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Boolean Modifier will triangulate 2 polyspheres on union
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System Information
Operating system and graphics card
winxp sp3

Blender Version
Broken: (example: 2.69.7 4b206af, see splash screen)
Worked: (optional)
Broken : blender-2.69-410613e-win32
Worked : blender 2.69 stable

Short description of error
Boolean modifier set to union will triangulate polysphere

Exact steps for others to reproduce the error
Based on a (as simple as possible) attached .blend file with minimum amount of steps

in Object Mode
-Add Subsurf modifier to Cube, set the View setting to 2 and apply the modifier
-SHIFT+D to duplicate the polysphere obtained and move it a bit
-Add Boolean modifier to the polysphere, set to Union and target the other polysphere
-Apply modifier , select the new object obtained and go to Edit Mode

The union zone is as expected from the new code having ngons but every other quads of the polyspheres have been triangulated.

screenshot in case i don't explain well :

Event Timeline

Sanc Tuary (sanctuary) raised the priority of this task from to 90.
Sanc Tuary (sanctuary) updated the task description. (Show Details)
Sanc Tuary (sanctuary) edited a custom field.

The thing is: boolean only works correct with flat faces and will triangulate non-flat ones to work correct.

Previously boolean used to merge split faces back because it only worked with tris and quads. Now then it does support NGons, merging stuff back is much more difficult and would take reasonable amount of computing power.

Further, SS is not guaranteed to produce flat faces even if your mesh had only flat faces.

Will type all this specifics to release notes, so it's clear what's happening.

Thomas Dinges (dingto) lowered the priority of this task from 90 to Normal.Feb 21 2014, 1:43 AM
Sergey Sharybin (sergey) changed the task status from Unknown Status to Unknown Status.Feb 21 2014, 7:05 AM

Previously the triangulation was confined to the immediate intersection are and so it was reasonably easy to clean up. The current behavior will sneakily triangulate quads in unrelated areas. I'd hardly call that more predictable, except in the sense that it's predictably dangerous to use. If the bug can't be resolved, the previous behavior, which was not great but at least it was manageable, should be rolled back. The new behavior is not an improvement.

However, it is puzzling to me that this is such a problem that it cannot be resolved. Judging from a quick test I can trivially (but not comfortably) perform the boolean without triangulating anything using the following manual procedure:

  1. Go into edit mode, select all and mark sharp on all edges, for both meshes you intend to boolean.
  2. Perform the boolean and apply.
  3. Select all.
  4. Deselect sharp edges
  5. Deselect the edges on the intersection of the two models.
  6. Dissolve Edges
  7. (Optionally) Select all verts with only two connecting edges and dissolve.

Surely this could be automated?

Made it so triangulation edges are being dissolved now in rB9643b2e. It works slower now, but at least no unwanted topology should happen now.

Works great now, thanks Sergey :)