I have recently had a problem where my scene crashes at the end of a long render.
So I decided that I should make it easier on the memory by rendering in cropped sections.
Below is the result. I don't understand why the two halves are of different color when all I did was move the crop down.
Edit: now I see that this has been answered in tile/bucket differences post, don't know how to delete this...
I have a idea now about the GI. How bought the ability to save out a GI pass and load this when rendering the sections.
Firstly, v nice render.
Second, there still is a little confusion over what helps, but I've found that, in the Advanced tab of the renderer setting the Ray detail region to "in camera" helps. EDIT - according to Matt, not really - see this answer http://forums.planetside.co.uk/index.php?topic=7019.msg74707#msg74707
Increasing GI blur may also help, but apparently at the expense of detail; and increasing GI samples also helps.
One other thing that sometimes helps is exporting different crops as EXR, and then exposing them slightly differently, e.g. 0 and 0.1.
Finally, a separate GI pass has been requested a number of times - I believe it's not straightforward.