NURBS sharp corner

[kai.scorpio]

Is there a way to make a sharp corner in NURBS curves, without using weights? The reason I don't want to use weights is that if I put them on one point, I have to put them on the surrounding ones as well, and it gets out of hand quickly. The help has this to say:

Make "degree" knots identical. When this is done, it is called raising the multiplicity of the knot. You can do this in the Refine sop by choosing the Subdivision option, or in the Model Editor by selecting the Refine state and dividing with the middle mouse button ( ).

A search around shows the model editor is something from an old version of Houdini? Is there a way to do this now?

The project is a similar thing to the F-18 that was in the WIP section some time ago, based on a piece of concept art lying around.

EDIT: Second question, is it possible to do two trims on a surface? What i'm trying to get is to trim a 'o' shape using two circles, but it gives me strange results.

EDIT2: Figured that one out, you need to write 0.0, 0.1 etc in the group bit on the Trim SOP.

(I'll just keep editing this until I get a reply )

NEXT: From stu's F18 WIP [here], the step uses a Box SOP and Carve SOP to even out the spans. Is he matching them up by eye?

If thats the case, how can I generate cross sections evenly spaced along say the z axis, if my UVs don't line up along this axis (does this make sense?)?

Kai

(also, why does it say 'fooey test' on the bottom of the page - EDIT: gone as of new design)

Edited July 20, 2010 by kai.scorpio

[old school]

Question 1 NURBs sharpness without weights. Why not use weights when sculpting your surface? You can always reubild the surface afterwards which strips the weights...

You can use the Refine SOP to add multiplicity to a knot. Really you are simply duplicating the knot/breakpoint, sort of subdividing the NURBs curve/surface at the break point from 1 to the degree of the curve (curve SOP defaults to order 4 which is degree 3). Anything above the degree of the curve does nothing.

You can see the knot basis using a Basis SOP. Append one and set to manual then press the read basis option. Here's an example where I used a Refine SOP to add a single breakpoint to the curve and adjusted the basis to 3:

0 0 0 0 1 1.77279 1.77279 1.77279 2 3 4 5 6 7 7 7 7

the first and last points also have multiplicity. Best to turn on breakpoint (knot) display and select breakpoints (knots) and edit the curve that way along with dragging the cv's.

Another way to add sharp corners in NURBs is to insert one or two extra cv's in the Curve SOP (shift click on the curve) then drag the cv's snapping on top of each other in a subsequent Edit SOP.

Question 2 yes. Make sure to turn on the display of trim curve numbers and there you will see the proper trim curve id numbers as prim_number.profile_number.

Question 3 I have no idea... I would imagine that Stu added the box SOP with divisions to get a visual cue as to where to cut/carve the modeled pieces to rebuild those surfaces with even divisions as close as the eye can get. The subsequent boolean operation would then be easier to clean up (and almost all boolean operations do need cleaning up...).

As for generating evenly spaced cross-sections, you can use a bunch of Carve SOPs with parameters evenly spaced. I attached my slicer asset that will cut any Polygon object in to equal divisions. Not perfect as you may get a couple nasty profiles at the ends that you have to either edit or delete.

slicer.otl

[kai.scorpio]

Ahh I thought this question had died. Thank you for your reply, I'll try this out as soon as I get back from my holidays.

Kai

[rd1010]

I do a lot of nurbs modeling but not in houdini so I'm not sure what "weights" are exactly but with nurbs basically its just that you need to add in as much detail as necessary until the corner appears sharp from the distance you are going to be viewing it so using a regular curve either through points or with control points this can be done.. you just need to add as many points as necessary close together (so zoom in heavily) around the corner... with nurbs I think you can never technically get a 90 degree angle but you can definitely make it look this way since the "curviness" would be only viewable when zoomed in at an extremely high level. I could be wrong in that you can get a perfect sharp corner but either way its not of much consequence, at least in my experience, since its not that hard to achieve the look of a sharp corner.

Edited September 7, 2009 by rd1010

[stu]

I would imagine that Stu added the box SOP with divisions to get a visual cue as to where to cut/carve the modeled pieces to rebuild those surfaces with even divisions as close as the eye can get.

Yes.