NURBS trimming issue

OpenGL OpenGL: Basic Coding
1

Hello,

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

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

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

2

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.