Grasshopper 0.8.0003 available for download

Replies to This Discussion

Permalink Reply by Andrew Heumann on December 5, 2010 at 6:15pm

As far as question 1, I can't reproduce this - divide surface works exactly as it should. Is it possible the surface you're working with is trimmed inboard of its UV domain? the divide surface command will return null values if the division points fall outside the trim.

Permalink Reply by Matt Gaydon on December 5, 2010 at 7:00pm

My Bad, Yeah the first one is not a bug it seems. I had Retrim ticked when i was rebuilding the surface in Rhino 5 after i had deformed it. Sorry

Permalink Reply by Andrew Heumann on December 7, 2010 at 3:03pm

it's fantastic to be able to double click components to access the help for that item, but it seems impossible to get the help in this way for the items that are hidden when "draw all components" is not enabled. Is it possible to have this kind of double-click interaction function when the menus are open?

Permalink Reply by Ben Sitler on December 9, 2010 at 2:38am

I opened a grasshopper file that I created in using 0.7..., but something's not right. The vb script component tells me that it does not recognize DocObjects.LayerInfo anymore. What gives?

Ben

Permalink Reply by to] on December 9, 2010 at 2:42am

Hi Ben
just use DocObjects.Layer ....DocObjects.LayerInfo is obsolete

Permalink Reply by Mårten Nettelbladt on December 10, 2010 at 2:59pm

Could it be that Min and Max are swapped for this component? (Both curvature and curvature direction). In the example above it says that Maximum principal curvature = 0. That seems unlikely for a cylinder. I looked at the directions in another test and they seem swapped too.

Permalink Reply by Chris Tietjen on December 10, 2010 at 3:28pm

Zero is certainly larger than -2.

 

Chris

Permalink Reply by Chris Tietjen on December 10, 2010 at 4:01pm

The sign of the principal curvature depends on the direction of the normal at the point of interest as well as the direction of the principal curve.  If the normal and the curve have an opposite sense the curvature is negative.

 

Chris

Permalink Reply by Mårten Nettelbladt on December 10, 2010 at 4:07pm

Yeah, this makes sense, and since nothing can curve less than Curvature "0", zero has got to be the Min Principal Curvature here, no?

Permalink Reply by Chris Tietjen on December 10, 2010 at 6:05pm

No.  Curvature is signed based on the direction of the normal at the point of interest and the 'sense' of the curves direction so it can be less than zero.  So one of the principal curvatures can be negative and therefore less than zero and also the min. principal curvature which would make zero a legitimate max. curvature.  Certainly though the absolute value of the reciprocal of any radius will always be greater than zero but that's not curvature as we're talking about it here.  In the case of the cylinder if the normal is pointing toward the central axis then the curvature would be positive since the curve of the cylinder and the normal have the same 'sense'.  If the normal is pointing outward then the curvature would be negative since the normal and the curve of the cylinder now have opposite 'sense'.  At least this is how I understand it.  Playing around with the curvature component and a referenced cylinder I notice however that changing the direction of the normal from 'outside' to 'inside' has no effect on the output of the component.  Swapping the uv directions however does change the sign...hmmm?

Chris

Permalink Reply by Chris Tietjen on December 10, 2010 at 9:44pm

Yeah I don't get it either.  It seems like the max and min curvatures should flip in the case shown in the screen shot attached.

Chris

Attachments:

• Principal_curvature.png, 218 KB

Permalink Reply by David Rutten on December 10, 2010 at 11:18pm

Surfaces have a strictly defined normal vector definition. So if you flip the U direction, the V direction, or if you swap U and V the normal vector will be inverted.

 

Breps on the other hand have an override for the normal direction. A Brep Face is allowed to invert the normal direction of the underlying surface. This is so we can join surfaces into polysurfaces and have their normals match up without the need to flip directions and trimming curves.

 

I'm not sure but it could be that the current confusion stems from this brep-face-normal-override flag.

 

--

David Rutten

david@mcneel.com

Seattle, WA

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