Get parent vertex group in geometry nodes

[Filat_Nicolae]

I am trying to use the new hair system and create a geometry modifier that would change the radius of the hair based on a weight map.

Unfortunately, the weight map is on the hair surface but now on the hair itself. I could not find a way to weight paint on the hair curve object.

As you can see in the image, the Plane has a Vertex Group and I would like to access it in the Thickness modifier that I am creating.

How I think it should work

As far as I understand when I weight paint there is a weight value in range [0,1] that is assigned for each vertex of the mesh. In my case that would be assigning weight values to the subdivided plane mesh.

I know that someone else did this before in an earlier version of blender but I don’t have the Transfer Attribute node now and I don’t what to use.

Quote from Access parent object vertex group more directly with geometry nodes

I can access it with an Object Info node, a Transfer Attribute node, and a string input with the name of the vertex group, and it works fine, but it isn’t very user friendly. The name must be typed manually, and I need an extra input with the correct object to pass to the object info node, and the string input can’t be used as a value by itself.

My conceptual idea is that for each hair I should find the closest point on the vertex group of the plane. After I find that point I should get the weight value associated with that vertex. I don’t have enough blender experience to implement my idea using geometry nodes.

[Filat_Nicolae]

My attempt is as follows

I am not sure what the index from SampleNearest gives me.
Any help would be appreciated

I created the second post because I can’t post two images in the same post.

[zeroskilz]

Hello

@Filat_Nicolae

I don’t have the Transfer Attribute node now

Transfer Attribute has been replaced with a Sample Nearest, Sample Index combo.

@Filat_Nicolae

I am not sure what the index from SampleNearest gives me.

Its sampling the nearest point… so, it is the index of the nearest point.
Sounds like you need to do some introductory tutorials.

Anyway, you can use Sample UV Surface to the get the weights of the vertex group on the surface object like so:

… none of that matters though if you’re using Eevee curve strips - Its good to be aware that Eevee curve strips only accept a single global width per curve object… setting the diameter per hair strand only works in cycles. For Eevee you’ll have to roll your own curve-to-mesh to show thickness.

Good luck.

[Filat_Nicolae]

Hi, thank you very much for taking the time to answer my post.

What you did works but only if I render the image with Cycles.
Is there a way to see the thickness in the viewport as I edit the vertex paint?

Here is what I currently see in the scene

Here is that happens when I render (F12)

I know that this question might be a noob question but how can I see the thickness change in real-time in the viewport?

[zeroskilz]

@Filat_Nicolae

I know that this question might be a noob question but how can I see the thickness change in real-time in the viewport?

Already answered before:

@zeroskilz

Its good to be aware that Eevee curve strips only accept a single global width per curve object… setting the diameter per hair strand only works in cycles. For Eevee you’ll have to roll your own curve-to-mesh to show thickness.

You can use a simple triangle profile for preview purposes…:

… stick it at the end of your modifier stack.

Good luck.