I posted something similar on the rendering part of these forums, but I have been investigating further.....   In the process of trying to understand the new Parallax bump map shader I ran into this peculiar problem with the displacement map shader. I made a brand new texture with a Renderworks brick png as a bump shader image. The First Column is an extruded rectangle.  The second is a Nurbs Surface The Third is the same Extruded rectangle converted to Nurbs Surfac