Complex sentence: The idea behind ray casting is to shoot rays from the eye, one per pixel, and find the closest object blocking the path of that ray--think of an image as a screen-door, with each square in the screen being a pixel.
Split into simpler sentences: The idea behind ray casting is to shoot rays from the eye, one per pixel, and find the closest object blocking the path of that ray. Think of an image as a screen-door, with each square in the screen being a pixel.