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Blue and green screening are common techniques used in production to place actors and objects in front of backgrounds that can be shot or created separately. One common example is the news weather-person standing in front of a screen that changes to show the temperature, weather conditions, etc. With a few supplies and a video editing program, you can try this technique yourself. Additional links are included at the end of this tutorial.
Background material - To be able to place your subject against any background, you'll first need to shoot them against a painted wall or material that has a solid color on it that you can remove later in the editing process. Blue and green are used for this, because these colors are less represented in skin tone than colors such as red and yellow. Photographic supply stores should stock blue and/or green paper. You may have to go to a special supplier to get chroma fabric or paint.
Subject - Your subject should not contain the color that is used for the background. For instance, if using a green screen, don't have your subject wear a bright green shirt that matches the color of the background, unless you want their torso to appear transparent. Subject should be far away enough from the green so that green light does not reflect back on the subject.
Background footage - You should know ahead of time what type of background you'll be eventually placing behind your subject. Your subject should be in scale with your background. For instance, a person placed against the backdrop of a city will look like a giant if the scale of the background is too large. Subjects are sometimes photographed larger, for better resolution, then scaled down.
Lighting - Good lighting is essential to a "clean" blue or green screen process. The background should be lit evenly, so that there are no dark or light patches, shiny spots or dark shadows (unless you want to keep the shadow). Your subject should be far away enough from the screen so that there is no blue or green light bouncing off the background and spilling onto the subject. You may want to backlight the subject to cut the spill from the screen, if there is any. You can cut the spill even more by putting a magenta gel on the backlight for greenscreening or an orange gel on the light for bluescreening. Doing all this will take some trial and error, so give yourself time. Check out this Basic Lighting Tutorial for more info on lighting.
Camera - You can use a still camera or any other kind of camera, depending on your situation and if you want to use moving images or not. For this process, anything that will allow you to get your picture or footage into a computer will do. Digital video cameras work great, because they can connect directly to most video equipped computers.
Computer hardware/software - Many different software programs will allow you to key out colors like blue or green. Premiere (PC/Mac) and FinalCutPro (Mac) are two of the more popular ones. AfterEffects (PC/Mac) has more sophisticated keying tools, as does Ultimatte (PC/Mac).
1) Set up your background - Put up your paper or fabric, or paint your wall, making sure that no edges, marks, creases, or wrinkles are showing. A cyclorama, or two curved walls to a floor with a curved corner between them, perhaps even a curve to a brief overhang, is the preferred background. Paper can be hung so that it curves off the floor, simulating this. Sharp dark corners will be difficult to key out. If you don't need to see people's feet on the ground, this won't be a problem.
2) Position your subject - Be creative. Subjects can walk through the frame, and even disappear behind objects covered with your blue or green material. Keep them away from the screen to mimize spill.
3) Frame your shot - Make sure that your subject is placed in the manner that you want it to appear in the final shot.
4) Light your scene - As mentioned above, pay special attention to shadows and bright spots on the background and reflected light on the subject.
5) Shoot your shot - Take as many takes as you need. This is a complicated processes, so give yourself enough footage to choose from in editing. If you move the camera, make sure that you avoid the edges of your background and that the change in perspective doesn't adversely affect the lighting. It can be easiest to reposition the subject. Change lighting if necessary.
6) Import your footage - Avid, Premiere and FinalCutPro have their own import capabilties. AfterEffects and other programs can import the footage once it's been captured by another program. See the individual program you're using for info.
7) Position your background footage and blue or green screened subject on different layers - You'll need to put your blue or green screened subject on a video layer above the background subject, to set up the transparency. You won't be able to see the background until you apply the key effect.
8) Apply the key effect - This process is different from program to program. In Premiere, it's called Transparency. In AfterEffects and FinalCutPro, it's called Chroma Key. Other options are Bluescreen or Greenscreen. See the individual program for more info.
9) Adjust the effect - Most of the time, you'll have to adjust the parameters of the effect in order to get the "cleanest" possible results. Premiere uses "Transparency" and "Cut-off". AfterEffects uses "Tolerance" and "Edge Thin". Experiement with different types of keying effects and different parameters. You may need to apply more than one keying effect if your background had shadows or bright spots. You may need to mask out parts of the foreground, like the wire holding up your subject or the edge of the blue or green screen material that got into the shot. You may also need to use a "spill suppressor" plug-in, rotoscoping (detailed masking out of parts), and/or frame by frame painting back of parts of the image to fine tune the shot.
10) Keep practicing! The more you do of this, the more you'll learn. Have fun, and keep on dreaming!
The Bluescreen Page/Online Bluescreen Store
Creative Mac: Video Tutorial: Keying Green in Adobe Premiere
Alice in Bluescreen Land - 355K PDF
Black Art of Bluescreenin - 55K PDF
Blue & Green Screen Fabric, Paint & Chroma Key Products
How to get the look of film in video with lighting, 3-02
Digital Dictionary for 3d and 2d software - Rosetta Stone Digital Dictionary
John Jackman's Green Screen Tips
Software:
Video Wave 5 (budget software for PC)
Adobe AfterEffects (advanced effects software for PC/Mac)
Adobe Premiere (advanced video software for PC/Mac)
FinalCutPro (advanced video software for Mac)
contact: "ralph at ralphdickinson dot com"