..sweet Jesus, my brain exploded. Hold on a sec *picks up brain*
This kind of bill-boarding is not what I require, and I've also achieved it through a waaaay simpler way (same rotation matrix of the camera applied on the mesh).
In other words, if I apply the matrix to a rectangular mesh, it rotates it toward the camera with the original position fixedly projected on a plane. That's not supposed to happen in my case, as the rectangle has to be a connection between two 3D points ;)
Not to be rude.. but I just don't get a single word.. as I said, I HORRIBLY fail at math in this point..
Could you make it simpler? If possible, can you use DirectX APIs to do that?