Digital compositing is the process of digitally assembling multiple images to make a final image, typically for print, motion pictures or screen display. It is the evolution into the digital realm of optical film compositing. Film is a term that encompasses individual motion pictures, the field of film as an art form, and the motion picture industry. ... In visual effects post-production, compositing refers to creating new images or moving images by combining images from different sources – such as real-world digital video, film, synthetic 3-D imagery, 2-D animations, painted backdrops, digital still photographs, and text. ...

Mathematics

The basic operation used is known as 'alpha blending', where an opacity value, 'α' is used to control the proportions of two input pixel values that end up a single output pixel. This example shows an image with a portion greatly enlarged, in which the individual pixels are rendered as little squares and can easily be seen. ...

Consider three pixels;

• a foreground pixel, f
• a background pixel, b
• a composited pixel, c

and

• α, the opacity value of the foreground pixel. (α=1 for opaque foreground, α=0 for a completely transparent foreground). A monochrome raster image where the pixel values are to be interpreted as alpha values is known as a matte.

Then, considering all three colour channels, and assuming that the colour channels are expressed in a γ=1 colour space (that is to say, the measured values are proportional to light intensity), we have: Mattes are used in photography and filmmaking to insert part of a foreground image onto a background image, which is often a matte painting, a background filmed by the second unit, or computer generated imagery. ...

c_r = α f_r + (1 - α) b_r

c_g = α f_g + (1 - α) b_g

c_b = α f_b + (1 - α) b_b

Note that if the operations are performed in a colour space where γ is not equal to 1 then the operation will lead to non-linear effects which can potentially be seen as aliasing artifacts (or 'jaggies') along sharp edges in the matte. More generally, nonlinear compositing can have effects such as "halos" around composited objects, because the influence of the alpha channel is non-linear. It is possible for a compositing artist to compensate for the effects of compositing in non-linear space. In statistics, signal processing, computer graphics and related disciplines, aliasing is an effect that causes different continuous signals to become indistinguishable (or aliases of one another) when sampled. ... jaggies are those sharp edges that you see in all the wii games Jaggies is the informal name for aliasing artifacts in raster images, often caused by non-linear mixing effects producing high-frequency components and/or missing or poor anti-aliasing filtering prior to sampling. ...

Performing alpha blending is an expensive operation if performed on an entire image or 3D scene. If this operation has to be done in real time video games there is an easy trick to boost performance.

c_out = α f_in + (1 - α) b_in

c_out = α f_in + b_in - α b_in

c_out = b_in + α (f_in - b_in)

By simply rewriting the mathematical expression one can save 50% of the multiplications required.

Software

The most historically significant nonlinear compositing system was the Cineon, which operated in a logarithmic color space, which more closely mimics the natural light response of film emulsions (the Cineon system, made by Kodak, is no longer in production). Due to the limitations of processing speed and memory, compositing artists did not usually have the luxury of having the system make intermediate conversions to linear space for the compositing steps. Over time, the limitations have become much less significant, and now most compositing is done in a linear color space, even in cases where the source imagery is in a logarithmic color space. Cineon was the first computer system designed by Kodak for digital intermediate film production. ...

Compositing often also includes scaling, retouching and colour correction of images.

Digital compositing systems

• Apple Shake
• Autodesk Combustion
• Autodesk Flint, Flame & Inferno
• Autodesk Smoke
• Autodesk Toxik
• Adobe After Effects
• Pinnacle Commotion
• eyeon Fusion
• D2 Nuke
• CompTime Industrial Light & Magic
• Industrial Light & Magic's proprietary Saber
• SideFX Houdini Halo (Houdini Master)
• Jahshaka

Shake is an image compositing package used in the post-production industry. ... To meet Wikipedias quality standards, this article or section may require cleanup. ... Adobe After Effects is a digital motion graphics and compositing software published by Adobe Systems. ... Commotion was originally released by Puffin Designs. ... Fusion (formerly Digital Fusion) is a full-featured, node-based compositing system created by eyeon Software Inc. ... Nuke is the Academy Award-winning compositing software used by Digital Domain. ... Industrial Light and Magics proprietary Sabre compositing technology is actually based on Discreet Logics Inferno and Flame. ... Jahshaka is a free video editing, effects, and compositing suite. ...

See also

• Gamma correction
• Bluescreen
• Digital cinema
• Compositing
• Alpha compositing

Gamma correction is the name of an internal adjustment made in the rendering of images through photography, television, and computer imaging. ... The bluescreen setup. ... This article is about digital presentation. ... In visual effects post-production, compositing refers to creating new images or moving images by combining images from different sources – such as real-world digital video, film, synthetic 3-D imagery, 2-D animations, painted backdrops, digital still photographs, and text. ... In computer graphics, alpha compositing is often useful to render image elements in separate passes, and then combine the resulting multiple 2D images into a single, final image in a process called compositing. ...

Further reading

• T. Porter and T. Duff, "Compositing Digital Images", Proceedings of SIGGRAPH '84, 18 (1984).
• The Art and Science of Digital Compositing (ISBN 0-12-133960-2)