fwiw steve, i do realize there’s an exact point which you’re looking for and it appears as if interpCrv could find that point…
mathematically, it’s beyond me… because as next point is being moved into place, the back end of the curve is also adjusting and/or somewhat free to move around (which doesn’t happen with an arc or line since they’re much more limited in their definitions and a lot easier ,for me personally, to understand the formulas behind them)…
but in this video, (or better yet, just copy what i’m doing in the video to see for yourself), it’s pretty obvious there’s an exact point which gets crossed where the interpCrv begins to cross to the other side of the circle… right before that crossing point, it appears there should be a tangent point which could be figured out by rhino instead of requiring the user to pre-define the point of tangency…
i haven’t really thought it through enough to know if this works out the same in other situations or if it only appears as if it should work in this particular drawing… but maybe pascal can pass it along to one of the über math geeks at mcneel for analysis ?