GeoNode "Points to Curves": how to make curves for each distinct vertex group in mesh

Question

I am newbie in geonodes and, after some googling, I still do not know how to find answer to my question.

My data: Mesh, made of vertices only. Each vertex is assigned to Vertex Group, therefore vertices of the mesh can be divided to separate subsets. (Example on pictures: I have mesh made of vertices, vertex_group_1 includes upper circle of vertices, vertex_group_2 includes lower circle of vertices)

My goal: to make curve from vertices from the same Vertex Group for each(!) Vertex Group and, as a result, to have a set of curves. If there were n Vertex Groups, I expect n curves as a result. (Example on pictures: as a result I want to have two circle-shaped curves, upper and lower, not a spiral!)

Expected solution: GeoNode "Points to Curves" is supposed to make what I want, as the documentation says "All points with the same Group ID value will be joined in the same curve".

Question: How can I convert my Vertex Group information from mesh to Group Id for PointCloud? I can convert my mesh to point cloud with "Mesh to Points", what is next?

Another expected solution: My mesh is the result of my own script, so I can also make a Point Cloud from it in my script too (I hope it is easy). BUT how should I add "Group Id" to it? Should it be a custom attribute with "group id" name? And how the "Points to Curves" geonode will derive this information, will it derive it automatically when I plug my point cloud geometry to "point" attribute?

Obviously, I do not understand something basic about geonodes.

Answer 1

(Using Blender 3.6.1) From the above picture, it seems that every Vertex Group is a different Attribute. For a vertex belonging to a Vertex Group, the value is the Weight set in Edit Mode; for the vertices not belonging to the Vertex Group, the value is zero. So the name of every Vertex Group is required a priori to make a selection, that can feed a Set ID node. Working with version 3.6.1, I can not check if it is a valid input for the Points to Curves node you are using. However, getting inspiration from the post What is the Group Id input in the "Index of Nearest" node?, I succeeded to make it work with an Index of Nearest node.

Answer 2

(Using Blender 3.6.1 and python)

Here is a piece of code to create the 'id' attribute and to set its value per vertex :

import bpy
import numpy

active_obj = bpy.context.active_object

# Split vertices in two groups (for demo)
count = len(active_obj.data.vertices)
GroupID = numpy.repeat([1,2],count/2)

# Create the attribute
active_obj.data.attributes.new(name='id', type='INT', domain='POINT')

# Set the attribute value
active_obj.data.attributes['id'].data.foreach_set('value', GroupID)

Reference : I took information from Python : Custom Attribute for Vertices and Manually setting custom attributes per edge / vertex / etc.