The standard haze and fog effects in Bryce are very basic render procedures and soon become unrealistic when used in conjunction with lighting. Bryce volumetric materials (and 'volumetric world') are much more complex and can interact with light to a significant degree. Real fog and haze is a suspension of particles, usually water and dust. Light illuminates those particles, and the result can range from a 'whiteout' like in thick fog, to highly visible flashlight beams whenever Mulder and Scully start snooping around a deserted laboratory. ;-) Bryce haze and fog is actually pretty hopeless, and totally fails to acknowledge light, either from the sun or from any extra lights added to the scene. It is basically a band of dead colour that can vary in height and thickness. In this simple set-up I just did, the only illumination is a white radial light placed close to the camera. There is no sunlight at all (disabled). The sky colour is set to black, and the floor and columns are a basic green with no ambient level (diffuse only). This first pic shows a mid grey haze set to 100: Note two interesting issues. First, although there is no light except the weak radial in the foreground (i.e no sunlight), the haze looks the same as if it was applied to a scene with bright sunlight on a beach! In fact, if you put the camera in a completely sealed black cube primitive with no lights at all in the scene, that haze will still show up grey! Second, although the radial light is illuminating the green floor and columns, the haze is completely unaffected. It is not illuminated, it doesn't block the light in any way, and no shadows are cast into it by objects. As you can see, this is not a realistic representation. This next setup is exactly the same, except the haze is set to zero and 'volumetric world' has been enabled. The setting is 100 percent density, 50 percent quality: As you can see, this is totally different, and more in keeping with what we'd expect to see in real life foggy and/or hazey conditions. The dense volumetric 'haze' is being illuminated by the radial light (and only that), and objects are casting shadows into the 'haze-particles'. If I had used a volume slab instead of 'volumetric world', I could have specified all kinds of settings (colour, particle size, varaible density, etc) to the atmosphere around those objects. This volume stuff creates a much more complicated scene for Bryce to figure out. It is simulating the presence of suspended particles, and accounting for them in its ray-trace calculations. The higher the quality setting, the more 'particles' there are for the render engine to consider. More calculations equals more render time. In this case, the scene with basic haze took a whole six seconds; whereas the scene with volumetric world took seven minutes, fourty eight seconds. A big difference. When used carefully, both methods can give good results. But for the ultimate in realism, especially in foreground scenes and with extra lights, 'volumetric world' or volumetric slabs offer the best solution. The price you pay is in enduring weekend renders!