Output from solid difference is an open brep when splitting closed breps

Grasshopper
1

I’ve divided a closed brep using solid difference with another closed brep, but cannot then subdivide the result.
I think this is because the output is an open brep?
How to close it?

AmmoniteVS2.2.gh (35.6 KB)

Or can I work around?

Solid%20diff%20snip
Solid diff snip.JPG1224×823 92.3 KB

2

I tried deconstructing the original brep, then extruding the relevant face to re-make it, but it’s still an open brep.
Surely an extrusion must be closed…
I think I’m missing something here?

3

See the attached, and some comments:

  1. always minimise your definition to make it read-friendly.
  2. when building definition, firstly make it work, then think about the efficiency and optimised approaches.
  3. if a problem can be turned into a pure math/coordinates, avoid geometrical calculation,
    if a problem can be worked out in 2D then avoid doing it in 3D.

BREP
BREP1374×727 87.1 KB

BREP.gh (25.3 KB)

4

Thanks for your help Mr Zheng, I really appreciate your taking the time.

Is it possible that I have a different version of Rhino/Grasshopper to you and this is making your addition to my definition fail? I have Rhino 5.

My ‘list item’ doesn’t have the +1 output, and the result, whilst being a closed brep, is what you see in the snip I have attached.

I really don’t understand why dividing closed breps outputs open breps.

Closed%20brep
Closed%20brep1495×668 144 KB

5

I think your list item do have +1 if you click on the +, alternatively put 1 at index, which does the same.

Try to bake the output brep into Rhino to see if it’s a display issue.

I think the reason dividing closed beep gives you open beep is that the profile boundary of the output beep is quite a complex curve that are potentially interpreted as being self intersecting?

As I mentioned before, try to solve it in 2d surface, then you will be assured that an extrusion from planar 2d element is definitely closed.

6

Hello Mr Zheng,
You put me on the right track.
Baking into Rhino showed the surface had not extruded successfully - no top.
Several attempts at different surface methods etc. were no good.
Eventually scaling the cutting surface by .9999 before solid difference did the trick.
I think it was the shared line that was the problem - intersection like you suggested.
Oh dear, I had been trying to do this for days and it had started to get to me!
All is good - for the moment…
Thanks again for your help.