In the attached, the closed triangular object refuses to make a planar surface because it’s not considered planar - one of the sides is an “arc” with a radius of 9.08173e+06. This refuses to simplify into a line segment…

In this case the arc’s maximum distance from line segment connecting its ends is .005 and the file tolerance is .001. So I don’t think SimplifyCurve should replace the arc. However, the way it work currently the ON_ArcCurve will never get replaced with a line segment. I think we should be using tolerance to make this decision.

i’ve got 2 cases in V6 where _SimplifyCrv does not merge linear segments which have zero tangent difference. This also applies to RhinoCommon using Rhino.Geometry.Curve.Simplify method.

Thank you Pascal, i did not pay attention to the seam. Maybe _SimplifyCrv needs an optional argument to include simplification of the first and last segment of the curve ?