Hello, can anyone explain how I can trim a cylindrical surface in opengl using a bspline curve ? I can trim nurb surfaces by using gluBeginTrim and gluEndTrim, so does anyone know how I can represent a cylindrical surface using gluNurbsSurface ie how do i calculate the knots and points I have the radius of the cylinder and the height, origin (in cartesian) and the direction vector ? Thanks in advance.

You can calculate your points mathematically, with trigonometry.
So now you need to decide on the resolution of your cylinder, ie: how many polygons do you use to complete the circle? so if you decide on a resolution of say... 18, then thats 18 quads or 36 TRIs for the main shell & then 18(or 36) more for each end cap as well. After you generate the cylinder points for a location & rotation of ZERO, you can then apply a view-matrix to those points if you want a certain origin & direction for the shape.