Resymmetrizologicalization

Since the dawn of time (i.e. since SharpConstruct) sculpt symmetry essentially works as though multiple brushes are operating on the mesh simultaneously. This is not necessarily the most efficient method, but it is easy to implement and is generally fairly robust. It is even serviceable with dynamic topology, which is somewhat surprising considering that no attempt is made to keep the topology symmetric. That said, imperfections do begin to pop up with dynamic topology, so a workaround would be nice.

Symmetric Suzannes
Naturally the first thing I had to do was go all Photo Booth on Suzanne.

While there are certainly fancy options such as dynamically mirroring the mesh while sculpting and constantly re-stitching the center, a simpler alternative is to add an operator that the user can manually invoke to symmetrize the mesh. (I was inordinately happy to discover that symmetrize is an actual word.)

I’ve just gotten a basic implementation working, code available as usual from my Blender Github repository, symmetrize branch.

To test it, just select a mesh and spacebar-search for the symmetrize operator. Only works in object mode for now. Once the algorithm is solid for meshes I’ll add a BMesh version. Note that right now it’s hardcoded to symmetrize across the YZ plane from X- to X+.

It’s actually a bit harder to symmetrize than you might expect; there are a significant number of cases to think through. A few examples follow:

A mesh consisting of a hexagon, pentagon, and three quads being symmetrized.
  • Any polygon that is already symmetric across the axis will end up just as it started. The pentagon and square demonstrate this.
  • In general, a polygon that is not symmetric across the axis gets chopped in half — everything on one side of the axis gets thrown away, and the rest is copied over and mirrored. The hexagon at left demonstrates this.
  • An exception is made for asymmetric polygons that have some symmetric edges. For example, the two quads adjacent to the square do not get split down the middle, as that would require the nice square to become a hexagon. Instead, a triangle is added to separate the asymmetric polygon from the symmetric one, and then a mirrored copy is added.

There are actually more cases — I’m still working on dealing with polygons that cross the axis multiple times. Loose edges should also be mirrored. It may be helpful to add additional cleanups too, such as optional merging of thin polygons that are sometimes generated near the axis.

As noted, this is just a regular mesh operator for now, not yet part of the dynamic topology branch. I’ll merge it in once the tool is nicely stabilized, but I should mention that I don’t currently have a lot of time to work on Blender.

6 thoughts on “Resymmetrizologicalization”

  1. Nick,

    you might consider in the case where a polygon crosses the center line, of just symmetrizing the vertices, and not the edge.

    So instead of splitting the quad into three triangles you’d end up with three rectangles.

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