http://blog.wolfire.com/2009/04/environment-shadows-step-4/
I just started working on baking ambient occlusion into the lightmaps. This is currently performed by accumulating the shadows from 64 random light sources in the sky, and using the resulting ambient occlusion map to modulate the indirect lighting contribution.
Here is what the Foothold map looks like with ambient occlusion enabled (above) and without (below):
It’s a fairly subtle effect, but I think it makes it easier to see the spatial relationships in complex scenes like this. Here is what the ambient occlusion map looks like by itself:
Here is a perspective that illustrates why indirect shadows are important. Even without direct sunlight, tunnels and interior areas should be a lot darker than outdoor areas.
There are a lot of other lighting features I could work on, such as radiosity and point light sources, but I think this is all the complexity we need in Overgrowth. It will mostly have outdoor scenes lit by the sun, so those extra features may not be worth the development time.