paint on 3D objects


I’m a newbie. And I have the following problem:

I have a object in 3D space. This object has a texture and the corresponding texture coordinates. Now I have a projector. That means I project a texture on this objekt. This texture is my paint brush. Also I vertex has a texture coordinate for the basic texture and for the paint brush texture. The image data of the paint brush texture should be copy to the basic image. Please note that the textures have different texture coords. I think the problem is very tricky.

Can everybody help me? Please ask when this problem is unclearly.

Cheers, Martin