Images, whether they are created in OpenDX or imported, are simply 2-D field with regular positions and connections. So Slab is a simple method to extract a rectangular subregion. If you want the rectangle with corner at (x,y) and size (w,h), use two Slab modules - one feeding the other. The first will have dimension=0, position=x, thickness=w, and the second, dimension=1, position=y, thickness=h. I could have the dimension values reversed. ClipBox doesn't actually edit the data; it identifies a volume of interest to be applied at rendering time, and thus does not affect the memory requirements of the refinement operation.
Greg
