1. A method of generating shadows in a computer graphics image, the method comprising: receiving a position corresponding to a surface of an object in a three-dimensional scene, the surface position visible from a virtual camera viewpoint; receiving a light fragment representing a portion of the scene from a point of view of a light source; defining, using a processor operatively connected to a memory, a frustum having an apex located at the surface position and a base defined by a size and a position of the light source; projecting the frustum onto an image plane associated with the light fragment to determine a projected two-dimensional frustum region, determining whether the light fragment is within an approximate projected frustum region that approximates but is not exactly the projected two-dimensional frustum region; and altering a shading value of the surface position at the apex based on the determination of the light fragment, representing the portion of the scene from the point of view of the light source, being within the approximate projected frustum region.