The meaning of chroma key is to remove some color from the image. Any image is a set of pixels that are in some specific colorspace (for example, RGB).
Colorspace can be represented as a color cube:
To implement chroma key you select a point in this cube and this color is removed from image. "color" here means not just a single point, but some spherical area around this point. So to set up this area means to set up a chroma key
The input image is processed with these settings and a result mask is created. The mask is a measure of transparency for pixels of the original image. If the color is into the sphere - it will be transparent. The closer pixel color to the center of the sphere the more transparent it is.
The meaning of Chroma Key parameters is described below. In the examples below this image is used:
Initial settings are made automatically with Auto Detect button:
As you see the green part from the image is disappeared. By design, auto-detect feature detects blue and green colors.
Power - common multiple for an axis of the selected color area. The greater is radius the more color points are included in its area. And the greater part of the image will be cleared.
Smooth - the level of result image smoothing. Each 0.1 value of this parameter uses an additional cycle of smoothing. The mask is limited by the outside border, then it is cropped on 1 pixel to get the area for smoothing. Then it is smoothed within this border according to Smooth value (0.9 means 9 times). And then Gaussian blur is implemented to the mask in the outside direction.
Minimum and Maximum Transparency - limitations for transparency range. By default, the transparency for each pixel is from 0 (fully transparent) to 255 (solid color). By using the minimal value you limit this range so if the transparency of pixel is lower than Minimal Transparency it is calculated as 0. And vice versa - if the transparency of pixel is higher than Maximal Transparency it is calculated as 255.
These settings are used to make chroma keying more accurate.
Exclude - you can exclude an extra area from the selected one. It contains Power and Transparency parameters that have the same meaning as basic parameters.
Power - contains values of selected area for each color axis (RGB, HLS, YUV). The total area is calculated as (basicPower*rPower, basicPower*gPower, basicPower*bPower). By modifying these parameters you change the keying area in the specified line.
Colors - operates colors on the border between image and background. You should set a non-zero value for the basic Color setting so these parameters take effect. In example below color = 0.2.
Range - the size of the border area.
Power - the amount of color that is subtracted on border area. To high value may increase color accent, to low value may make border blue
Chroma -the chromaticity of the border.
Luma - brightness of the border.