U count in Surface from Points

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Permalink Reply by David Rutten on December 25, 2014 at 8:30am

A nurbs surface requires a rectangular grid of control-points; it needs to have a fixed number of columns, and a fixed number of rows. However instead of putting these control-points into a table structure, Rhino treats them as a single flat list. For example a surface can have 10 rows and 6 columns of control-points, resulting in a list of 60 points.

But 10 times 6 isn't the only way to get to 60. If you want to make a surface out of a list of 60 points, you'll also have to tell Rhino how those 60 points should be interpreted in terms of a grid. It could be 2*30, 3*20, 4*15, 5*12, 6*10, and all of the aforementioned products the other way around.

Sometimes there's only one way for a number of points to fit into a rectangular grid. For example if you provide 49 points, then 7*7 is the only way to make it work, but these cases are rare so we always demand you give us all the information required to actually make a rectangular grid of control-points from a linear collection.

As for "Why is it, sometimes we need to attach additional value into it?", this is usually because when you divide a domain or a curve into N segments, you end up with N+1 points. For example take the domain {0 to 5}, and divide it into 5 equal subdomains. You end up with {0 to 1}, {1 to 2}, {2 to 3}, {3 to 4} and {4 to 5}. However there are six numbers that mark the transitions between these domains 0, 1, 2, 3, 4 and 5. This is why you often have to add 1 to the UCount, because the number that controls the UCount often results in N+1 actual points.

Permalink Reply by esha hashim on December 29, 2014 at 1:44am

thank you !  :)

Permalink Reply by vijay on June 4, 2016 at 5:48am

Dear Sir,

PFA images showing the U and V direction means ?

Kindly help how to fixed it to make mesh.

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