Chroma key compositing, or chroma keying, is a special effects / post-production technique for compositing (layering) two images or video streams together, used heavily in many fields to remove a background from the subject of a photo or video - particularly the newscasting, motion picture and videogame industries. A color range in the top layer is made transparent, revealing another image behind.
The chroma keying technique is commonly used in video production and post-production. This technique is also referred to as color keying, color-separation overlay (CSO; primarily by the BBC), or by various terms for specific color-related variants such as green screen, and blue screen - chroma keying can be done with backgrounds of any color that are uniform and distinct, but green and blue backgrounds are more commonly used because they differ most distinctly in hue from most human skin colors and no part of the subject being filmed or photographed may duplicate a color used in the background.
In Downfall Parodies
Many FX-based Untergangers use this technique to either put various items or persons in a Downfall scene or to remove certain elements from the video (superimposition). Sony Vegas is the most known software that uses this feature, although there are many others. FX parodies are made by enough untergangers that it is considered one of the styles in parody making.
Green screens can be easily made by cropping the desired part of a photo (or video) and filling the rest with a specified colour (usually green or blue).
The technique does have its disadvantages, though, as it is known to produce artifacts compared to rotoscoping.
A tutorial for chroma keying can be found here .
Known Untergangers that use Chroma key: