After you have created a mask using the Primatte Chromakey plugin, you may see a colored tinge around the foreground image. This fringe is called 'color spill'.
Color spill occurs when the color reflects off the back screen and casts a noticeable tint on your subject. Certain areas of your model absorb or reflect that color, which is typically blue or green. It’s kind of like throwing a ball against the wall and the ball bouncing off to hit another wall. In this case, light bounces off the chromakey screen, then throws its color on the subject.
Color spill is especially noticeable -- and difficult to remove -- in semi-transparent areas like blond hair or a wedding veil. Usually you see color spill on the back of the shoulders, side of the arms or legs, and through the hair. This often happens during the shooting process and needs to be corrected afterwards.
The goal of your setup is to reduce color spill as much as possible, so that you don’t have to remove it with software afterwards. This is why choosing a non-reflective back chromakey screen and being careful of your lighting is so important.
If the walls or ceiling are close to your subject, it is helpful to paint them black or hang black fabric. Light bounces all over the place and that light can turn your background into a big green or blue light. The black material helps soak up that light and avoids color spill on the front of the subject, which can be especially problematic.
Dealing with color spill after the shoot can often take longer than the photo shoot itself. It’s the main reason that people give up on green screen photography. However, if your shot is set up correctly, color spill mostly avoidable. You just need to be aware of spill situations and make adjustments as necessary BEFORE you shoot.
You need to remove color spill in order to create a natural looking composite of subject and background. Otherwise, your composite will look 'cutout' and fake. Any color tinge will be especially noticeable in semi-transparent areas, like wispy hair and materials such as glass and water.
The challenge is to remove spill without losing details that you need to keep, like hair strands or the edges of a glass cup. You also want to correct the extra blue or green tinge without shifting the color tones of the rest of the image.
The Primatte Chromakey plugin gives you a dozen ways to remove color spill while maintaining edge details and color fidelity.
Step Five: Lighting the chromakey screen