What's the extent of SelectById2()'s dependence on the viewport state? I have the exact coordinates of the vertices I want (through GetExtremePoint() ), but sometimes I can't select them when they're behind something else. It seems to work okay when the viewport is set to wireframe with all of the part inside the view. But what if the viewport is zoomed very far out? Or contain many vertices very close to each other? Is it known to have problems with those cases?