The graphics card of a modern computer is like a 'computer within a computer', with (often more than one) graphics processor and dedicated on-board graphics memory.When opening or updating one of the OpenGL graphics features in ZEMAX (Shaded Model, NSC Shaded Model and NSC Object Viewer), you may encounter the following error: 

What this error is indicating is that, given the complexity of what ZEMAX is trying to draw in the Shaded Model/Object Viewer, there is not enough video memory available to render the image. The best solution is to add more memory to the graphics card, so it has enough memory to render the scene you are modeling! If that is not possible, there are a variety of methods to reduce the memory requirements.