Negative CP Weight

Hello,
This issue was recently presented to me and I could not really find a way to resolve it. The person asking the question was making a CreateClampedUniformNurbs crv from 3 pts, modifying the center control pt's weight with a slider. By using the same points, and using the same cp weight but negative, resulted in the completion of the positive curves ellipsoid.

The crv with the positive CP bakes and is selectable in rhino while the crv with the negative CP can be baked but not selected in Rhino. They can both be joined, but you can only select the baked joined curve, if you select the segment defined by the positive CP crv. In GH it can also be revolved, but again, when baked, only the area defined by the positive control point can be selected, even though it is one surface, the area defined by the negative CP cannot be selected. Why is this the case?

Attachments:

crv1.png, 8 KB
crv2.png, 9 KB
crvWeight2.ghx, 47 KB

Replies to This Discussion

Permalink Reply by David Rutten on May 23, 2009 at 5:13am

Hi Luis,

does this some from De Mesa? Indeed, Rhino does not offer any tools to set negative weights on Nurbs curves and surfaces even though the nurbs functions can obviously deal with it. I suppose nobody who's working on Rhino core ever suspected that people would want negative weights, and as such they are not supported by many functions.

Note that curves with negative weights will partially exist outside the control-point polygon, which is not true for curves with positive weights. This in itself could already be a large problem.

I cannot help out here since the problem is in Rhino, not Grasshopper. You'll have to take it up with Seattle HQ, probably Dale Lear. Trust me, it will carry more weight (no pun intended) if a user complains as opposed to an employee.

Permalink Reply by Luis Fraguada on May 23, 2009 at 11:18am

You reply is very much appreciated. I will contact Dale Lear @ HQ. Yes, indeed this is from De Mesa :)

Permalink Reply by Dimosthenis Stogios on June 21, 2009 at 8:16am

Hi... I am very interested in this topic as well, so after digging in for a while I found by chance that the curve with the negative weight is selectable, though only at its ends! Also, if you turn on the cpoints and select the point with the -w it will show its value in the "set cp weight" dialog althought you will not be able to give it another negative one.

dimosthenis

Permalink Reply by Dirk Anderson on June 21, 2009 at 8:29am

I don't really understand the theory of curvature other than the amount of influence of curvature away from a 1 degree polyline. Is it naive to plot the degree CV vertex and reverse the vector position to get the -negative weight?

Permalink Reply by Luis Fraguada on June 21, 2009 at 8:47am

This was Dale Fugier's response from the newsgroups:

> Why does Rhino not support NURBS curves with negative weight values?

Our geometry library does support negative weights. But the Rhino UI does
not allow for them because the applications what we export to do not support
them. For example, neither STEP nor IGES support negative weights. Plus, you
can create ellipses and arcs of greater than 180 degrees without having to
use negative weights.

Dale Fugier
Robert McNeel & Associates

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