Since it had been asked several times recently how to achieve semi transparent objects like smoke or fire billboards (or spinning helicopter rotors) I decided to post a simple example file. Actually it's Ulco's idea, so credits go to him!!
You might notice that I used an inverted mask and checked "invert mask" in the surface layer. For some reason it looks better (denser) than using a regular mask (white visible-black invisible) and "invert mask" unchecked.
There are two planes. One for the smoke itself with the appropriate transparency shaders (set to not cast shadows), and another one set to invisible with the same alpha mask in the opacity slot of a simple default shader. TG can create at least semi transparent shadows, so you can play with the opacity to make the shadow even more transparent.
The ray detail multiplier is set to 1 to avoid pixellation behind the plane object. Takes a bit longer to render, but it's necessary.
