How is fog of war implemented in RTS games such as StarCraft II or AoE? I'm making an RTS at the moment, and plan to have circular vision up to a certain radius from every unit on the field. 

My first thought was having a mask that is black for every point except for points close to units, where it is transparent, but on unit movement the mask would have to be recalculated, and it is very processor heavy. 

Is there any simpler way to manage this type of fog of war? Thank you very much in advanced.