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Hi @surfactant !
I have a suggestion, but not sure it's better than the 2 you have already discounted.
First, measure the location of the first endpoint. E.g. -10, 10, 0.
Now, in the Studio Spline Edit, select the mid-pole (#2) then set your Movement to be constrained to the ZX plane, by selecting YC:
Now when you edit pole #2, edit the X and Z locations to be -10 and 0, just like the starting point. The Y will not be changed (n fact, cannot be changed since you constrained the movement to the ZX plane).
One possible problem with this is that you are not explicitly controlling the magnitude, but I assume you want to leave it alone (in reality, simply making the tangent vector parallel to Y leaves an infinite number of possible splines, since you can have any magnitude on the tangency).
Another method would be to go ahead and set the Tangency on pole #1 to be -YC (which is fast to do), then Remove Parameters on the spline.
A possible downside to that is the Spline will cease to be a feature and show now as a Non-Timestamp curve. I say "possible" because for some workflows this might also be advantageous.
But if you need to align to a vector other than global XYZ, my suggested method will be nearly impossible without creating helper geometry, which also said already that you don't want to do.
Since you don't say what you want to do with the Spline next, nor why it's a problem to have the Tangency on the first pole, or why construction geometry is to be avoided, I cannot really provide any opinion on which methods might best suit your situation.
Hi @PatMcManus Thanks for your long reply!
I post the question since I'm a little unsatisfied with the pole handling capability of commands like Studio Spline.
In editing a Studio Spline, if I drag a spline pole to an existing point, the pole would be directly made coincident with the existing point, no matter which plane the spline is confined to. It would be better if the pole can be kept in the predefined plane and aligned to the existing point in a projection way.