Lunchbox Triangular surface

See these threads. What you are dealing with is are trimmed surfaces. Trimmed surfaces are almost always evaluated by their untrimmed underlying Nurbs(4 point) surface, it is how Nurbs works. This isnt a Grasshopper knowledge issue, it is a Rhino knowledge issue. Check this: https://www.rhino3d.com/download/rhino/5.0/Rhino5Level2Training/

http://v5.rhino3d.com/forum/topics/trimmed-versus-untrimmed-surfaces

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