Generating a composite video signal

A composite video signal VO is formed from a foreground signal FG and a background signal BG in accordance with the luminance self-key. A key signal k is derived from the luminance foreground signal in dependence upon clipping levels YU, YO. To avoid a quadratic dependence of the luminance signal during product formation of the key signal k with the foreground signal FG (referred to as "double multiplication"), the linear key signal k should be replaced by a control signal h which is hyperbolically formed in such a way that the unwanted quadratic dependence is eliminated, h being defined by h=1-(1-k)*YU/YFG, where YFG is the foreground luminance and YU is a lower clipping level and VO is defined as VO=h*FG+(1-k)*BG.