Key color effect is commonly used in video and film editing.In this paper

the concept of Alpha channel has been described and algorithms of the key color effect have been thoroughly studied theoretically.A mathematical model that foreground image can be seen the overlay of the video object and background of this image has been established.Based on this model

the matte equation and its general formula are obtained.By discussing the special case that a desired foreground video object is separated from two backgrounds of constant color (blue and black)

a double foreground technique has been presented.Corresponding theorems that can pull a matte on any color object against random backing have been given and proved.We pulled the foreground video object from a complicated background and even object that includes backing color from constant color background by this technique with C language programming.Any other key color effects cannot realize these.