into any part of the screen for compositing with other elements. The background that is composited behind the keyed-out animation frames can be a still photograph, a painting, or any type of imagery. A particularly unique use of green-screen compositing was used on a short student film called For Sock’s Sake, made by animator Carlo Vogele at CalArts. The film uses real clothing, like pants, socks, and shirts, as a cast of characters who go on a journey to save a runaway sock. Carlo shot real clothes on a flat green screen with a digital still camera pointing down from above (Figure 9.34). To move the clothes, he placed magnets inside them and moved matching magnets underneath the green panel, which allowed them to be moved without breaking the continuity of the fabric’s folds and wrinkles. The backgrounds were drawn in Photoshop with photo collage and digital drawing, and the animated clothes were composited into them with After Effects (Figure 9.35). The film can be seen on Carlo’s blog (http://carlovogele.blogspot.com).
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.
Many different software programs, such as Photoshop or TVPaint, can be used to composite the different layers together in each frame as long as you can create transparencies for the negative space around the puppet and adjust the contrast and brightness. Independent animator Nick Hilligoss has his own method for compositing front-lit/back-lit images together using LightWave 3D. His foreground animation elements are captured as their beauty shots (Figure 9.39) and back-lit silhouettes (Figure 9.40). Each image sequence is separated, with the beauty shots applied to a rectangular flat object’s surface in LightWave as a color image map and the back-lit sequence as a transparency map. In this environment, black is solid, white is fully transparent, and shades of gray are partly transparent, so if there are any see-through objects or motion blur in the animation, they will also show up. Directly behind this rectangular object, with the beauty shots and mattes, is another flat object with the background applied to it (Figure 9.40). Both the background and puppet screen objects are exported together from the same camera view to create the final composite (Figure 9.41).
Further tutorial details for front light and back light compositing can be found on the YouTube channels for Nick Hilligoss (http://www.youtube.com/user/StopmoNick) and Ron Cole (http://www.youtube.com/user/animatorIsomer), as well as further tips searchable through Stop Motion Animation (http://www.stopmotionanimation.com).
Advanced Compositing for Ava
In Chapter 4: Digital Cinematography, I included some information that was shared with me about a moving camera shot from Lucas Wareing’s student film Ava, made at Emily Carr University of Art + Design. The camera move itself starts by pointing up at the ceiling of the film set, and then begins tilting down and moving forward through the set. The camera moves through a bunch of real foliage and eventually ends in front of the monster, Charlie, who is sleeping between two cliff facades built as part of the stop-motion set. Meanwhile, there is the effect of a moving sun being animated behind to create a real lighting change throughout the whole set. The background behind the set was a plain white backdrop, instead of a green or blue screen. This was done deliberately to better match the lighting throughout the scene and to avoid worrying about blue or green reflections spilling into the set. The original footage from this particular shot in the film (Figure 9.43) went through a complex process of compositing afterward in post-production. The visual effects for this shot would involve a digitally created matte painting of a night sky from which the camera would tilt down, eventually revealing a sunrise in the far background behind the stop-motion set, including an animated sun that followed the path of the lighting change created physically on set (Figure 9.44). Lucas and his compositor, Henrique Moser, shared with me some of the steps involved to complete this shot.
The first step was to use a steadiness plug-in to minimize some of the camera shakes that resulted from the physical stop-motion camera move. Next, the camera move was tracked to create a virtual 3D version of the movement of the camera on the physical set. This was done using PFTrack, a software program used for match-moving 3D elements to live-action plates with exact precision. The shot starts with the camera’s position extended past the beginning of the stop-motion camera move, pointing up at the matte painting. The virtual camera tilts downward across the sky portion of the painting, and eventually merges with the tracked version of the physical camera moving through the set.
The matte painting itself, originally done in Photoshop, had its various middle-ground and background elements of mountains and clouds separated onto different layers (Figure 9.45). These split-up layers of the painting were projected onto separate cards in the 3D environment to create a parallax effect as the camera moved through the set. This basically means that there is a change of perspective in the background elements that creates more depth and simulates how it would look in a 3D space, as opposed to just one static background element that stays the same through the whole shot. In the 3D environment, there was one camera that tilted upward and moved down to match the physical camera move, and another camera that was locked down and acted as a projector for the various foreground and background elements in the virtual set (Figure 9.46).
The plants and various bits of foliage were keyed out through a combination of various keying techniques to extract mattes from them. Also, in front of the virtual 3D version of the camera move, certain elements that existed in physical space on the stop-motion set were projected onto 3D cards in the exact places they appeared within the set. Certain things like the branches, plants, and cliffs would go through various stages of movement and overlap with each other as the camera moved past them, so on certain frames, sections of these set pieces had to be matted out and projected to all match together. Many things in the set needed to be painted out, including the black disc that was animated throughout the set to line up with the moving sunlight and cover it up. The projection of mattes onto the 3D cards helped in the particular frames where the disc passed in front of the cliff, for example. To cover up the disc, a section of the cliff from another frame could be matted out and placed in the space it needed to be (Figure 9.47). The color grading would also need to be manipulated to blend in with the rest of the shot since the light changes throughout the whole scene.
In addition to the basic process of compositing and removing the various elements from the original footage so that the composited background could show through, other subtle effects were added to enhance the atmosphere. One example was to enhance and exaggerate the highlights created by the sunlight at the end of the shot. This was done by keying out the bright highlights in the frame itself (Figure 9.48) and then separating the foreground and background into an alpha channel matte (Figure 9.49). The highlight shapes were blurred and given a warmer color tone, which could then be layered over the original shot to exaggerate the highlights, making them brighter and softer in the rim of light along Charlie’s body and next to his shadow on the ground (Figure 9.50).
Many other effects and subtle details were executed within the advanced production method of completing this shot, all coming together to bring to the audience a beautiful and unique approach to the art of stop-motion filmmaking. Check out the movie called Ava Footage.mov on the CD to see the original footage from the stop-motion set and breakdowns created by Henrique Moser of a few steps taken to create the final shot.
Ken A. Priebe has a BFA from University of Michigan and a classical animation certificate from Vancouver Institute of Media Arts (VanArts). He teaches stop-motion animation courses at VanArts and the Academy of Art University Cybercampus and has worked as a 2D animator on several games and short films for Thunderbean Animation, Bigfott Studios, and his own independent projects. Ken has participated as a speaker and volunteer for the Vancouver ACM SIGGRAPH Chapter and is founder of the Breath of Life Animation Festival, an annual outreach event of animation workshops for children and their families. He is also a filmmaker, writer, puppeteer, animation historian, and author of the book The Art of Stop-Motion Animation. Ken lives near Vancouver, BC, with his graphic-artist wife Janet and their two children, Ariel and Xander.