I've been thinking how to paint a 3d model texture in 3d.
I am thinking of actually how a modelling program would do it not a game. So it needs to modify the texture.

I have failed to find anything on this topic from my searches. That is probably more to do with having the right terminology.

My current thought is rendering to texture the uv coordinates so you have the uv locations in screen space. Though the texture could be more detailed than the screen space and would miss some pixels. Also I am not sure what would be the best way. using cpu to manually edit these values into the texture or using a shader.

I'm hoping there is simpler way that also hopefully works across seams.

Any help or ideas are much appreciated.