A mix of two pictures which is controlled by their luminance levels relative to each other, as well as a mix value K (between 0 and 1): e.g. the position of a switcher lever arm. A and B sources are scaled by factors K and 1-K but the output signal is switched to that which has the greatest instantaneous product of the scaling and the luminance values. The output of any pixel is either signal A or B but not a mix of each. So if K = 0.5 (lever arm moved half way), in areas where picture A is brighter than B, then only A will be seen. Thus two clips of single subjects shot against a black background can be placed in one picture.
The term has also come to encompass some of the more exotic types of picture mixing available today: for example to describe a mix that could add smoke to a foreground picture – perhaps better termed an additive mix.
See also: Digital mixing