Page "Chroma key" Paragraph 37

Most keyers use far more complicated functions.

A popular approach is to describe a closed 3D surface in RGB space and determine the signed distance the point ( r, g, b ) is from this surface, or to find the distance the point ( r, g, b ) is between two closed nested surfaces.

It is also very common for f () to depend on more than just the current pixel's color, it may also use the ( x, y ) position, the values of nearby pixels, the value from reference images, and values from user-drawn masks.