Cameras have gotten 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 in order to save on the costs associated with storage of footage.
Avoid Too Much Compression
A bigger image does not necessarily mean better chroma keying. Do your best to avoid shooting on formats like DV or HDV as these apply heavy compression to the footage when writing to tape. Similarly, options that use SD cards often compress the footage heavily to fit on affordable storage media.
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 rise significantly). These cameras improve the quality of the footage by using less compressed recording formats and also offer greater color fidelity which can improve keying.
Progressive versus Interlaced Frames
Video footage has traditionally been shot using interlaced technology. Historically, interlaced video allowed for smoother image quality on CRT-based devices (such as traditional television sets). The technology was first implemented in the 1930’s as cathode ray tubes became brighter (and subsequently flickered more).
To improve the appearance of footage on these tube-based devices, the image was split into fields. Using this process half of 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 as it produces a clearer image that will work better for chroma keying tasks and playback smoother on modern displays.
With Cameras, Auto is a “Bad” Word
When shooting chroma key footage you’ll want to turn off all the Autos on your camera. This means no auto exposure, auto-white balance or auto-focus. If any of these are left on, the footage you are trying to key will constantly be changing as your model moves.
Even if you’ve hired a professional videographer, don’t make assumptions. Double check that he or she has turned these things off or you will waste time trying to fix it.
While in the camera settings you should also turn off sharpening. Most consumer (and even prosumer) cameras have a sharpening filter that is turned on by default. This has the effect of increasing in contrast in the edges but will destroy subtle edge detail.
There are many choices to be made 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 of the action needed? If full-length body shots are needed, especially for walking scenes, then a studio approach is generally needed. 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 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 Keying Systems. Much of the material shot for this chapter uses a Reflecmedia Chroma Key system. This approach relies on a LED disc attached to the camera lens 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.
- If 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 upon the subject matter you are shooting. In fact, other colors are sometimes used (such as red) for specialty situations.
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. We can offer some important advice though that will make your keying easier.
- Keep your subject and your camera as far away from the screen as possible: It is better to increase the distance, even if it means non-green edges are showing in the shot. 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. 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 foreground.
- Go vertical. Consider rotating your camera 90˚ to get a taller shot. This works great for a single presenter and allows you extra resolution to work with. You can now get a wide and tight shot from the same piece of footage.
- Shoot 4K Deliver HD. Another strategy is to shoot 4K and deliver HD if you want flexibility. This gives you extra resolution so you can scale the shot for different composition options.
- Avoid feet. If you can skip shooting feet, do so. It’s much harder 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.
Rich has published over 100 courses on Lynda.com. Rich has authored several books including From Still to Motion, Understanding Photoshop, Professional Web Video, and Creating DSLR Video.