I have tried to avoid using any kind of Global illumination approach, as I wanted more control over the image and I also did not want to wait ages for renders. Therefore I have used key-fill light mixture as the main light rig and I have also used a dome-light rig to generate ambient occlusion map to add more reality. So first of all I have created the main light rig consist of the main Sun light and a couple of fill lights to soften-brighten up the shaded areas. I have created a dome light rig from 64 lights where 48 was acting as skylight, and the rest (16 lights) were the ground lights intended to simulate the bouncing lights from the ground plane. Using the ambient occlusion render as a mask in composite, I had more control over the shaded parts of the image. I could darken/color correct specific parts as where wall are connected to each other and therefore less light to bounce into those areas. I have rendered an ID pass as well to be able to separate the objects during compositing for the final tweaking.

and finally a making of image with the flat shaded render, the final image and the colored occlusion map.

click image for large version	click image for large version	click image for large version

and the final render:

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