How to obtain barycentric coordinates? (Wireframe Shader)

It seems like the approach I outlined above isn't really going to work, as two corners of the triangle can be the same color.

actually the code I posted above works under certain conditions. it seems that some models (like those I exported from magicavoxel) have different (messed up?) vertex winding orders, so there are some faces with R,G,G, which messes up this process.

@cyberreality if you could post or email me your version I would appreciate it. (jasons aat novaleaf doot coom)

heres my code to process the mesh (C#)

	public override void _Ready()
	{
        var testNode = FindNode("Test") as MeshInstance;
		//var mesh = GD.Load<ArrayMesh>("res://asset/fish/Fish1.obj");
		mesh = testNode.Mesh as ArrayMesh;

		var mdt = new MeshDataTool();
		mdt.CreateFromSurface(mesh, 0);



		float vertexCount = mdt.GetVertexCount();
		for (var i = 0; i < vertexCount; i++)
		{
			var channel = i % 3;
			var color = new Color(0, 0, 0, 1);
			switch (channel)
			{
				case 0:
					color.r = 1;
					break;
				case 1:
					color.g = 1;
					break;
				case 2:
					color.b = 1;
					break;
			}

			mdt.SetVertexColor(i, color);
		}
		//replace our mesh with modified version
		mesh.SurfaceRemove(0);
		mdt.CommitToSurface(mesh);

		testNode.Mesh = mesh;
	}

and here is the shader

shader_type spatial;
void fragment() {	
	if(COLOR.x < 0.01 || COLOR.y < 0.01 || COLOR.z < 0.01) {
		ALBEDO = vec3(0.0, 0.0, 0.0);
	} else {
		ALBEDO = vec3(0.5, 0.5, 0.5);
	}
}

here's an example of the same workflow on my magicavoxel mesh.

it seems like I need to rebuild the mesh, any ideas?

Your code is a bit complex, so I spent a good number of hours implementing my own version, but I kept running into a pathological case. I then realized that this is basically "Graph coloring" and found an algorithm called "Welsh-Powell" that seems to work at reasonable speed: https://iq.opengenus.org/welsh-powell-algorithm/

I'm going to implement that, and if I still get problems I think the vertex may have too many edges. I have an idea to color alpha in that case and have the shader ignore where alpha ==0;

i'll post when done!

Thanks for the help @cybereality , I finished, at least good enough for now. The above is an example output and configurable shader.

My MeshDataTool code is written in C# so not sure how usable it will be, but you can find it and a test scene here: https://github.com/jasonswearingen/godot-csharp-tech/tree/dev/demo-projects/scratch/demos/WireframeMeshDemo

The MeshDataTool processing is done in WireframeMeshDemo.cs and I did my best to comment the code. I ended up using 5 color channels for vertex ID's, and 1 channel to hint what side is longest (for interrior edge removal). I was able to have these 6 channels by encoding 5 choices into red (20% each) and 1 into green. The shader reads this encoded data and generates the barycentric coords for the rest of the shader to use. The shader is in WireframeMeshDemo.shader

While this took a lot longer than I expected (everything does) I sure learned a lot about shaders! hope I don't forget it for next time it's needed.