That’s not the problem. The degree 3 curve in the attached file was created using InterpCrv with the StartTangent option and selecting only the end points and the start tangent direction. Inflection.3dm (32.2 KB) The third control point is put in on a straight line from the start to the end curve. That is what causes the inflection. The inflection can be eliminated by moving the third control point.
To duplicate what I did:
InterpCrv with Degree=3
Select StartTangent option
Pick the start of the curve at the end of a line.
Choose the start tangent direction the same as the line.
For next point select the end point.
Enter to finish the command.
Here is the command line history from creating the curve in the attached file:
Start of curve ( Degree=3 Knots=Uniform PersistentClose=No StartTangent ): StartTangent
Start of curve:
Choose start tangent ( Direction ):
Next point ( Degree=3 Knots=Uniform PersistentClose=No EndTangent Undo ):
Next point. Press Enter when done ( Degree=3 Knots=Uniform PersistentClose=No EndTangent Undo ):