Last updated (14-Apr-2005)
Let's have a look:
I try to model a scene which comes originally from the Radiance book:
http://radsite.lbl.gov/radiance/book CHAPTER 3 - Scene 2 Tutorial
If you want to see other images I have done so far:
http://www.maxwellrender.com/forum
http://www.wahn.demon.co.uk/Maxwell/Images
Or compare it to the original scene and renderings:
http://radsite.lbl.gov/radiance/book/ch3/img
Basically there is a kind of a "chimney" in the main area of the gallery room and a triangular shaped obstacle which makes light coming from the sun scatter around a couple of times before it hits the floor, walls, or the artwork. Some artificial lights are illuminating the gallery because there are only windows (and a door) in the lobby, no other openings to directly illuminate the main room. It's kind of hard to see how much difference the "Physical Sky" simulation make in the main gallery (the room to the left) but we can do a falsecolor difference to see more:
The white areas are obviously from the direct illumination caused by sun light coming through the windows. In the gallery sign the rectangular area over the letters "ALLER" are illuminated by the reflections of the door opening behind the camera. The red areas are a bit less intense and the reason why the direct influence of the sun on the sofa is not as powerful as it is on the walls or the tabletop is the material used and the yellow color of the sofa which doesn't reflect the bluish frequencies as much as e.g. the wall. In areas where the artificial light sources are close to the wall the difference isn't too obvious because the main contribution is the artificial light source but there is still contribution of the daylight. Only the black parts of the diffuse only material of the picture on the wall of the main gallery is enirely "black" in the falsecolor difference image, which means that there is no difference at all. But even the non-black parts of the same picture are influenced by the diffuse-only contribution of the daylight. Most people would think that if you want to fake the lighting with traditional (non-GI) methods (using a lot of fill lights) there wouldn't be too much difference in the main gallery. But they forget the reflections visible on the legs of the chair and the "waves" artwork with golden material ...