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3D compositing is a process by which various elements, often including both live action film footage or photographs and virtual computer-generated images, are layered and composited together into a single image or scene. This type of compositing is often done to create still images that include various components into a single image, though it can also be used in the post-production process for film or television programs to composite different elements into a video sequence. 3D compositing often differs from two-dimensional or 2D compositing in the way in which the different layers can be made to interact and more realistically affect each other.
The process of 3D compositing typically utilizes computer software developed for such compositing, often with input from various sources. This process typically involves multiple input files, including still images and video files, which are assembled and layered during the 3D compositing process. A simple example of this process would be the assembly of an image depicting a boat on the water in front of a large cliff, with other cliffs and clouds and sky in the background. The final image would contain all of these elements together in a single, essentially seamless image, though each piece could have come from a separate source.
An image created through 2D compositing typically treats each input source as one layer of a flat image, which can allow for excellent compositing for certain purposes. The use of 3D compositing, however, allows the separate layers from different sources to be pieced together in a way that allows the elements to overlap and interplay more realistically. In the above example, the boat should cast a shadow on the water below it; this may be difficult to achieve if the boat and water were separate layers in 2D compositing.
Using 3D compositing, however, these separate layers can be pieced together, and a shadow can be generated for the boat onto the water. This is typically done by 3D imaging software that can create a shadow based on the boat, or by creating the boat in a 3D imaging program in the first place, since generating a shadow in that program can also be composited into the scene. Other effects can also be more easily applied to a scene or still image through 3D compositing, such as having objects in one layer affect lighting in another. This type of compositing is also typically used to create more realistic images generated in 3D graphics software without overtaxing rendering computers, by rendering multiple passes for a scene or object that are then composited together for a final image.