Green Veil:

Adjusts the amount of green that will be subtracted during the screen subtraction process.

Veiling is a term used to describe the discolorization of the background scene in a composite caused by an incomplete subtraction of the backings color components.  If the backing is a consistent color, the automated algorithms will remove all traces of the backing color in the processed foreground and in the composite.  The Veil controls provide a means for overriding this function in order to acheive a special effect or to compensate for slight inconsistencies in the color of the backing or its lighting.

For example, a blue set that includes a blue floor will not represent a uniform blue color to the camera.  A thin dust layer, plus the flat angle of the floor to the camera, generally creates a washed-out blue-gray look to the floor.  The Veil controls may be used to remove floor veiling, but it is recommended to first view the composite, since the background scene may be able to "hide" the veiling problems.

If the Veil controls are advanced to far, other areas of the backing are subtracted below black, which may cause dark edges on the foreground subject.