How do I Vertex-Paint over a transparent shader?

Question

I was wondering if it would be possible to use vertex-paint over a transparent material?

I have tried plugging in different sockets into the Mix Shader node but, have not gotten the result that I was hoping for. How do I make sure my vertex-paint is not see through when I overlay it on a Transparent Shader?

Here's a sample to play with

Blender 4.1.1

Answer 1

Vertex painting might not be the best tool for this job, since vertex colors don't have an alpha channel. (See the manual section on texture painting for an alternative, where you can assign a proper image texture to your plane with an alpha channel, and then paint on the texture.)

However, if you want to stick with vertex painting, you could try modifying your node setup by feeding the vertex color through a color ramp before connecting it to the Mix node Factor input. This should look okay if you're mostly painting with bright colors, but dark colors will be "more transparent".

Here's the node setup I'm talking about:

Answer 2

You can use Vertex Paint over a Transparent material when you set up the Color Attribute to be used, with a $0.0$ Alpha value.

For the Color Attribute node within your Shader Material, you can connect the Alpha output socket with the Fac input socket for the Mix Shader node.

All Vertex Colors including Black can thus be used to paint over the Transparent Shader.

As a final note, make sure the Blend Mode and Shadow Mode are set to anything other than Opaque for your Shader Material.