Acquisition Strategies for Greenscreen
Video cameras have become significantly more affordable in recent years. Consumer-grade, high-definition video cameras can be found at prices below $1,000 US. Unfortunately, these cameras may not do the best job for keying. The footage of many consumer-grade cameras is heavily compressed to save on the costs associated with storage of footage.
A bigger image does not necessarily mean better chroma keying. Do your best to avoid shooting on formats like DV or HDV because these apply heavy compression to the footage when writing to tape. Similarly, options that use SD cards or DVDs often compress the footage heavily to fit on affordable storage media. This compression leads to blocky edges that can destroy edge detail and makes for poor composites.
This is not to say you can’t “make do” with the camera technology you have, you just may need to “work harder” to get acceptable results. For professional projects, many multimedia producers and motion graphic artists will utilize higher-quality cameras (that generally start at $5,000 US and increase significantly). These cameras offer 4:2:2 chroma subsampling (or better) and improve the quality of the footage by using less compressed recording formats and also offer greater color fidelity, which can improve keying.
Progressive vs. Interlaced Frames
Video footage has traditionally been shot using interlacing (Figure 5.7). Historically, interlaced video allowed for smoother image quality on CRT-based devices (such as traditional television sets). The technology was first implemented in the 1930s as cathode ray tubes became brighter (and subsequently flickered more).
Figure 5.7 The image on the left shows the “tearing” that is visible with interlaced footage when viewed on progressive displays. The image on the right is a cleaner plate and shows the benefits of shooting progressive when keying.
To improve the appearance of footage on these tube-based devices, the image was split into fields. Using this process, half the image loads onto the CRT display from the top-left corner to the bottom-right corner. The process then repeats for the second half of the image. This approach is ideal for CRT displays but produces jagged-looking footage on other display types (especially computer displays).
Fortunately, video technology evolved. Cameras are now readily available to shoot video using progressive formats. Choosing a progressive format is highly desirable because it produces a clearer image that will work better for chroma keying tasks and plays back smoother on modern displays.
Tape or Tapeless
The use of tape has been the traditional acquisition approach for cameras. Tape offers a cheap way to archive footage. It does have its drawbacks though, because it requires more (and often expensive) hardware for playback and loading of the footage into a computer. It also must often compress the footage further to store the acquired information. Another drawback is that loading tape is a real-time process; it takes at least one hour to load an hour of source material.
These limitations have been largely responsible for the increasingly popularity of “tapeless” cameras (Figure 5.8). Manufacturers have chosen to approach tapeless acquisition in different ways. Some use hard drives built in to the cameras, whereas others favor removable storage. These solutions can speed up the process of loading material into the computer. Additionally, some models offer storage of HD video with less compression than their tape-based counterparts. Generally speaking, tapeless solutions are preferable for a modern HD workflow.
Figure 5.8 Many cameras use tapeless acquisition (such as this camera from Panasonic). Storage options like P2 cards make capturing high-quality HD video easier.
You’ll often find that many cameras offer a live video stream via an SDI or HDMI port. This output is often uncompressed and can be captured directly into a tower via a professional capture card for an even better chroma key.
You’ll have many choices to make when shooting chroma key. The backdrop you use will greatly impact the quality of the key you perform. Here are some considerations when selecting a backdrop:
- Size. Is the backdrop large enough to accommodate all the action needed? If full-length body shots are needed, especially for walking scenes, a studio approach is generally needed for controlled lighting in a large space. If tighter shots can be used, portable backdrops are a much more affordable choice.
- Fabric. There are many choices to be had. The most popular backdrops use polyester fabric stretched by a metal frame, which offers an easy to light surface that avoids wrinkles and shadows. These backdrops can be easily folded and transported. Muslin backdrops are also used but may require more attention to lighting to avoid wrinkles and bad keys.
- Chroma key systems. Much of the material shot for this chapter was shot using a Reflecmedia Chroma Key system. This approach relies on an LED disc attached to the camera lens (Figure 5.9) that reflects light on a special fabric containing millions of glass beads that reflect the lower-powered light and create an even-colored surface. Systems like this cost more but are popular for their ease of use and portability.
- Color. You should use a color that is the opposite of the foreground color. Blue or green are usually chosen, because there is very little of those colors in human skin. But if you’re shooting a product that has a lot of blue and green in it, you might be better off using a red screen.
Figure 5.9 A Reflecmedia Chroma Key system is a good option for its ease of use and portability.
If you’re shooting DV or heavily compressed HD, definitely choose a green backdrop. These video formats show less noise in the green channel, which makes for a better key. Otherwise, the choice of blue or green is really based on the subject matter you are shooting. In fact, other colors are sometimes used (such as red) for specialty situations.
The color you choose is really a balance between finding something that works for the subject matter (such as skin tones) and the technology driving your key. Some keyers depend on green or blue to perform a difference key, whereas others are more flexible in their keying approach.
The last step to shooting great chroma key is the actual shooting (yes, good keying takes work). Our book can’t teach you to be a top videographer in a few paragraphs. But we can offer some important advice that will make your keying easier (Figure 5.10).
Figure 5.10 An approach to shooting great chroma key. Always remember that you are -lighting both the subject and the backdrop (and that those are two separate things).
- Keep your subject and your camera as far away from the screen as possible. It is better to increase the distance between your subject and camera, even if it means nongreen edges are showing at the edge of the frame (not behind the subject). You can always crop these out with a garbage matte.
- Avoid fast movement. You’ll see better results if you can avoid motion blur. This is where keys typically become “obvious.”
- Use shallow depth of field. If your camera supports it, lower your aperture. You want the background as blurry as you can get it so that wrinkles, seams, and hot spots blend away.
- Use a garbage matte. You don’t need to key everything in the frame. In After Effects and Motion you can use the Pen tool or a mask to crop out portions of the background. This makes it necessary to only key the most active areas, which can hide issues. It also means that the green screen needn’t fill the entire frame when shooting.
- Minimize camera movement. Try to avoid moving your camera, as well as zooming or panning (Figure 5.11). These tasks are easier to do in post and won’t involve having to do motion tracking, image stabilization, and match moving to synchronize the background to the foreground.
- Avoid feet. If you can skip shooting feet, do so. It’s very difficult to key a full-length shot and get the shadows right. Some folks will cheat and use a treadmill surface to show someone walking with a key. Objects like turntables and treadmills can also be painted green so they can be keyed.
Figure 5.11 Keep the camera steady when shooting with a green screen. You’ll want to avoid as much camera movement as possible because it just makes keying more difficult.