Dirty masking..

I have a problem with masking in Blenders compositor. Look at this screenshot:


Notice the white rectangle around the television at the left? It is created at the edge of a mask I use for when the ball passes behind the television. Quite ugly.
The mask itself is just a white rectangle, nothing else.

Also take a look at the crude (grey big pixels) maskng around the seat. Brrrrr.

Is there maybe some kind of precision value or anything I need to set to get decent masking?

Problem solved.

It turned out to be my own mistake. Well, kind of.
While the brush I used to draw the masks with in the Gimp had a maximum hardness, it seemed to still blur a bit the edges of the drawings. Hence the compositing quirks.