impoly: constraining vertex placement

I'm trying to control the placement of vertices in "real time" when using the impoly function... basically I want to force a "snap to grid"-like behavior.

Using 'PositionConstraintFcn' I'm able to constrain the vertices' positions in my polygon after it's been completely drawn, but I'd prefer for this to happen as the user places the vertices instead. The same position constraint function allows me to do just this when placing the end points with 'imline', but for some reason, it seems to delay execution of the constraint function until all the vertices are places when working with impoly.

Does anyone have any advice on how to do this? Is it possible to do what I'm hoping? Thanks in advance.

[Pieter]

I am really interested in this. I want to do this with imrect. Are you willing to share the code with the imline? Do you already found a solution for impoly? Thx!