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I have an object with a bunch of Vertex Groups. I need to use those Vertex Groups "after" an elaborate nest of Geometry Nodes. In general, how is this done?

As for what I've tried, sadly my typical approach of "do a bunch of work and watch Blender kindly overlook what I technically asked for and instead provide what I meant" is not going well. I see in the manual that Geometry Nodes genericizes all attributes (why?!). True enough, I see populated columns in Spreadsheet→Evaluated→Mesh→Vertex with the same names as my Vertex Groups. But I can't figure out how to then access them. I started down the road of tossing Capture/Store Named Attributes all over my GN trees, but only seemed to encounter additional problems—I'll write a separate question if that's the right road.


Here's a sample file created in v4.1, in which a Join Geometry node causes the above effect on two copies of incoming geometry. An Icosphere with a Vertex Group has two modifiers added; Geometry Nodes to pass through the original, plus create and move a duplicate icosphere, and a subsequent Mask modifier to hide part of the original Icosphere mesh by Vertex Group.

alternating screencaptures showing the situation described below.

Unticking the ✓Join box in Icosphere→Modifiers→JoinerGN only gives the Join Geometry node one copy (translated and no-op joined, yet VG-intact?), and lets the lower Mask modifier work as intended, revealing the green "Go" object. How could the setup be altered such that, for example, the Mask modifier can affect the all of the Geometry Nodes-manipulated geometry?

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    $\begingroup$ I can confirm that Geometry Nodes tend to destroy vertex groups when adding geometry. It would help if you could emphasize in your question what you are trying to achieve exactly. So that we can find a workaround. $\endgroup$
    – Lutzi
    Commented Apr 19 at 14:25
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    $\begingroup$ I have follow-up Questions to post if this goes anywhere, sourced from my actual project. That's a sufficiently awful mess I tried for a Minimal Reproducible Example sample file instead, hoping to get at the concept itself, and correct workflow. I'm desperately itching to expand it, but for this question as written, getting a Mask modifier to operate upon prior GN-duplicated geometry would ✓Solve it. $\endgroup$
    – Joel Reid
    Commented Apr 19 at 14:48
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    $\begingroup$ After constructive modification, the vgroup disappears, but does survive as an attribute, which can be used by modifiers accepting attributes as parameters further down the stack.. (edge_bevel_weight survives..) I'll bow to superior knowledge.. but this looks to me like a stage in the evolution of Blender, which will eventually see the end of vgroups altogether? $\endgroup$
    – Robin Betts ♦
    Commented Apr 19 at 18:29
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    $\begingroup$ After playing with applying the "joining" GN, I am afraid a workaround is to develop your own GN mimicking the Mask modifier, but reading the cutaway attribute, as the vertex group is lost. $\endgroup$
    – StefLAncien
    Commented Apr 19 at 18:48

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For any future readers, do not do this. As noted in the question's comments and indeed in the manual, Geometry Nodes make everything generic. Vector Groups will not survive as VGs, but as a float field. Plan your VGs and UVs and Modifiers and workflow accordingly.

I "solved" this problem in line with StefLAncien's suggestion—I've recreated my entire Modifiers stack in Geometry Nodes. Most Modifiers went without too much fuss. The Mask mod in the question, for example, was even improved when replaced with a Set Material node pulling a transparent material; letting renders omit the mesh and its shadow, but the geometry still be present.

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