These are two holes I cut by using bool tool, my question is why the edges 1,2,3 are necessary, I have a rough idea that all the shape needs to be at least N-gons, so if there is a standalong hole without any connection to the mesh, it is not allowed, but why should we follow this rule, in other words,

1.why the feature is designed in such way, is this reasonable or not, I think this question will help me better understand the mesh, so thanks in advance!

2.Is edge 1,2,3 has to be at the same location as shown on the screen shot? Can the edge 1 replaced by edge a?

enter image description here

  • $\begingroup$ it's just the way blender's topology system works. I don't know any of the code behind it so I couldn't really tell you the reasoning behind it, but yea. $\endgroup$
    Feb 10, 2021 at 0:49

1 Answer 1


There's evidently some critical conditions that an ngon can't exist under.

If you take an example with only one hole, you'll see you are completely unable to dissolve these edges.

enter image description here

When I cut two holes I got a different result from you, and I was unable to dissolve any edges.

enter image description here

Until I added a new edge in the center like you have, and then you are able to dissolve a couple. I get one less edge than yours.

enter image description here

But each of these remains a shape with a definite inside and outside, and that allows a face to be constructed from it.

I'm unaware of the exact limits placed on ngons, but my best guess is that these are very close to the simplest shapes possible that still allow you to draw triangles without drawing through an area where the mesh shouldn't have a face.

enter image description here (I kind of misplaced the X here, maybe)

You can use a Triangulate modifier in Wireframe view to visualize.

enter image description here

  • $\begingroup$ Thanks for the answer, one thing I could not understand is what do you mean by leaking out into space that should be open $\endgroup$ Feb 11, 2021 at 14:24
  • $\begingroup$ I've edited to state "without drawing through an area where the mesh shouldn't have a face." $\endgroup$ Feb 11, 2021 at 17:26

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