OK, I'll tell you that I always use GI for outdoor scenes. GI spreads shadows in a far more realistic way than standard lighting, especially on plants. Try it on a simple scene with a few trees, you'll see. Choose any atmo, and check GI, you'll see your scene gets darker as Zak said, because it's like I said GI spreads shadows. You'll need to raise the skydome lighting gain ( at least 60%) so your scene gets brighter. The shadows will also take a nice bluish color if you use a sunny atmo, just like in the real world. Then I usually set the sun's shadow to 80% only, so the scene is less dark. (100% shadows is not real world accurate anyway, because the colour black is used in Vue for shadows, and it's not the case in outdoor lighting.) Tip: move the GI quality slider to -1, speeds up render times without affecting quality. GR is different, and is better used for indoor scenes. GR simulates how the indirect lighting coming from light sources bounces from the objects touched by the light rays. Objects hit by light will absorb some of the light, but not all of the light ( depending on the optical properties of your object's materials), so the remaining light bounces off the surface of your object, and reaches the surrounding objects. And so on. This is the most accurate method for photorealsim in CG, and I think Vues engine, although really slow is very good at GR.