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Voronoi Fracture Create Artifacts with Concave Geometry

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Hi everyone !

I'm having issues with the voronoi fracture on concave geometry.

I'm trying to fracture a pillar which was created in C4D. To do so, I create a VDB and scatter points in it, then I use the Voronoi Fracture SOP on the base geometry. It works quite well overall, however on some curved concave areas the voronoi fracture fills the area, which I would like to avoid.

I attached images to the post; on the second picture the darker parts of the fracture fill the hollow space.

Does anyone know how to fix this issue ?

Thanks !

 

PS I have not attached the .hip file since it's based on a heavy alembic file, but I can do so if necessary.

Pillar.PNG

Voronoi.PNG

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I can't say exactly why this is happening but my guess would be that maybe your base geometry is not watertight or has some other issues like non-manifold geometry etc. From what I can see there might be some strange Ngons where the concave part starts from all the vertical lines not continuing through the concave part.
Maybe try checking your geometry for these kinds of anomalies. To check a meshes "watertightness" I like to just place a "group" node down, connect the mesh I want to check to it, turn off "Base Group" and enable "Include by Edges", then check the "Unshared Edges" option. If there are any members in your group after that then you know that you that there are holes in the mesh that should be closed before you do any kind of fracturing. An added bonus to this is that if you are visualizing the group node then it should highlight where on the mesh these holes are since they are in a group now. 

Just as a proof of concept I made a super quick and dirty mock-up in Houdini and it seems to work for me.

concave_voronoi_frag.hipnc

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Thank you very much !

I searched a bit for where the problem was coming from, and it was indeed stemming from the base geometry. It was created using a C4D boolean, which doesn't automatically stitch together adjacent edges. I modified it, and now it works just fine.

Out of curiosity, if I wanted to close the hole by stitching together adjacent unshared edges, is there a node that can do that inside of Houdini ?

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The "fuse" node should be able to fix that. It does the same thing that many other 3D applications might call "Welding" of points/vertices. 

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