Network Surface Without Bulges

Problem59.3dm (413.9 KB)
I am trying to create a network surface using the red shapes. Ideally it should be tangent in the horizontal and vertical.

For whatever reason, the surface insists on bulging out.

Is there any way to eliminate the bulge without inserting hundreds of frames?

I’d also like to know better the how the concept of how surface building works behind network surface…

I came to this after splitting the lines to do networksrf 3times instead of one.
As with Edgesrf, it seems the order of selecting surface changes the outcome
quite much when there are only three edges to work from.
It’s some way that the isocurves connect, but would like to know a little more in detail…
(left is selecting the bottom edge last, and right is selecting from bottom edge. The right create a more bulge than the left… )

Problem59_Split.3dm (533.4 KB)

Problem59.3dm (324.9 KB)

My way to build that shape is to

  1. explode upper srf and split bottom surface in 3 parts
  2. blend middle surfaces from both sets
  3. patch corners

I am still looking for a general solution to this problem. Even where I have tangency set, the networksrf’s want to bulge.

With nurbs you only want to work with 4 sided surfaces. However sometimes it’s necessary to trim one back. To achieve that with difficult topology like this.

I run into the problem of triangles whenever I hit the end of something (e.g. bow and stern of a ship). I would be great to have a simple solution.

Here’s one more idea. The strategy is to use a blend when the edge conditions are critical.

That creates the best looking part I have seen. The problem I have is doing anything with it.Problem62.3dm (218.0 KB)

Now I need to attach the top to the bottom (shown) whose curves are the same over the part where they intersect.

It won’t boolean add or join whatever I do.

Likely cause of Boolean not working is the coincident surfaces.

To join trim the coincident surfaces. DupEdge the lower edge of the upper solid. Use those curves to Trim a portion of the upper surface of the lower solid, and to remove using Trim the lower surface of the upper solid.

BooleanUnion or Join the trimmed polysurfaces.

Problem62 DC.3dm (290.4 KB)


What I ended up doing was to cut the middle (Cobra Head) piece back to that it ends where it intersects with the top piece. Joining the top and bottom is a PITA because the boolean union creates non-manifold edges. After I got rid of those and caped, I could add in the cut back middle piece.