Chroma key compositing or chroma keying is a production technique where two or more separate visual elements are combined together with the aid of a certain colour range. During post-production, this colour from the raw footage, typically green or blue, is replaced by other visual elements not present during the initial shoot. These are usually 3D special effects, such as an explosion or another background.
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A chroma shoot is widely used in Hollywood to create 3D chroma key video tricks. This type of shoot generally requires the use of a green or blue background; then the background is deleted during post-production and replaced with the special effects and other background elements the scene requires. A number of chroma key video tricks make it possible for out-of-the-ordinary scenes to look seamlessly real in the movie. When combining them with live-action shots, the images replacing the chroma background may look photorealistic, surreal or magical, depending on the storyline. Essentially, the images should have a three-dimensional perspective to match the sense of reality of the real actors, sets and props. The background serves as a matt, which allows a particular scene to combine two or more images in a single frame. For instance, when putting a space background over astronaut characters, the matt could be a background painting of a large scenic expanse of space.
Motion Control Photography
Motion control photography is a video trick that requires shooting very precise camera movements and creating specific shots and effects that are impossible to do with regular shooting techniques. The process involves shooting the actor with a chroma screen background; then the motion control camera rig shoots separate backgrounds for the scene, which should look like one continuous shot otherwise impossible to shoot together in a single take. Using the motion control technique, this set-up allows the separate scenes to look seamless, as if they were shot in only one take. For instance, the production needs to shoot a 360-degree shot of a room in a castle. However, shooting continuously would reveal the other side of the scene behind the camera -- filled with lighting equipment and crew members. The motion control camera is locked into position, the production rearranges everything needed to complete the shot, then the camera shoots what's left of the scene to create a 360-degree shot. The shot of the actor walking around is placed in the final scene during post-production.
There are times that a Hollywood movie requires an actor to do stunts that are impossible to do in real life. From flying to jumping scenes that clearly defy gravity, actors typically use harnesses and wires so that they can do such stunts. After shooting the action scenes in chroma background, the wires and harnesses on each scene are conveniently erased with special effects programs. This provides the illusion of flight and makes extraordinary stunts for the movie. After erasing these unnecessary elements, the 3D background of the actors is also placed over the scene.
Virtual Sets and Real-time Effects
When shooting very challenging scenes that require lots of photorealistic images that actors don't actually see when filming, the technique primarily used in TV news and talk shows involving real-time showcasing of the chroma screen's replacement footage, such as a weather map or virtual set, can be used during the film shoot. After the actor finishes initial filming over a chroma screen, he can readily view from a monitor a raw example of the previsualization special effects, often referred to as "previs." This allows the actor to visualise the scene better so that he can shoot the scene again with more suitable emotions, a perspective of the props and sets and an accurate line-of-sight when having to look at CGI elements. Although the previs material is not yet final, present technology already allows rendering of effects that are very near the intended final material.
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