colors however with k 2 at zero all blue flare in the camera lens and on wardrobe and flesh tones is entirely eliminated equation 1 e b is implemented by entity 15 of fig2 differential amplifier 16 provides the term g r through gate 21 differential amplifier 17 provides the r g term through gate 20 linear 0r gate 18 passes whichever term is positive the b g term of equation 1 is formed at the and input terminals of differential amplifier 19 which also subtracts the g r or the r g term potentiometers 14 20 and 21 provide the k k 1 and k 2 of the equation having removed by subtraction the 3 58 vector that represented the hue of the blue backing the signal leaving amplifier 6 via conductor 8 and entering multiplier 10 contains only luminance black and white information which represents the 34 brightness 34 of the blue backing it is necessary to remove this luminance so that it does not cause a gray veil over the entire background scene chroma key apparatus removes the luminance and the chroma of the backing by switching off the foreground scene in the blue backing area in the present invention the luminance of the blue backing area is removed by arithmetic subtraction the luminance component of the blue backing and of the foreground objects resulting from secondary illumination from the backing are removed from the video signal by the connection of the e b output into the negative terminal of the x input of multiplier 10 luminance bias control 23 allows the proper amplitude to be selected thus the output of amplifier 24 has zero.