paint on 3d objects, uv size question

Unity Engine
1

I have a tool called TexturePaint up on the store. I just submitted an update that adds support for painting on complex 3d objects. It works very well, but I never found a truly dynamic solution for one specific item. Maybe you guys have an answer I overlooked.

Here is the issue:
I need to translate the world size of an object into UV texture space of another object… So for instance, my TexturePaint tool has multiple features to keep world scale of objects relevant when painting and they all work great… but I cant find a way to accommodate for UV layout. For instance, if the UVs use the entire UV space, no problem. but if the UVs use just a smaller section of the entire space they are effectively “scaled” up or down.

I could get the UV verts and determine how much space they use, but that doesn’t really tell me much they “should” use. I guess this is really just getting into arbitrary information areas and there may not be a great solution. I have one idea about using a new UV set and doing texture re-projection, but that would just be really expensive.

Hopefully someone out there has a great way to achieve this.
Thanks

Here is a screen shot to give you some context:

2

I realize this thread is a couple years old, but I’m in the same boat except I’m implementing a decal system of which one of the decal types is directly painting on the target texture. Did you ever resolve this issue?

Also, a related issue you may also have encountered/overcome, have you been able to isolate UV Islands so you don’t accidentally paint on things that are beside the target on the texture, but UV’d to a completely different spot on the model?