NURBS trimming issue


I made part of sphere using gluNurbsSurface() function. It looks like this:

I want to trim it using gluNurbsCurve() function. Trim curve will go counter-clockwise. These curves will be at the each side of NURBS surface. It looks like this:

So finally, I put everything together:

gluBeginSurface(nobj);  //begin my surface

  gluNurbsSurface(...myNURBS...);  //draw my part of sphere

    gluNurbsCurve(...curve_1...);  //first trimmig curve
    gluNurbsCurve(...curve_2...);  //second trimming curve
    gluNurbsCurve(...curve_3...);  //3rd trimming curve

gluEndSurface(nobj);  //end my surface

After this procedure, I should get same image like first, full NURB, but I get just part of my NURB. It looks like this after trimming:

Any ideas, what I am doing wrong? I will wait any suggestions and ideas. Thank You

When the trimming curve is in counter clock wise direction then the external part is discarded and the internal portion is retained so if u want to make the whole in your nurbs surface then rotate ur nurbs curve in the clockwise direction.