1. A method for generating a clean clipping signal α for a chromakey function or a video composition where still and moving subjects shot against a solid color background by a video camera are clipped out and overlaid on a new complex background, comprising the steps of: identifying background colors which are formed by the solid color background, shadows cast by the still and moving subjects, non-uniform reflection caused by spot lighting and non-flat backdrop or flaw wall, and translucent foreground objects, with a 3D volume in a 3D color space; automatically determining parameters which define the 3D volume by using a dirty alpha α; generating a clean clipping signal α shd for the background colors, which occurs to the colors identified as the shadow colors or confined by the 3D volume for the shadow colors, and a clean clipping signal α tsl for the translucency colors, which occur to the colors identified as the translucent areas' colors; identifying foreground colors which are formed by the still and moving subjects with a 3D volume in a 3D color space; classifying into transition colors those which are not identified as either the background or the foreground; and generating an area map which labels each pixel as one of the areas such as background, shadow, translucent area, foreground and transition areas.