A technique used in computer graphics to create realistic images by calculating the paths taken by rays of light entering the observer's eye at different angles. The paths are traced backward from the viewpoint, through a point (a pixel) in the image plane until they hit some object in the scene or go off to infinity. Objects are modelled as collections of abutting surfaces which may be rectangles, triangles, or more complicated shapes such as 3D splines. The optical properties of different surfaces (colour, reflectance, transmitance, refraction, texture) also affect how it will contribute to the colour and brightness of the ray. The position, colour, and brightness of light sources, including ambient lighting, is also taken into account.