Is it possible to create and export a NURBS with negative weights?

[Anonymous]

I have an application that should work with a NURBS model exported from AutoCAD. Is it possible to export a surface with negative weights?

[john.vellek]

Hi @Anonymous,

I see that you are visiting as a new member to the AutoCAD forum. Welcome to the Autodesk Community!

I have little experience working with NURB surfaces but my understanding that it is not a best practice to work with 0 or negative weights.  Do you have a sample that I can examine?  I tried creating a simple planar surface and then used Convert to NURBS. Then I elected to edit one of the CV's and dragged it in a negative direction. I am not sure if this is what you are looking to do.

Please select the Accept as Solution button if my post solves your issue or answers your question.

[john.vellek]

Hi @Anonymous,

I am checking back to see if my post helped you with your problem or question. Please mark my post as the solution if it resolved the issue or give me a bit more detail on this issue so we can continue to work towards getting this solved.

Please hit the Accept as Solution button if a post or posts solves your issue or answers your question.

[john.vellek]

Hi @Anonymous,

Can you give more specifics on what formats your other program can accept?  I would like to test one of your NURBS models if you can provide it on a post.

Please select the Accept as Solution button if my post solves your issue or answers your question.

[leeminardi]

@john.vellek FYI, the OP also posted this question on the 3ds Max forum.  You can get a little more insight into his goals there.

https://forums.autodesk.com/t5/3ds-max-forum/is-it-possible-to-create-and-export-a-nurbs-with-negati...

AutoCAD's splinedit command for weights states "Value must be positive and nonzero."

Lee

[SEANT61]

I suppose another detail worth mentioning is that the "-1" CP weight (displayed more often than not) in the Properties Pallete is just an AutoCAD convention to indicate that all of the CPs have a weight of 1.  If even just one other CP has a weight  other than 1 (always Positive), then none of the weights will have the Negative sign.

[Anonymous · Accepted solution]

I did some research on file formats and came to the conclusion that some of the most popular ones (such as STEP, IGES and BREP) don't support negative weights for NURBS at all (the files specifications explicitly forbid them). Although I'm not 100% sure that this is the case for other file formats, that's enough for me for the time being. Also, your insights tell me that is probably impossible that a user can set negative weights manually while using AutoCAD. Thanks a lot for your help!