Rebuild or which command to keep curve ends tangent?

I have drawn a curve which starts tangent along X axis and ends tangent along Y axis, its v similar to a quarter of an elipse.
Having traced a working curve with InterpCrv I wish to lessen its points now and also improve the curvature graph a little.

Rebuild did fine but I then noticed the tangent along Y axis was lost.
When I mirrored the curve as I need this one but a bit different for the half an elipse sort of shape, the graphs overlapped.

Which command will retain the tangents whilst improving the curve to iron out any subtle variations, no S bends etc.

Otherwise the option would be to then trace this with InterpCrv starting and ending tangent !


Here is perhaps an important bit of info for you concerning tangencies:

The last two control points on a curve determine end tangency. The end tangent lies along a theoretical line between the end control point and the one next to it. So in order to keep the end tangent fixed, this line must not change. You can move the points anywhere ALONG the line and the end tangent direction will stay the same, but not OFF the line.

Any command that moves control points may thus upset this relationship. Since Rebuild only keeps the end points fixed, that will definitely do it. If you need to smooth out curves while keeping tangency fixed, you will need to either manipulate only the interior control points, or you will need to reset the tangency afterwards (either manually or with something like Match).

I can imagine something like Smooth on only selected interior points might help smooth out the curves, but if you need to reduce the number of points, I think you’re stuck rebuilding and then re-adjusting the tangency.


Hi Mitch,
I thought of Match afterwards and went with that, strangely enough match and join to the two halves of the elipsish type shape didnt see the graphs also become mirror image at that point, curvature did, so I went with that for the join and tangent for the ends onto straight lines.

It would be nice to have rebuild with option to select points at ends to indicate those that can run to/fro retaining tangent.

Most of my curves start and end tangent, nature of the work, so I would like that option now that I am making use of Rebuild.