I spent much time to create and place acurately my background skies on half transparent planes inside Blender, to render them in Octane like any other object, to avoid any compositing. Background can be added as HDRI or LDRI or as object inside the 3D application and rendered within Octane. Any other method requiring to a third software to add the background, even if this background has been done with Octane, IS compositing.
Compositing is the easy way because it is easy to move or change the background afterward, but it is not allowed in the rules.
@Yoyoz : My airplane IS NOT as Stampe ! It is very similar in the shape, but if you look at the signature on the image with the airplane alone, you will see that it is a De Havilland DH-82 "Tiger Moth
". It is a british airplane constructed and used for training the pilots of the Royal Air Force before and during the second world war !
Some of them had anti-spin boards on the tail, but most have been removed on the airplanes that are still in used, because they were heavy and not very efficient. The original model had no tail wheel, but as the tail is heavy, most owners added one to their machine.
Of course, I modelled it myself ! I always do everything from scratch, and all my textures are made by hand or from photos taken by myself. I have purchased the Arroway texture pack, but I keep it only in case I would have to do a paid work for archviz project. I hate using third party materials, and I couldn't show on my website something including even a small part made not by me ! That's in my nature !
I have spent so much time studying this airplane that I could almost buid a real one !
French Blender user - CPU : intel Quad QX9650 at 3GHz - 8GB of RAM - Windows 7 Pro 64 bits. Display GPU : GeForce GTX 480 (2 Samsung 2443BW-1920x1600 monitors). External GPUs : two EVGA GTX 580 3GB in a Cubix GPU-Xpander Pro 2. NVidia Driver : 368.22.