Alpha mapping
Alpha mapping is a technique in 3D computer graphics involving the use of texture mapping to designate the amount of transparency/translucency of areas in a certain object.
Alpha mapping is used when the given object's transparency is not consistent: when the transparency amount is not the same for the entire object and/or when the object is not entirely transparent. If the object has the same level of transparency everywhere, one can either use a solid-color alpha texture or an integer value.
The alpha map is often encoded in the alpha channel of an RGBA texture used for coloring instead of being a standalone greyscale texture.
