Creating NURBS curve caps

[bART Janssens-2]

Moving this discussion about NURBS curve caps to the list, so everyone
can benefit.

In the original GSoC project, Carsten implemented NurbsCreateCap using
a ruled surface between two halves of the curve loop. When updating
ExtrudeCurve, I tried a simpler approach, using the same method as in
NurbsDisk.

Unfortunately, both methods fail when the curve is convex, so I tried
using a Coons patch. This does not work in the general concave case
either, however, so I propose we follow Ayam's method completely:
- If the curve to cap is planar, create a NURBS plane over the
bounding box and trim that using the curve
- Use Coons patches if the curve is non-planar

This would be the behavior of the "create caps" option we have on i.e.
ExtrudeCurve, and NurbsCreateCaps would have a flag to enable Coons
even on planar curves.

Alternatively, we could drop the "create caps" flag everywhere and
require the use of the NurbsCreateCaps plugin whenever caps are
needed.

Cheers,

Bart

On Tue, Nov 3, 2009 at 9:17 PM, Bart Janssens <bart.janssens@...> wrote:

> On Tue, Oct 27, 2009 at 11:39 PM, Daniel Scott Matthews
> <dsmatthews@...> wrote:
>> What about some sort of triangulation of the space enclosed by the
>> curve set. Like the sort of routines used in FEM pre-processor
>> utilities, http://www-users.informatik.rwth-aachen.de/~roberts/software.html
>>
>> Or a set of quads that then have a trim curve?
>>
>
> I'm currently looking at Coons (or Gordon) patches, that seems to be
> the way Ayam does it:
> http://ayam.sourceforge.net/docs/ayam-4.html#ss4.27
>

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[Carsten Haubold]

2009/11/8 Bart Janssens <bart.janssens@...>

Moving this discussion about NURBS curve caps to the list, so everyone
can benefit.

In the original GSoC project, Carsten implemented NurbsCreateCap using
a ruled surface between two halves of the curve loop. When updating
ExtrudeCurve, I tried a simpler approach, using the same method as in
NurbsDisk.

Unfortunately, both methods fail when the curve is convex, so I tried
using a Coons patch. This does not work in the general concave case
either, however, so I propose we follow Ayam's method completely:
- If the curve to cap is planar, create a NURBS plane over the
bounding box and trim that using the curve

That was exaclty the reason why I chose the other approach, I didn't want to have two different methods doing the same thing just for different end types. But true, my solution fails at convex shapes.

- Use Coons patches if the curve is non-planar

I had and have no idea of Coons paches so I can't help you there.

This would be the behavior of the "create caps" option we have on i.e.
ExtrudeCurve, and NurbsCreateCaps would have a flag to enable Coons
even on planar curves.

Alternatively, we could drop the "create caps" flag everywhere and
require the use of the NurbsCreateCaps plugin whenever caps are
needed.

Is is possible to attach or detach a NurbsCreateCaps node from within lets say a NurbsExtrudeCurve plugin when (un-)checking  the "create caps" option?
Then that would be nicest way to handle that.

Regards,
Carsten

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[hoako]

Carsten Haubold wrote:

> 2009/11/8 Bart Janssens <bart.janssens@...
> <mailto:bart.janssens@...>>
>
>     Moving this discussion about NURBS curve caps to the list, so everyone
>     can benefit.
>
>     In the original GSoC project, Carsten implemented NurbsCreateCap using
>     a ruled surface between two halves of the curve loop. When updating
>     ExtrudeCurve, I tried a simpler approach, using the same method as in
>     NurbsDisk.
>
>     Unfortunately, both methods fail when the curve is convex, so I tried
>     using a Coons patch. This does not work in the general concave case
>     either, however, so I propose we follow Ayam's method completely:
>     - If the curve to cap is planar, create a NURBS plane over the
>     bounding box and trim that using the curve
>
> That was exaclty the reason why I chose the other approach, I didn't
> want to have two different methods doing the same thing just for
> different end types. But true, my solution fails at convex shapes.
>
>     - Use Coons patches if the curve is non-planar
>
> I had and have no idea of Coons paches so I can't help you there.
>
>
>     This would be the behavior of the "create caps" option we have on i.e.
>     ExtrudeCurve, and NurbsCreateCaps would have a flag to enable Coons
>     even on planar curves.
>
>     Alternatively, we could drop the "create caps" flag everywhere and
>     require the use of the NurbsCreateCaps plugin whenever caps are
>     needed.
>
> Is is possible to attach or detach a NurbsCreateCaps node from within
> lets say a NurbsExtrudeCurve plugin when (un-)checking  the "create
> caps" option?
> Then that would be nicest way to handle that.

I think that logic would be at the GUI level. Since the
NurbsExtrudeCurve shouldn't care about it.
So probably those plugins should inherit a "iCapsNurb" interface that
the GUI should detect and react.
Probably another GUI plugin should take care of creating and deleting
the NurbsCreateCaps plugin.
Well, I am just guessing. Tim will say correctly.

Cheers!
Joaquín





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[bART Janssens-2]

On Sun, Nov 8, 2009 at 12:20 PM, Carsten Haubold <CarstenHaubold@...> wrote:
>> Unfortunately, both methods fail when the curve is convex, so I tried
>> using a Coons patch. This does not work in the general concave case
>> either, however, so I propose we follow Ayam's method completely:
>> - If the curve to cap is planar, create a NURBS plane over the
>> bounding box and trim that using the curve

Both the trim and Coons patch method are in my repository now, though
I haven't merged with sf.net in a long time, so there may be conflicts
if anyone is brave enough to try pulling in the changes.  I'll merge
with sf.net once all NURBS plugins are converted to the new structure.
In the mean time, I'll see if I can make some screencasts and put them
on my blog.

> That was exaclty the reason why I chose the other approach, I didn't want to
> have two different methods doing the same thing just for different end
> types. But true, my solution fails at convex shapes.
> Is is possible to attach or detach a NurbsCreateCaps node from within lets
> say a NurbsExtrudeCurve plugin when (un-)checking  the "create caps" option?
> Then that would be nicest way to handle that.

I've removed the flag, and added a "NurbsCapPatch" action script
instead, that uses a (new) NurbsExtractCurve plugin to extract end
curves and then applies NurbsCreatePatch.

It would be nice if in 0.9, we could add a way to bind a UI to such a
script. In this particular case, we really need control over:
- What method to use (Coons or trimming)
- Should the caps be created in the U or the V direction?
- Should both ends be capped?


Cheers,

--
Bart

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[Timothy M. Shead]

Bart Janssens wrote:

> Both the trim and Coons patch method are in my repository now, though
> I haven't merged with sf.net in a long time, so there may be conflicts
> if anyone is brave enough to try pulling in the changes.

Builds cleanly here ...

Cheers,
Tim

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