# Fixing a messy boolean operation

I'm splitting an object with a plane using a boolean modifier as described in Split a mesh with a plane. Roughly:

1. Duplicate the object.
3. Apply an intersection Boolean modifier to the original object.
4. Apply a difference Boolean modifier to the duplicated object.

I did this twice to pull a thin slice out of my object, and the resulting mesh is much more complicated than it could be. I'm slicing with unmodified planes so the big main faces are actually flat, big end up broken up into many crazy smaller faces.

I'm imagining the cleanest slice would be a topological cylinder. Is there some way to get a cleaner slice than this (python manipulation is fine) - either a method to make a nice slice directly, or a way to clean up the slice I've already got?

Here is what I imagine as an ideal mesh:

• maybe you could show a mock-up of the kind of geometry you expected or wanted Jul 5 '13 at 12:30
• Will do tonight - as much as I'd love to do Blender at work, I'll have to wait until I get home. Jul 5 '13 at 13:07

I don't believe you can get a better result from a boolean modifier, they are nearly always messy, however, there are several ways to clean up a messy mesh.

Method 1:

Rather than fixing it by hand it might be easier to just add a 'Remesh' modifier which will try to convert the mesh to a quad based mesh.

Setting the remesh modifier to 'smooth' mode and upping the Octree Depth (think of it as resolution of mesh) will result in a cleaner mesh.

However, to get the the mesh as smooth as you want it may mean the resolution of the remesh would have to be quite high, but even at a lower resolution it can be a good starting point.

You could use a lower level remesh modifier and then a subsurf modifier after that to smooth it out again.

Method 2:

Another method is to select all the faces that make up the bottom of the mesh press F to make an ngon and then choose Ctrl + F > Poke Faces to subdivide them again. You would then also have to repeat this for the top of the mesh.

This should result in the 'topological cylinder' you mention in your question.

• A third method (though its effectiveness really depends on the starting model) would be to add a thin cylinder to the scene (without top/bottom faces) and shrinkwrap it to the original model using the shrinkwrap modifier. The top and bottom faces can been added as giant ngons after the shrinkwrap modifier is applied. Jul 4 '13 at 12:47
• @Ray I was able to get better results in my model with a remesh + sharp. But thanks for pointing me in this direction.
– HRJ
Nov 17 '13 at 14:22

Then subdivide the plane (or flattened cube) first, and you will end up with (possibly) more appropriate geometry.

If the surface is flat and you don't intend to deform it, it should not matter how it is constructed, even if it isn't aesthetically pleasing topology flow. The example on the left is already optimal considering the number of faces it creates.

• The only reason I care about the topology is because I want to apply a UV mapping. I really just want to draw an image on the flat surface.. maybe I'll find that there is a way to do that without unwrapping the mesh? Jul 5 '13 at 19:27

In the case where the geometry is valid but there are too many redundant cuts in the mesh, Limited Dissolve works well.

Select all, go to: Mesh -> Delete ->Limited Dissolve. Try set the angle very low (1.0 or so).

Main thing to watch out for here is you'll get n-gons which might not be what you want.