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Boolean operator creates non-manifold meshes when the input meshes contain some vertices with the same coordinates.
Open, Confirmed, MediumPublic

Description

System Information
Windows 10, n/a

Blender Version
Broken: 2.79

Short description of error
When two meshes are joined with a union boolean modifier and both meshes share at least one vertex with the same coordinates, the resulting mesh will contain non-manifold vertices. Some of these can be repaired by removing doubles, some can't.

Exact steps for others to reproduce the error
Open the mesh where the boolean modifier has been applied in edit mode, then select non-manifold, several vertices will be selected. Then select all and remove doubles. Some vertices will be removed (I think 6), but non-manifold geometry remains.

However, if the two meshes are moved apart slightly before applying the boolean modifier (this can be done with the middle mesh, where the modifier is still unapplied) so that no vertices share the same coordinates, the problem disappears. There are no double vertices and no non-manifold geometry.

.blend upload:

Details

Type
Bug

Event Timeline

ronan ducluzeau (zeauro) triaged this task as Needs Information from User priority.

Boolean modifier is described into manual as a geometry modifier with known limitations.
https://docs.blender.org/manual/en/dev/modeling/modifiers/generate/booleans.html#known-limitations

Mentionned issue seems to correspond to the point Overlapping Geometry.

Known limitations are not considered as bugs to report. Bugtracker is focusing on supported features that does not work as expected.

I marked this report as incomplete to let you the opportunity to attach a .blend and develop explanations if you still think this report is valid.

I thought overlapping geometry meant within the same mesh, rather than between the two meshes (surely they have to overlap to boolean at all?) but if that's what it means and developers are aware of it then I'm fine with it being marked incomplete.

I thought I had uploaded the .blend, but apparently it didn't work. I've reuploaded it now. It contains two meshes in three stages (separate, boolean created, boolean applied) and clearly labels all three. If it would be useful you can take a look, it might illustrate the problem more clearly.

ronan ducluzeau (zeauro) raised the priority of this task from Needs Information from User to Confirmed, Medium.Jan 30 2018, 4:06 PM

It looks like Carve Boolean handles this case pretty well.

You can switch Solver of Boolean modifier to Carve.
Bmesh solver is in development. It does not seem to be able to produce appropriate N-gons as Carve solver does.

I let choïce to close or archive report to developer in charge of Bmesh solver.

Sorry, yes, you're correct, in the file I provided the carve solver does indeed work. It doesn't work consistantly however, I was able to produce this error using bmesh, carve and the intersect modifier.