Chroma Green Color

What do you do with the chroma background once you have it? First, put a subject in a foreground and shoot footage of the subject in front of the chromakey background. Second, process the footage using special software (or in old days hardware) to remove the background from the footage. That is it, delete! Once you deleted the green background the footage does not have a background anymore. Now is the really fun part, you can bled anything on the background now.  If you blend in a photo, it would look like the subject was shot against that background. To see how it works, just tune in into a weather report on any TV news station. It is very likely you will see a weather man in front of a gigantic weather map. The Weather Man looks very cool in front of this map. The real life shaman weather men (very often women though) were rally manikin weather for real. Today they do not even try. They just pock their finger into green chromakey background. TV technicians do the rest. But what you see, dear reader, is a complete illusion (as anything else, in fact, but this is a bit different story).

The word “chroma-key” means that color (“chroma”) is used as as a key to remove the background. The key can, in fact, be of any solid color which is not present in the foreground.  If the foreground contains green, the backgournd key is often blue. If foreground contains both green and blue, then chromakeying might not be the solution. In this case it is possible to use opaque black or pure white background. In this case the technique is called “lumakey” because “luma”, or amount of light, is used as a key.

In all cases it is very important to make sure that the background and foreground are  well lit. Make sure that there are no shadows cast by the foreground on the background and there is no light spill from the background to the foreground. It is easy to get rid of the shadows cast on the background by lighting the background with its own source of light that simply fills-in the shadows and overpowers them.  Inverse squire law is to your rescue here.  Just make sure that distance between the foreground lights and foreground is about the same as distance from the background lights to the background. At the same time, make sure that the subject is at least 45 feet (1.5 meters) away from the background. Increasing the distance from the foreground from the background is limited by space in your studio and the size of the chromakey background. So, effective lighting of the chromakey set is easier in if you have a lot of room and a huge chromakey background. In fact you can get a 20×24 feet backgound relative cheaply on eBay. But were would you hang it? Do you have an animal farm? Get those goats out on the pastures, clean the barn, and viola, you’ve got yourself  a great movie set.

If the first problem can be resolved by adding more lights and space, second problem is a bit more tricky. This problem comes from stubborn reflectiveness of the backgrounds. Background light reflects from the colored background and casts a tint of that color on everything around it. The foreground now catches some of that reflection, particularly around the edges, hair and fine details.  Now the foreground has some of the color it did not have before!

There are different ways of dealing with the second problem. This would be a subject for my future post. Stay tuned!