This is my setup:
I have a line and two points. The line has a custom modifier with two pickbuttons, choosing one of each point.
I have a script controller that keeps the end of the line at each point. When I put this script controller in a dummy custom attribute, teh viewport displays correctly but when I render it doesn’t update correctly.
The strange thing is if I throw a Twist modifier (or other standard modifiers) and place the script controller in one of those parameters, the line updates in the viewport and the render. Its like the the Twist has a deeper call than the custom attribute. I know the custom attribute is being updated at rendertime – i can control other things like render_thickness just fine, but it won’t respect the setKnotPoint.