Shadows cast on a back-projection screen. The light is from three projectors fitted with primary red, blue and green filters respectively. Where the light from all three projectors falls on the screen, you get white; no light, you get black; from one projector only, you get that projector's primary colour; from two projectors, one of the intermediate colours, i.e. yellow, magenta or cyan. Taken on Agfa artifical light film, first shown at my RPS retrospective in 1984, improved today in Photoshop.