New Uv map Alpha - not work?

When you create a new uv map you have the option of setting an alpha channel. However when it comes time to to use the compositor the alpha channel is just black. There are work arounds, but it would be nice to use the alpha map to mix textures. Obviously im missing something.

Does anyone know how to get the alpha channel to work on a newly created uv map.

I haven’t done something quite like this, but offhand it sounds to me like you’re talking about something that would be done using texture or material nodes, not compositing nodes. Exactly how are you trying to use the alpha-information, from what source, and where do you expect the alpha-information to wind up such that it can be used in the compositor? (Several possibilities come to mind, but it would be a pure-guess which one you might mean, so …) “Details, please.”

Yes sorry, material nodes.

On the most simplistic level, you have a white box. You create a new uvmap alpha checked and unwrap model. you simply want blue dots painted all over the model but keep the underlying colour or texture. Basically i use an add node to mix the 2 but the colour is not exact. You can adjust this further with other nodes to get it right, but it would be great if could simply use alpha.

I hope this makes sense.

Add will add the color values together, so that would of course change them. You want mix, with the alpha channel of the overlayed image used as the mix factor.

Thats what i do, but the alpha channel is just black?

Just worked it out, simply invert the alpha channel in the image menu, then start drawing, now you have photoshop type alpha map!

Thanks for all the suggestions!!