How is 'u' and 'v' determined for a surface?

I am trying to use the SubSrf(isotrim) module. It wants input of a range for u and v. I can input those and get a portion of the surface, but what I don't know is how to determine the region I want. The domain numbers do not seem to correspond to anything I can think of. Not to the number of control points for the NURBS surface. The surface is a direct input, so clearly GH has a default method of setting the uv grid for a surface.

I even did a trial where I divided the original surface into a grid of uv points, and then generated a new surface from that point grid. The new surface was close to the original, especially with a high enough number of points. The SubSrf module gave a completely different region of the surface for the same domain parameters as before, but the new portion did not seem to correspond to the uv grid I used to make the second surface. So I need to know the underlying GH determination of the number of uv for a surface. Please enlighten me.

Replies to This Discussion

Permalink Reply by David Rutten on October 13, 2009 at 6:15pm

Hi Damon,

UV domains are indeed utterly meaningless. When we create a new surface we of course need to come up with some Domains, so usually we'll pick something that corresponds roughly to the dimensions of the surface, or the number of nurbs spans, or the sweep angle or...

What you need to know about domain is that they can be practically anything. You can see the UV domains of a surface in Rhino by using the _What command. You can see UV domains of a surface in Grasshopper by pluggin the surface into a 2D interval parameter (then either look at the tooltip or feed the interval parameter into a Text panel).

Every Surface parameter in Grasshopper has an option called "Reparameterize" buried in the menu. When you check this option, it makes sure that all surfaces that pass through that parameter get a UV domain of {0.0 To 1.0} in both directions.

--
David Rutten
david@mcneel.com
Seattle, WA

Permalink Reply by Damon Caldwell on October 13, 2009 at 8:54pm