Datenbank Glossar – Compositing

Glossar CGI	Glossar Compositing	Glossar Special Effects

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Compositing

Compositing is the combining of visual elements from separate sources into single images, often to create the illusion that all those elements are parts of the same scene. Live-action shooting for compositing is variously called “blue screen,” “green screen,” “chroma key,” and other names. Today, most though not all compositing is achieved through digital image manipulation. Pre-digital compositing techniques, however, go back as far as the trick films of Georges Méliès in the late 19th century; and some are still in use.

Basic procedure

All compositing involves the replacement of selected parts of an image with other material, usually, but not always, from another image. In the digital method of compositing, software commands designate a narrowly defined color as the part of an image to be replaced. Then every pixel within the designated color range is replaced by the software with a pixel from another image, aligned to appear as part of the original. For example, a TV weather person is recorded in front of a plain blue or green screen, while compositing software replaces only the designated blue or green color with weather maps.

Histoy

In 1857, the Swedish-born photographer Oscar G. Rejlander set out to create what would prove to be the most technically complicated photograph that had ever been produced. Working at his studio in England, Rejlander selectively combined the imagery from 32 different glass negatives to produce a single, massive print. A reproduction of this print, which was titled The Two Ways of Life , is shown in Figure 1.1.

It is one of the earliest examples of what came to be known as a “combination print. "

Motion picture photography came about in the late 1800s, and the desire to be able to continue this sort of image combination drove the development of specialized hardware to expedite the process. Optical printers were built that could selectively combine multiple pieces of fi lm, and optical compositing was born. Many of the techniques and skills developed by optical compositors are directly applicable to the digital realm,and in many cases, certain digital tools can trace not only their conceptual origin but

also their basic algorithms directly to optical methodologies. Consequently, the digital compositing artist would be well served by researching the optical compositing process in addition to seeking out information on digital methods.A number of early examples of optical compositing can be found in the 1933 film King Kong.

Importent points

1.Alpha Blending: blending foreground and background

2.Keying: separating foreground and background

3.Luma, chroma, difference keying

4.Rig Removal: removing unwanted elements,

5.Matching real lighting with CGI

Matching lighting which is part of a photographed background is a staple art of Technical Directors and FX artists. It is an art more than a science.

Alpha Blending:

C = á F + (1 – O) B

F : foreground image

B : background image

C : composition

O: opacity or transparency

An image of á values is called a matte.

The above operation is performed on each corresponding pixel.

Keying

Separating foreground from background,creating a matte of foreground.Also called pulling a matte (of foreground),

Basic methods:

Luma keying: based on luminance (i.e., intensity)

Key out the background based on luminance.Useful when background has a uniform luminance that is very different from foreground luminance.

Chroma keying: based on color (i.e., bluescreen, greenscreen)

Key out the background based on color.Useful when background has a uniform color that is

very different from foreground color.

Difference keying: requires a clean plate, i.e., a background image without the foreground element.

More general than luma and chroma keying.Key out background based on pixel-wise color difference between foreground and background footage.

Can be used when Blue screen is not perfect, e.g., many shades of blue.Background is not blue screen.

Blue spill: Light reflected from blue screen;must be removed.

Applications

In TV studio practice, blue or green screens may back news readers so that stories can be composited behind them, before being switched to full-screen display. In other cases, presenters may be completely within compositing backgrounds that are replaced with entire “virtual sets” executed in computer graphics programs. In sophisticated installations, subjects, cameras, or both can move about freely while the computer-generated environment changes in real time to maintain correct relationships between the cameras, subjects, and virtual “backgrounds.”

Virtual sets are also used in motion pictures, some of which are photographed entirely in blue or green screen environments; for example, Sky Captain and the World of Tomorrow. More commonly, composited backgrounds are combined with sets – both full-size and models – and vehicles, furniture, and other physical objects that enhance the “reality” of the composited visuals. “Sets” of almost unlimited size can be created digitally because compositing software can take the blue or green color at the edges of a backing screen and extend it to fill the rest of the frame outside it. That way, subjects recorded in modest areas can be placed in large virtual vistas. Most common of all, perhaps, are set extensions: digital additions to actual performing environments. In the film, Gladiator, for example, the arena and first tier seats of the Roman Coliseum were actually built, while the upper galleries (complete with moving spectators) were computer graphics, composited onto the image above the physical set. For motion pictures originally recorded on film, high-quality video conversions called “digital intermediates” are created to enable compositing and the other operations of computerized post production. Digital compositing is a form of matting, one of four basic compositing methods. The others are physical compositing, multiple exposure, and background projection.