Keying (graphics)

Keying is a technical term from the video editing and describes the cropping of pixels from the background (usually based on a key color, so keying). Here, a mask (alpha channel) is created that defines a transparency for each image region or pixels. Keying is therefore an important technique of visual effects to create image compositions ( compositing ).

Keyingmethoden

There are different keying techniques, approaches have their different advantages and disadvantages. The goal of all keying approaches is to generate the alpha mask as precisely as possible. This semi - transparent elements must be considered (for example, glass or water ), fine contours ( for example, hair) and motion blur of the foreground object.

No technology is inherently better than any other, but instead the right Keyingmethode be selected depending on the situation and starting material. In many situations, it is also advisable to apply multiple keys on individual image areas (eg, head, shoulders, hands), and the results to be combined to obtain the final mask.

Chroma Key

This approach is based on a color ( the color key, the key) to select from a color space. In addition, tolerances for similar shades can be defined. Prerequisite for the Chroma Key is thus a plain background like the blue screen. Colloquially, there is also the name Colorkey.

A typical application is the television presenter in front of a weather map.

Color Difference Key

The approach of the Color Difference Key is also provided for single-color backgrounds. In this case, the differences in the color channels of red, green, and blue are analyzed, to create an alpha mask. For a blue background, the value of the red or green channel ( the higher of value ) is subtracted from the blue channel. For a green background, the value of blue or red channel ( the higher of ) from the green channel is subtracted.

3D keying

The basic idea of ​​the 3D - keying is the idea to map all pixels of an image in three-dimensional space and include a 3D object to the pixels of the keyenden background. For that pixel representation of the X-, Y- and Z - axis is normally the R, G and B values ​​of an image are used. About the deformation of the 3D object pixels may be specifically included or excluded in this 3D space. All pixels whose RGB or XYZ value is within this object are considered to be completely transparent. Keying in to the semi -transparent image areas, a second, larger 3D object is used, which includes the semi-transparent pixel values. Pixel values ​​that are outside of the small, but within the large object is calculated by the distance from the pixel value to the surface of the small object.

Luma Key

The luma or luminance key is based on the brightness values ​​of an image. The exemption a brightness value (possibly with a tolerance value ) is defined. Image areas that have the defined brightness value are transparent set, not other areas. A Lumakey is used when the indemnified article has very strong brightness contrasts with the background.

Difference Matte

In this method, the difference between two images is calculated. With this approach, an identical image area of two images will be set transparent. Thus, only image differences remain visible. For a Differencekey ie two identical settings of the same scene are needed to cut out an object. This turns out in practice as complex as film grain, wind, light and shadow or a pumping camera are parameters that prevent the absorption of two identical pictures.

Depth Key

It has a picture of accurate depth information (Z- channel ), a Depth Key to exempt these values ​​image areas. For this purpose, a low range is selected and defined as visible. All other image areas are so transparent.

Implementation

Many electronic or digital image mixing devices offer implemented keying methods. About controls, the keying type ( Chroma Key, Luma Key, Linear Key ) are selected, and the parameters are set.

In the field of digital editing systems and video editing now offer nearly all known video editing applications integrated keying solutions. In addition, special keying plug-ins are available that allow extensive adjustment possibilities and more complex keys. Known keying tools are

• Primatte (3- D keyer )
• Ultimatte ( Color Difference Keyer )
• Keylight keyer ( Color Difference Keyer )

Mask refinement

The mask formed of the keying process can be refined by other methods to obtain the desired effect.