Geometry Nodes: Extrdue Mesh, Faces, Individual is non-deterministic. #109429
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Reference: blender/blender#109429
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Blender 3.6
The indexing for new faces/edges when using extrude Mesh with individual checked is in the wrong order.
When individual is not checked, everything is in the correct order.
https://youtu.be/tSQ5b7zBOa0
Solution:
I noticed you have two completely different functions that are called for "individual_Mesh_Faces" and "Mesh_Faces".
You have everything you already in place.
Inside " poly_selection.foreach_index"
37eabb171a/source/blender/nodes/geometry/nodes/node_geo_extrude_mesh.cc (L1184)Have a function call for that one face to "static void extrude_mesh_face_regions"
37eabb171a/source/blender/nodes/geometry/nodes/node_geo_extrude_mesh.cc (L701)This will result in the same mesh as currently, but with the faces/edges having the correct new indices.
While I know what to do to fix it, I don't know all the syntax, or how to use GIT. (Every attempt at using Linux and/or GIT results in weeks of frustration and failure...)
Before you say "this is not a bug, it's a feature request". It's a bug. Ask Hans. Having indices in a consistent and correct order is a requirement for Nodes.
Thanks! ^_^
Hey, thanks for report. Just to be sure, can you added to extrude selection mask not just one but 2~3 polygone (connected) and deduce: if rings of extrusion now have changed topology, did we have to randomize indices of new faces in new rings?
Not entirely sure what you're asking.
But the bug persists regardless of number of faces extruded and if they are physically connected before extruding. The indexing gets worse the more faces there are.
Just having the "extrude_individual_mesh_faces" Call the function "extrude_mesh_face_regions" one face at a time will result in the same mesh that we get now, but with the correct face/edge/vertex indexes.
Thanks for the report and for digging into the code, but this actually isn't a bug. The two functions use different implementations for performance reasons; the "individual" mode is much faster than the face region extrude.
The guarantee we make about indices for geometry nodes is that they will be the same given the same inputs-- nodes don't necessarily have to produce "good" indices, they just have to be deterministic. The output of this node is deterministic because it's the same every time with the same settings and input mesh. The order of the faces around each individual extrusion is based on the winding order of the face.
Ok, I'm trying to write an Addon that translates Blender Geometry Nodes into Unreal's Dynamic Mesh system. Could you explain how it comes up with order of edges being extruded?
I'm not saying change it, just explain it so I can replicate it. (If that's not to big an ask) Thanks Hans.
Never mind. I figured it out. It starts with the edge that has the lowest summation of edge indices. Then goes to the edge connected to B vertex of "current edge".