slab mask edges

Hi,

i want to know what the "edges" thing in the jit.gl.slab does exactly. What i want to do is reshape the geometry of the slab in a way so it is the contour of the thing that's projected on it, i.e. the background is gone. The 'shape' message seems to be functioning like a slab geometry modifier, but i am at a bit of a loss what it does exactly...any help would be appreciated.