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I know why mac- I spent the last 2 months experimenting trying to work out a system. But, alas, it's image specific. It's the firefly engine. In you image it's the walls reflecting light. Your room consist of more then 2 models. The doors, frames, walls ect. Firefly and P4pro with only one single model in the scene create the EXACT same shadow pattern. But if you add any other model you get instead of sharp edges on the shadows, softer more realistic shadows. It's because of reflected lights. That is why it's so much brighter too. Heres a trick on fakeing a HDRI simular effect. Make a curved prop. Load it into P5. Set the node to white black black white like normal. Then set it to reflect. Move it back behind your camera (not between any light source). You can treat it as a brand new 'type' of light source. Soft defused light. It's really make a tremendous differance on the shadows (do a side by side to see). It is NOT a ray traceing event, it's the firefly render engine itself that is doing this. This is just another trick in our arsenol. Shape, color, distance and enviroment intensity all have effects on the final output. The brighter the lights, the more it reflects. I myself tend to use very bright lights so it's not a big deal for me. Geo