# A question about single span surface

hi dears, I
have a question, in rhino3d if i want to
create a surface class A(single span surface)is there any difference between
edge srf command with point count=degree+1 in both direction and networks srf
command that have a rebuild to a surface with point count=degree+1 in both
direction? Thanks a lot

Yes there is a difference. One way to learn about the difference is to experiment.

NetworkSrf creates a surface of degree 3 in both directions, and uses a sufficient number of control points in each direction so that the surface will be within the tolerances in the command of the edge curves and interior curves. A set of single span edge curves, even degree 3 edge curves, may result in a multi-span surface.

EdgeSrf creates a surface as a composite of the edge curves. If the edge curves are single span then the surface will be single span in both directions.

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Hi Shayan - yes - EdgeSrf will always use the input curve structure in the surface - so matching single span, uniform curves will make the cleanest surfaces. NetworkSrf never uses the input curve structure to make the surface. Surfaces are always degree 3 and seldom, if ever, single span.

Loft, Sweep1, Sweep2, ExtrudeCrv, ExtrudeCrvAlongCrv are other commands which use the curve structure in the resulting surface.

-Pascal

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To add, would be great to have the ability within the EdgeSrf command to match to adjacent surfaces. At the moment you can build up to the edge of adjacent surfaces and then need to Match Surface separately.

Sach

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thanks dear

thanks dear

hi dear davidcockey, thanks for your attention
i understanding from your answer if i use networksrf even my edge curve was single- span may the result is a multi-span surface but if all edge curve are single_span (point count=6 &degree=5)and then i create a surface with networksrf:
( “U”: degree =3 CV count = 88 (0 <= U <= 1179.27)
(“V”: degree =3 CV count = 11 (0 <= V <= 304.114)
then i rebuild my surface to :
(U": degree =5 CV count = 6 (0 <= U <= 1)
( “V”: degree =5 CV count = 6 (0 <= V <= 1).
even the second surface(surface have rebuild) not single-span surface?or this is a single span surface but not clean as surface that make from EdgeSrf?

thanks a lot

shayan, in your example the second surface created is single span. However using Rebuild will usually change the shape of the surface so that the edges of the rebuilt surface will not exactly match the original single span edge curves.

EdgeSrf with a set of single span edge curves results in a single span that exactly matches the original single span edge curves.

If you need a surface to exactly match a set of edge curves use EdgeSrf.

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dear davidcockey,
appriciate for your attention to my question and giving me a very useful answer to it

Hi dear pascal
you said,Loft, Sweep1, Sweep2, ExtrudeCrv, ExtrudeCrvAlongCrv are other commands which use the curve structure in the resulting surface, it means these commands can produce single-span surface ?or they can produce single-span surface but not as clean as EdgeSrf?
And i have another question ,i have to single-span surface which is better ?
1)“U”: degree =5 CV count = 6 (-753.627 <= U <= -222.453)
“V”: degree =5 CV count = 6 (2.77556e-17 <= V <= 0.78484)

2)“U”: degree =3 CV count = 4 (-1 <= U <= 0)
“V”: degree =5 CV count = 6 (1.38778e-17 <= V <= 1)

Well, yes and no - Loft > Normal always makes a degree three surface in the lofting (U) direction - so, if you give it two single span curves, of any degree, it will have whatever the highest degree of the two curves by degree three and single span. (This is not different from EdgeSrf with the same two curves). More than two curves in Loft(Normal) will add a span per curve.

Loft > Loose will make up to degree 3 surface - that is, two curves will make a degree 1, 3 > degree 2, 4 or more > Degree 3. More than four curves in a Loose loft will have two or more spans.

EdgeSrf with four single span curves will result in a surface with the highest degree from the input curves in each direction.

Sweep 1 & 2 will follow the structure of the cross section curves, and the rail curve(s) if Simple Sweep is available and checked.

ExtrudeCrvAlongCrv will match the two input curves in the resulting surface.

I don’t know if either is better exactly, it depends what you are trying to do. Sometimes fewer points is advatageous. I would probably use Reparamterize > Automatic to get the parameter range to roughly match the 3d size, but I am not sure that really matters in general.

I don’t know if any of that helps.

-Pascal

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dear pasca
appriciate for your attention to my question and giving me a very useful answer

I’m not sure if this is the right place to post this request, but:
I wish we could have an improved version of _EdgeSrf that preserves the degree of the curves, but also has the option to set position tangency or curvature to the edges selected.

Hi Joaquin - it is on the pile. @GregArden was doing some chipping away at this idea.

-Pascal

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thanks! this a really good news!

Hi Joaquin - sorry, don’t get too excited, it only means there is, or has been, some typing in that area, not that the feature will actually be in Rhino 6…

-Pascal

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OK, I understand, it’s good to know this feature is on the pile.

Thanks,
Joaquín