adding a background image for fulldome images in Blender

This turns out to be not so simple. My first try was using a texture for the World in Blender. Using either Global or AngMap Co-ordinates, and making sure the Influence -> Horizon is checked. Result is not too bad, except that the background image is a bit zoomed in.

This was the background image, and after using it, the result was like this:

Well, not too bad. Perhaps if I fiddled around with the background image, making it bigger, it might improve things? Anyway, the method of using the Alpha channel as given in the tutorial here doesn't work too well because of the fisheye camera rig. When rendered with the fisheye camera, the alpha channel includes the whole circle - since that is what is being reflected on the fisheye rig's mirror. So, the scene when rendered will not include the background image inside the circle at all - only in the corners, which is not useful at all.

One of the comments in the above tutorial mentioned simply using a paper sky in World settings, and then enable Horizon in Influence settings. That works well with normal cameras, but had a problem with the reflection from the fisheye rig - with a ring around the horizon of the fulldome - it looked like this:

Anyway, going through the Blendercookie/CGcookie tutorial was a good learning experience, especially since it took me to his previous tutorial, and I learnt the reason for the scale - dimension problem I noted earlier. Apparently if you change the dimensions of an object in Object mode, the scale goes out of whack. To correct it, he does Ctrl A, and chooses Scale. Then - et voila - the scale goes back to 1.0 and the dimensions are preserved.