My housemates and I were having a discussion on how projectors work, and how they can project text on a white background. One idea was that white light is projected around the text with an absence of light where the text is, so it appeare relitively black. Then my friend posed the question of if you project a black frame around a white rectangle, it would still appear black. With an absence of projected light, surely it would appear the same colour as the background screen, which would be white, the same colour as the screen with projected white light on it. So how does this work?