but you save output image as single image, there is nothing special to obtain a pixel coordinate in image from any perspective and do whatever you like

lol, he wants to paint an object. You are suggested something like a decal system. How does a single quad wrap around a sphere? That is not painting and it is a broken decal system as well.

I still don't get why you can't use a normal artist pipeline. 3D artists do this all the time and then its all good to get loaded in at startup. If you really need it to be inside of your project then it is a bit more hard to explain. A basic overview that should work:

UV unwrap the object (you have to do this).
In game, you need run all your vertices through the modelviewprojection to get the vertices final 2D Screen pos.
Use those 2D texture coords with your "paint brush" image, and you have to draw those on top of the original texture coords.

That's as in depth as I can go, I haven't seen many articles about it but you should decide why you need this. There are plenty of other 3D paint tools. If the user/gamer is not going to be painting objects in the simulation/game, then use an existing tool.

:0

Well, I'm still at trouble with this feature I'd like to add to my application.

I have the unwrapped mesh and I know how to get reference to it, so that if I click a point on a model in the 3D viewport, I can get the corresponding point in UV space.
So in practice I am able to draw a pixel directly on the uv texture map (and apply it on the model so that it shows the painting).

However, I don't understand how I could actually do the same for all the pixels inside the cursor brush area (the attached image should explain better than my words).

How do I calculate the UV points for all the pixels inside the green cursor brush area? Using glUnproject would be and option but it would be slow as hell. Any idea?

Do you know pixel shaders well?

Not much. Would they be helpful in this situation?

It would be much easier with shaders yes.
The idea would be this:
1Render the brush to say a stencil buffer.
2While drawing the current model, if a pixel on the current triangle passes the "inside brush/stencil", then it knows it needs to be written back to the UV textures
3For given pixel, you already know the 2D UV coordinate in the image you are about to paint. Paint it based on the brush applied.

Given that you don't have shaders working then I dont know. As always my suggestion is: Open up maya/blender/zbrush. Paint, export, import into your game.

I came up with a genius, non-shader way:

To start, you need a texture that holds pixel coordinates. What this means, pixel 0,0 is not red or green or blue, but it is 0,0,0 (its position is 0,0. If your texture is say 512x512. the pixel in the middle (256,256) holds again not a color but 256,256,0. Ok in gl everything is scaled 0 to 1. So divide by 512, and the middle pixel = .5,.5,0.
With me yet?
How to use it:
When drawing your model to the screen, you will write to the framebuffer, the model with that texture applied. What this means is every pixel you see on the screen currently, you have their positions in the UV map. You can then call glReadBuffer, read the pixels back and if then apply that brush by getting its window position on the current image. And applying that over your array of pixels.

Will this be slow? No. You only need to readback the current framebuffer if the camera has moved. Well sending the new texture every frame might be slow if it is big, but for 512x512 should be no problem.

Get it?

If it wasn't obvious, you would then just clear the screen and draw the model with the painted texture applied so that the user never sees the other stuff you painted.

Hello Adam, that's certainly a very interesting approach, thanks. I'm going to try to do something with it and report the progress.