An object which is hidden from light source is a shadowed object. Hidden surface algorithms are used to locate the shadowed objects. In these calculations regions are marked with shadowed and unshadowed groups.
Another method is shadow volumes. Shadow volume is defined by light source and an object. It is bound by a set of shadow polygons which are invisible. Visible polygon and invisible shadow polygon is compared to locate the area which lies in or out of the volume. Areas which are inside of volume are shadowed and areas which are outside needs to be further checked if they are shadowed by any other polygons.