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