New Wiki page describing B-splines

[uwestoehr]

I created a new Wiki page:

https://wiki.freecadweb.org/B-Splines

that
* explains what B-splines are
* how one can use them in FreeCAD
* shows some use cases

I hope this helps especially new users.

Every constructive comment is welcome.

For developers the section https://wiki.freecadweb.org/B-Splines#Limitations might be interesting. I know that ironing out the limitations is not easy, but maybe there are some libraries around that actually provide e.g. the calculation of a tangential to a spline.

.

[onekk]

At a first glance it seems very weel explained, impressive are the animations, explanation is clear (after a quick reading).

Well done!

It lacks of a part describing scripting, it wil be very useful, expecially if it is taylored on Part WB, as it is more near the OCCT implmentation.


Regards.

Carlo D.

[abdullah]

For developers the section https://wiki.freecadweb.org/B-Splines#Limitations might be interesting. I know that ironing out the limitations is not easy, but maybe there are some libraries around that actually provide e.g. the calculation of a tangential to a spline.

The problem are not static calculations. In fact, the solver, unlike for any other geometry, relies on OCCT for calculation of the B-Spline. The solver only sees points (control points, i.e. poles). That is the problem. What is needed is to implement in the solver the parametric calculation of the B-Spline along with all its partial derivatives, then implement the constraints (such as point on object) where they need to be geometry specific. DeepSOIC has given it a thought:

Thanks. You reminded me that I need to come up with an n-th derivative algorithm for b-spline, with parameter derivative support. That's like d2x(t)/(dt dp), d3x(t)/(dt2dp), and so on...

He also posted another post where he asked help for a discussion, which he did not get (and which I cannot find...)

Personally, I have not found time to undertake this yet.

[UR_]

under Limitations you wrote:

4. You cannot create an offset curve for a B-spline using the tool Draft Offset.

This sounds a little bit desperately

Perhaps you can add:

Please use Part 2D Offset instead

[jonasb]

He also posted another post where he asked help for a discussion, which he did not get (and which I cannot find...)

Are you referring to this endeavour: https://forum.freecadweb.org/viewtopic.php?f=20&t=40525

[uwestoehr]

Perhaps you can add:

Please use Part 2D Offset instead

This does not help. At least in my use cases I need the offset spline within the same sketch. With Part's 2D offset I can only offset the whole sketch and I never had a use case where this helps.

But in general the Sketcher WB should offer an offset tool -> I know the whishlist is long, therefore I don't complain but continue to contribute

[uwestoehr]

You reminded me that I need to come up with an n-th derivative algorithm for b-spline, with parameter derivative support. That's like d2x(t)/(dt dp), d3x(t)/(dt2dp), and so on...

He also posted another post where he asked help for a discussion, which he did not get (and which I cannot find...)

Personally, I have not found time to undertake this yet.

Many thanks that you are aware of this. Since the post from DeepSOIC is 3 years old, maybe he already worked on this or would like to?
-> inviting DeepSOIC

.

[DeepSOIC]

well, the nth derivatives were halfway there when i abandoned constraintsolver. As for offset bspline in sketcher, i'm not sure. I've heard that an offset spline is possible, though it will be of a higher degree. I suppose, if general constraints to bspline become a reality, it will be possible do knock out an approximation of an offset spline with just these regular constraint, something like constrain the distance between them at all knots for example.

[Vincent B]

Great job!

Limitations
At the moment (FreeCAD 0.19) there are some limitations when using splines you should know:
You cannot set tangential constraints.

...
but can be got around with several bspline.

[abdullah]

He also posted another post where he asked help for a discussion, which he did not get (and which I cannot find...)

Are you referring to this endeavour: https://forum.freecadweb.org/viewtopic.php?f=20&t=40525

Not that one. But, there might be a link to the one I refer hidden in it.