my problem :
i wanted to draw some 2d-trees, so i did this :
draw mask of tree (only a black "shadow" of the tree visible)
draw the tree on the mask
-> everything looks good.
no problem so far. but if i draw a second tree behind the first one, the invisible part of the nearest mask (in this case, the first tree) contained the background i've drawn before - the second tree was ...i dont' know what happened to him. everything inside the first trees rectangle was simply not drawn...