atmosphere

In the atmosphere editor of Vue you can choose among five different lighting models. They differ in how the effects of indirect lighting is calculated. I will compare the effects of these models on a sample scene. But first here is what each mode does:

• Standard Model
  A uniform ambient light of pre-defined color is adde to the scene. You can choose how much will come from the sky and how much will come from all directions. Horizontal surfaces will be brightened more by light from the sky
• Global Ambience
  This takes into account the color variations of the sky. A surface facing a blue part of the sky will receive a little blue light, surfaces facing clouds will receive white or grey light.
• Ambient Occlusion
  Every point in the sky acts like a tiny light source. An point on an object receives a bit of shadow if another object is occluding the path to the sky. Only objects within a pre-defined range will cast these shadows. If the path is free, the point receives light from the sky.
• Global Illumination
  This is basically the same as ambient occlusion, but there is no range limit. So all objects influence each other.
• Global Radiosity
  Instead of calculating shadows as in the AO and GI models, the radiosity models asumes that any obect that recieves light will reemit a bit of it according to its physical properties. This light affects the surrounding objects. This is done recursively. Vue includes an option to “optimize for outdoor rendering”. This effectively cuts down the number of indirect contributions because usually in outdoor scenes this effect is less relevant.

Comparision of the Lighting Models

To compare the different lighting models I used a simple nature scene with a basic grassland terrain and an ecosytem of red maple trees. I used two different atmospheres for the test: “Default” for a clear sky and “Glasgow” for bad weather. No other settings were changed. The images are 640×480 renders on “Final” setting with Vue 8 Complete.

Default Atmosphere – Clear Sky

The standard model is easily recognised by its uniformly bright coloration. In GA the shadows within the leaves of the background trees are very dark. They lighten up with AO and even more with GI which is unusal according to the manual. GR is almost as bright as the standard model, but the coloring of the leaves is much better differentiated. Optimized GR is a bit darker than standard GR

“Glasgow” Atmosphere – Bad Weather

The results here are slightly different than with the default atmosphere. Global Illumination is now noticeably darker than Ambient Occlusion and outdoor optimization for radiosity results in a significantly brighter picture.

Conclusion

The standard model should only be used if render times would get too long otherwise. All other models give significantly better results. Where there is no sunlight, Global Illumination will darken the picture as compared to Ambient Occlusion, with much sunlight it seems to brighten the shadows more than AO. Global Ambience is a good and reasonably fast alternative for scenes without much direct light. Global Radiosity works best in all atmospheres, but I would recommend it only for detailled scenes where objects are close to each other. I don’t think that vast open landscapes will profit much from radiosity. Regarding the difference beween the two radiosity variants, I think it is up to personal taste which version looks nicer. I couldn’t honesttly say that I would prefer one over the other.

For a comparison of lighting models for indoor scenes look here.

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