The alpha channel is the technology behind transparency in computer graphics. It first evolved in the late 1970s at the New York Institute of Technology and then continued to be refined at Lucasfilm and at Pixar. When you employ alpha channels embedded into footage layers, transparency data can travel seamlessly between After Effects and Flash.
The original footage is used to generate a matte file, which can be stored with the layer for export as an alpha channel or can be used to composite over a different background layer in After Effects or Flash.
You can create alpha channels in several ways. One common technique you can use for video footage is chroma key technology. By shooting elements against a blue or green screen, you can easily remove the background. This technology is hardly new, making its first appearance at RKO Radio Pictures in the 1930s. The popularity of chroma key technology has grown immensely, bringing the technology and tools within reach of most content creators.
Shooting Chroma Key Right
Successful use of chroma (or color) key requires properly acquired footage. While software tools are fairly forgiving of poorly shot footage, the most professional results start with well-executed acquisition. With this in mind, let’s explore effective techniques for creating footage.
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 be the best for keying. The footage of many consumer-grade cameras is heavily compressed to save on the costs associated with the 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 they write to tape. Similarly, options that use SD cards or DVDs 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 utilize higher-quality cameras (that generally start at $5,000 US and increase significantly).
Progressive vs. interlaced
Video footage has traditionally been shot using interlaced fields. 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).
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.
For better appearance on these tube-based devices, the image was split into fields. When this interlaced process is used, 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 has evolved. Cameras are now readily available to shoot video using progressive formats. This means the camera records full frames instead of interlaced fields. Choosing a progressive format is highly desirable because it produces a clearer image that will work better for chroma keying tasks and will play back smoother on modern displays.
Tape vs. tapeless
The use of tape has been the traditional approach and a cheap way to archive footage for cameras. It does have its drawbacks, though, because it requires hardware for playback and loading of the footage into a computer. It also must 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 increasing popularity of “tapeless” cameras. Manufacturers have approached tapeless acquisition in different ways. Some use hard drives built into the cameras, whereas others favor removable storage. These solutions can speed up the process of loading material into the computer. Additionally, some offer storage of HD video with less compression than their tape-based counterparts. Generally speaking, tapeless solutions are preferable for a modern HD workflow.
The frame rate of video indicates the number of still images shown every second to create the illusion of motion. This phenomenon is referred to as persistence of vision and means that the human eye can connect a series of still images into smooth motion due to the chemical transmission of nerve responses.
The frame rates used to create smooth motion often vary. Video frame rates tend to rely on frame rates between 24 and 60 frames per second for common uses. Film has traditionally been shot at 24 frames per second, whereas cartoon cel animation is most often drawn at only 12 frames per second. In Flash animation you may even encounter frame rates as low as 8 frames per second.
What does this all mean? Quite simply, computers are not televisions (there we’ve said it). Video cameras shoot video with the intention of playback on video devices like television sets. These higher frames rates are desirable but will often need to be simplified for use in Web deployment. This process can be easily accomplished in the conversion from After Effects to Flash.
Video is often acquired using non-square pixels, meaning that pixel width is not the same as pixel height. There are numerous reasons for this discrepancy ranging from the need for flexibility in supporting multiple frame sizes within the same camera to limitations in digital formats. It is important to realize that you will often need to reshape video when preparing it for use in Flash; if you don’t, your images may appear heavily distorted when viewed on computer displays. You’ll learn more about this later.
You’ll need to make many choices 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. Be sure the backdrop is large enough to accommodate all the action. If full-length body shots are needed, especially for walking scenes, a studio approach is generally the solution. If tighter shots can be used, portable backdrops are a much more affordable choice.
- Fabric. Many fabric choices are available. 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 uses a Reflecmedia Chroma Key system. This approach relies on an 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 are costly but are popular because of their ease of use and portability.
Color. Use a backdrop color that is the opposite of the foreground color. Blue or green is usually chosen because human skin contains very little of those colors. But if you’re shooting a product that has lots of blue and green in it, you might be better off using a red screen.
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 special situations.
Effective keying relies on good lighting. It is essential to minimize variation in colors for the backdrop, meaning that you must evenly light the background to avoid hot spots. Additionally, you’ll want well-lit subjects that don’t cast shadows on the backdrop. This might sound a bit challenging, so here are a few tips on doing it right:
- Spill is bad. Your backdrop must be evenly lit. You’ll also want to place the foreground person or object far enough away from the backdrop (with their own lighting) to avoid spill. When subjects stand too close to the backdrop, the color can spill into lighter areas of their hair or clothing, which causes keying problems. Additionally, if the foreground and background are too close, you’ll get shadows on the backdrop.
- Get rid of hot spots. Avoid variation in brightness for the backdrop so you can get a better chroma key. To do this, look for hot spots. Simply turn down the exposure of your camera and look through the viewfinder. Your hot spots will be quite visible. Adjust your lighting by softening it, and then set your exposure back to a normal level.
- Light with softboxes or fluorescent lights. The use of these specialty light types will better enable you to get an evenly lit backdrop. The goal is to spread an even amount of light across the surface of the backdrop.
- Match the foreground to the background. If possible, you should know what your keyed footage will be composited with so you can adjust lighting. The most believable keys try to match the lighting of the replacement background in the final composite. Make sure that the light is shining from the same direction (and with the same color and intensity) as the light shining on the background plate you plan to use.
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:
- Keep your subject and your camera as far away from the screen as possible. It is better to increase the shooting distance, even if it means nongreen edges appear in the shot. You can always crop out these edges in After Effects.
- Avoid fast movement. After Effects is quite good at handling motion blur, but you’ll see better results if you can avoid it. Motion blur is typically where keys become “obvious.”
Use shallow depth of field. If your camera supports it, lower your aperture. You want to blur the background as much as possible so that wrinkles, seams, and hot spots blend away. Adjusting your camera’s aperture setting allows elements farther away to fall out of focus.
Use a garbage matte. You don’t need to key everything in the frame. In After Effects you use the Pen tool or a mask to crop out portions of the background. So, it is 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 you’re shooting.