INTERSECTION OF CYLINDER WITH A PLANE

 
     The section is ellipse in the common case.                                                           
          In case the plane is perpendicular  to the axis , the section is a circumference.
  Technical description :

 
   

  1. The plane closes an unspecifed angle with the axis 
      of the cylinder.  
   2. You can have either unfold or section or
      both of them.