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Video compositing is a process that combines images from multiple sources into a single video. A portion of one video is edited away and layered over another image. The most common method of editing out part of an image is the chroma key, where all objects of a single color are removed from the picture. Bright green or blue are most common. Typically, an actor or presenter is recorded in front of a blue or green backdrop, and for this reason video compositing is frequently referred to as bluescreening or greenscreeening.
When making a recording for video compositing, a suitable backdrop is essential. This backdrop, either blue or green, must be smooth and properly lit for the video compositing process to work well, and the screen must be large enough for the entire shot. Clothing and objects in the foreground similar in color to the backdrop cannot be used.
The backdrop is critical when recording a video for compositing. It must appear as an even field of either blue or green in order to edit away cleanly. Any bright spots or dark shadows may be dissimilar enough to remain in the image. A backdrop needs to be smooth and evenly colored, with a matte finish and no discernable pattern. Painted walls are simplest, but if a portable option is required, cloth backdrops specifically designed for video compositing are available.
Size must also be considered when choosing a backdrop. The backdrop must be large enough to fill the image for the entire shot. For a stationary presentation, a small backdrop will work fine. But if the subject needs to move around, a much larger background is required. In some cases, such as when the subject is to appear to fly, it may be necessary to chroma key the floor and other objects as well.
Proper lighting is also required to give the backdrop the even appearance required. Most importantly, the subject being filmed and the backdrop must be lit separately to prevent the subject’s shadow from appearing on the backdrop. For best results, three diffuse lights are recommended for the backdrop, with one overhead and one to either side.
Either blue or green is suitable for chroma key video compositing, but it is important that whichever color is chosen does not appear in the portion of the remaining image. For example, green clothing and objects should not be recorded in front of a green screen unless they are intentionally being removed from the shot. Accessories and ties are most likely to be overlooked, often with unintentionally humorous results.