I get a curve then find the domain with ON_Curve::GetDomain(*t0, *t1).
I then add a point to the document that is located 3/4 of the way though the domain using the PointAt method.
doc->AddPointObject(MyCurve->PointAt(t0 + (t1 - t0) * 0.75));
The results for 3 curves are shown below.
The first curve is a single straight line with the added point shown 3/4 of the way along the line in model space.
The next line is a polyline with 2 segments created with the Rhino “_Lines” command. The mouse points used for selection of the ends are shown with an “x”. The returned point is consistent with the first line.
The third line is the same as the second except it was created from 2 individual lines with the Rhino “_Join” command. The returned point is different.
Shouldn’t all these lines return consistent point locations?