Curve interpolate points start and end tangent bulges beyond tangent line

[Steve1]

Hi,
V5

I start tangent at ‘a’ and end tangent at ‘b’ aim was to have a curve sitting above ‘a’ and to right of ‘b and c’

steps:-
Curve Interpolate Points tool
hit S
go ctrl and pick OnCrv, select curve at ‘a’ and pick end of line to right of the ‘a’, pull tangent to left and click on that line to left of ‘a’.
I can see its starting tangent as I move mouse around.

type n and hit enter for end tangent,
ctrl and pick OnCrv and pick line at ‘c’ and click at ‘b’ then click on that line beyond ‘c’

curve bulges beyond line a bit.
so why is it whipping out then back in to go tangent ?,making life more difficult for itself one might say ?

I wanted curve remaining to rhs of that line.
start and end tangent bulges beyond.3dm (26.1 KB)

Cheers

Steve

[Helvetosaur]

This is why:

To fix it, move the third control point to be vertical in line with the first two:

[davidcockey]

@Steve1

so why is it whipping out then back in to go tangent ?,making life more difficult for itself one might say ?

Because of where you are moving the control points. The control point next to an end point determines the direction of the curve at that end. In a degree 3 curve the next control point affects the shape of the curve away from the end. Move the third control point to the other side of the vertical line and the reverse curvature will be eliminated.

If you need more control over the shape of the curve you need more control points. Either a higher degree curve or a multispan curve. One method to increase the number of control points is to use InsertKnot which will add control points (and spans) without changing the shape of the curve.