I have seen that technique. It only works when the source light is visible.

Here is a link what volumetric lighting is. http://en.wikipedia.org/wiki/Volumetric_lighting
Essentially it is known as light beams,god rays and some other stuff.

I get how to do it with a mesh...

I have heard of ways to do this with shadow maps. I remember also reading something not to long ago where a shadow map was used on a tessellated quad to make a mesh that contained only light area....

I was hoping there would be more solutions.
reading and writing pixels sounds pain-full and not efficient.
I can not think of any glsl solution that could write to a texture in this situation.
...

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...