What determines seam points in polysurface from extrude closed curve?



I hope I am expressing this question clearly! (if not please forgive me!)

“what determines seam points in polysurfaces created?”

In other words, let’s say:

  1. I create a curve using interpcurve and blend, and then join it into one closed curve.
  2. then I extrude the curve (straight/ both sides= no) to 1.0mm high.
  3. the extrude creates a polysurface/ open- 3 surfaces
  4. this polysurface doesnt seem to be able to be “joined” into one surface…why is this?

my question is: how is it determined how many surfaces are created, and where the seams are placed?

Thank you in advance and best regards,

(Wim Dekeyser) #2

Exactly what is going on is hard to tell without seeing your curves.
Normally, when you extrude a polycrv, a surface will be created for each segment and the seams will be where the segments join. This is not so when the segments join curvature continuously - that will create one single surface. Also, depending on the setting in the ExtrudeCrv command (SplitAtTangents), tangent continuous segments can also create one single surface. Then there are cases where one single segment will produce two surfaces - that happens when there are kinks in the curve.

Why do you want one single surface?


the way NURBS surface is constructed is tied to the way you create the curve. if let say you have 1 Curve Degree 3, extruding the Crv will result in degree 3 Srf as well. (u can change it using rebuild command if you want to)

Curve Control points will be your Surface control Points and if there is a kink on the curve, your Srf will have Kink as well.

different Curve degree joined into 1 will result in Polysurface. (for better understanding, try this method, create Curve A (Degree 3) and Curve B(Degree2). join them together and type “What” on your command line. you will see your curve is actually consist of 2 segments (Degree 2 and Degree 3 Crv) this what makes u have polysurface on your extrusion.

for Surface seam, it will follow your curve seam if it is a closed Crv.

hope that helps.


Hello Ronnie,

Thank you sooooo muuuuch! I totally understand what you said! I feel smarter now! yay!

p.s. your reply stimulated my brain in the right direction, and I was able to solve some filleting difficulties after reading it! It made me stop feeling the futility of the filleting command, and instead made me look at the geometry and “solve the problem…'looking at what was actually happening” and by splitting and creating new surfaces and extract isocurves, doing minor surface and curve surgury, and sweeping new surfaces! one step forward, two steps back, one step forward, etc!

Thank you!

I love this forum!!

Best Regards,