making fog of war

i’m currently editing the RPG Tactical module by giving it an extra large blank map since most of the supplied maps aren’t big enough to hold a full dungeon, but what i’d like to know is how to create fog of war to cover all of the map, except for the areas selected by the DM.
rules that apply to it:
it must cover the entire grid, but still be partially visible by the DM, and also has the ability to be turned off by the DM so he can create his maps with no interference
individual squares can be selected by the DM to be shown to the players (ex. they enter a new room and the dm selects and removes the FOW squares that cover his map)
the players cannot be allowed to move the FOW and they cannot see what is underneath it
and finally, and most importantly (even though i just said this), it must cover the entire grid so that the shape of the rooms cannot be given away.

this seems to need some coding to create this effect, and i’m no coder, and i’m also no expert when it comes to vassal, i only know the basics. so if anyone can help me to perfect this module, it can become a very handy tool for all rpgs allowing groups of people to fully play online!

Something like this? [url]] :wink: