having decided to add a kind of cart to the right hand side of the scene, i got to the point i figured was coming, and mental ray 'out of memory'. so... started having a little read and look for alpha channel info. I see there is a checkbox in the render settings, and as you say genny, it is checked by default. I saved out my test renders as Tiffs. Its a bit strange because im thinking its hard to isolate elements if theyre all casting shadows. And also in the first few quick renders I did, all thats really happening is the background (if there is none) gets isolated from all the actual 'positive' objects. im guessing theres a way to give individual geometry its own alpha channel? But then Im thinking...shadows. I first of all isolated the cart in my scene and rendered it, then added it to a previous wip, but obviously it therefore had no shadow. So i rendered it with all the lights and anything that would be affecting its lighting or vice versa, but then that includes most of the stuff in the seen so... all i left out was the dragon. which was enough to get a render, but when compositing them I wasnt really sure how to go about it. I basically cropped half of one picture and then stuck it on top of the other, which left a blatant vertical line down the middle of the picture where theres a subtle shift in tones. Im sure i went about the process in a completely crude amateur way. Will have to look into this more.... and i see there is a way of rendering shadows in their own alpha channel involving switching off primary visibility and using the use background shader... again... much to explore.