thanks for this idea. I did not thaught of this command.
it would be possible for me to use it but a shader effect would be better.
The problem with this is: if the player builds a certain building, a circle bitmap gets attached to the terrain. thats fine. but if the player builds another certain bulding IN this circle, they both will reduce the alpha in the area where both bitmaps are visible.

is a shader effect like this even possible? again: i am a totally shader-noob :-S