Vertex Group grows after applying Subdivison Modifier

[derSpunk]

Hi there,

I have a question about how Blender handles Vertex Groups.

I have a couple of vertices assigned to a Vertex Group, like in the follwoing image
(To make it easier to recognise I also assigned a different texture to this group)

Now I add a Subdivision Surface Modifier to the model

As long as the Modifier isn’t apllied, the vertex group stays the same.
But as soon as I apply the Subdivision Surface Modifier, the Selection “grows” the amount
of subdivisions, that were added to the model.

Shouldn’t the vertex group just be subdivided, but by and large stay the same area?
How can I avoid this?

I’m curious about your explanations.

Daniel

[stray]

Hi.
As far as I can tell this has to do with… weight interpolation? Unlike materials which are assigned to faces, vertex groups work only with vertices. If you turn on weight overlay, you’ll see there’s a gradient between vertex group and its neighbors. So new vertices in-between groups will probably belong to both and have averaged value.

No idea how one would normally handle this - those new vertices has to belong somewhere.
But, you can use “Vertex Weight Edit” Modifier to handle case by case scenarios, for example, by removing vertices below certain threshold.