Making A Green Screen With Local Materials
One of the more interesting things you can do in video is to define something as a key on a video track so that the content underneath the track will show through. These can be luminance keys, a channel or you can use a color to define as the key, too. This is a chroma key.Chroma keys are also frequently referred to as blue screen (or bluescreen) and green screen (or greenscreen), simply because those are the two best-known colors for keying, although you can key any color. I prefer green.
The local news weather report generally relies heavily on chroma key. The animated weather map behind the reporter is generated and reporter is in front of a wall painted a single color to be keyed out.
A few years ago, we had a project here in Kalispell, Montana, that required a color key, so I searched the Internet for some paint. A gallon of the good stuff was over $80 then, and to get it to me quickly pushed the cost to nearly a hundred bucks.
It struck me that there might be a way that was as good and much cheaper.Wal-mart has a paint department, and I shuttled off to pickup some paint sample cards. The green range that I thought would work fairly well spanned about 40 distinct hues of green.
I laid out the samples and video taped them in a series of different light setups in both incandescent and outdoor light.
Arugula (number 91322) looked fine keyed in Premiere.
I had a gallon mixed in the Flat finish. There was a sale at the store and the cost was $6 a gallon, a considerable saving. While you might not have a Wal-mart discount store near you, the same techniques can be used in any store where paint samples can be taken and video taped and the color keyed to see which color yields the best results.
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We got some flat sheets, 4 feet x 8 feet to paint. It is import to have very smooth texture for the paint, and paint the surface so there are no blemishes. We used three coats, and did a fine sanding between coats. The flats we used were insulation cores that cost about $15 apiece. We were able to have 3 portable screens completely painted for a bit over $50.Lighting for greenscreen is important. The nature of light and color become quite apparent the more you key. A single light source to either side of a surface will show a gradual change of color from lighter to darker hues from the light source.
By placing multiple lights on either side of the screen, and maintaining flat and even light on the screen, you can get a better key. Even keen eyes will have trouble seeing the color differences – I use a light meter and try to keep the light in about a half, even quarter f-stop throughout the screen.
The subject in front of the key has to be lit well, too. If you are setting up a shot that you want to be seen as reality, then the subject in the key needs to have the same light direction as the background. So if the light in the background is coming from the left and high of the camera, then the subject in the key should also be lit from the left and high of the camera, too.
Subject placement needs to be off the screen, too. Shadows from the subject falling on the screen destroy the effect.In Premiere the key material is on V2 or higher and the Transparency is applied. Threshold refers to the amount of transparency in the key in the clip, Cutoff to the amount of opacity in the subject.
You can also select *Mask* to just see the key. The corner *Handles* can be used to make a garbage matte.DV suffers from the nature of 4:1:1 color sampling. I use a Canopus DV Storm, and it has an excellent keyer. Other companies make plugins that work with Premiere to improve DV keying, too. And of course, Adobe After Effects and Boris have a great keying with in them, too.
So get to the local discount paint store and key away!
About the author:
Mike Gunter, is host of the Premiere, Educators and Canopus DV Forums. He retired from the Army in 1988, and from the Air Force Civil Service in 1995. He was the Editor of Airpower Journal and the creator of USAF's Air Chronicles, the Department of Defense's first on line publication. He has taught digital imaging and digital videography at Flathead Valley Community College. He has won numerous Telly and Aegis awards for his video work. He is a Adobe Certified Training Provider, and his clients include the US Army, The National Guard Bureau, the University of California, Abbot Laboratories, the State of Oklahoma and others. He has training assignments throughout the United States. He lives with his lovely bride, Shirley, and their miniature rat terrier, Wolfgang Pup (AKA, Wolf), in Kalispell, Montana. 
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