One of the most common and popular techniques for producing shadows is called shadow mapping. In its basic form, the algorithm involves two passes. In the first pass, the scene is rendered from the point of view of the light source. The depth information from this pass is saved into a texture called the shadow map. This map will help provide information about the visibility of objects from the light's perspective. In other words, the shadow map stores the distance (actually the pseudo-depth) from the light to whatever the light can see. Anything that is closer to the light than the corresponding depth stored in the map is lit; anything else must be in shadow.
In the second pass, the scene is rendered normally, ...
