I'm trying to use weights from a vertex group in cycles nodes to change the transparency of the mesh. But as far as I know this is still not supported.
Is there any workaround that I could use?
This is what I want to achieve:
As you can see, I use a VertexWeightProximity modifier, to paint the vertex group, and I would like to use it in the node tree to affect the transparency of the mesh. The attribute node doesn't seem to read vertex groups yet.
This method doesn't use weights, but it seems that your real goal is to get the proximity into Cycles. If this is the case then you can get the proximity using a different method called Dynamic Paint. This will output an image sequence that can be used in Cycles.
Below is a rough example of what dynamic paint can do:
This will output a vertex colour layer which can be used in cycles like this to produce to the image on the right:
Miikah, the creator of dynamic paint has a full tutorial here:
http://www.miikahweb.com/en/articles/blender-dynamicpaint-basics
The basic steps on the physics tab is to set the circle segment to be the brush like this:
The circle that will become transparent is set up in the same way as the brush but is set to be the canvas with vertex colours set in the output panel:
You need to translate vertex weight into some proxy that nodes can read. Consider using UV as a proxy. It can be read by nodes and it can easily be mapped to vertex group weight.
Create a new UV mapping for your mesh and set all vertices to 0,0 on that new map. Add a UV warp modifier editing this new mapping, modulated by the vertex group you want to reference, and set it to use two empties, one located at 1,1 and one at 0,0. Now, by accessing these new UV coordinates, you're accessing the vertex group modulating the modifier.
Since Blender 3.0 using weight paint in shaders is pretty easy, you ony need to convert the weight paint with geometry nodes and push the result to the desired/empty vertex paint. After this the vertex color will store the weights and will automatically update. By this method it is even possible to store multiple weights in one vertex color by using the rgb channels
An alternative to weight paint is Vertex Paint. Vertex paint can be, unlike weight paint, accessed in cycles. After applying your modifier, you can use the information in this answer to convert your weight paint data to Vertex Colors. Once this is complete, simply use an attribute node:
