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Home | Modeling | Animation | Rendering | Compositing | Motion Capture | Software | Physical Effects | Resources | |||||||||||||||||||||||||||||||||
Compositing
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| What is it? |
Compositing is the process of creating new images by combining several different images. These images can be from digital film, 3D images, still photography, and text. Ever since the 1990s, compositing has been done digitally on computers in film post production. Software packages such as Adobe After Effects and Apple's Shake provide extensive tools for doing different types of compositing. In this section I will cover how greenscreens are used for set replacements, alpha compositing, and matte painting. |
| Greenscreen Compositing |
Greenscreen, also known as choma keying, is a technique in which a green (monochomatic) background is used with the intention of replacing the background with an image (see Sin City image). The most classic example of greenscreen use is the weather on television where the weatherman is recorded on a greenscreen which is replaced by the digital background. The reason a green or blue screen is used is because their is little to none green and blue pigment in human skin. Therefore it is easy to extract the green screen without disturbing the actor's appearance once a digital background is put in. |
| Alpha Compositing |
| Alpha compositing is a process in which an image and a background are combined to create the look of partial transparency. An important concept to alpha compositing is the idea of an alpha channel. In an image, each pixel stores information on the color that it retains and an alpha channel value which ranges from 0 to 1. This alpha value allows us to determine if a pixel in the image was drawn or is empty. A value of 0 means that there is no color contribution and a value of 1 means that the pixel is completely opaque. With this alpha channel information. a composited image can be formed using compositing algebra (see image below). |
From this image you should be able to see how two images are operated upon to create a single composited image. The compositing algebra consists of : over, in, out, atop, and xor. |
| Matte Paiting | ||
Matte painting is another compositing technique in which certain areas are masked to control the areas of the picture that are exposed. The four pictures below explain the matte painting process well. In the first image the building is extracted by masking out all other elements, such as the sky, adjacent building, and trees. Once the building is extracted, it is then overlayed, or composited, with the dark sky to create a final image. This same technique can be used in film to mask out (extract) actors and composite them with digitally rendered sets. |
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Examples |
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 Final Composition |
 Building used in final composition |
 Sky (reversed) use in final composition |
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 3D Modeling and compositing in Night at the Museum |
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Greenscreen compositing used in Sin City |
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