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Multiple scattering in clouds

We’re developing a new lighting model for clouds that simulates multiple scattering without using the GI cache. I rendered this turntable video to test how it responds to light with different camera and lighting angles.

In the real world, large, dense clouds of water droplets scatter light many times, absorbing almost none of the light as it is scattered. This gives rise to phenomena such as bright cloud centres that contrast with dark edges (especially when the sun is behind the camera) and interreflections between masses of cloud at scales both large and small. Light reaches deep into the shadows and beautifully illuminates the clouds in ways that a single-scattering simulation does not.

Thin clouds are easier to render because they can be approximated with only one scatter (because most light exits the cloud after the first scatter). But if we try to render optically dense clouds with only single scattering then not enough light reaches deep into the shadows and it’s difficult to consistently achieve all of the subtle effects that make clouds look like clouds. Most renderers try to approximate these effects in ways that often fall short of producing photo-realistic results. Terragen Classic had a simple ‘darkening’ parameter to artificially control the rate of self shadowing, which became ‘light propagation’ in later versions. Terragen 2 added cache-based GI, which simulates multiple scattering but often lacks detail. To render many different kinds of clouds photo-realistically, we need a lighting model that works for large and dense clouds as well as for thin ones, whether wispy or smooth, and everything in between.

The new model in Terragen 4 produces the bright centres and dark edges that we’re looking for, and scatters light deep into the cloud with a smooth falloff and a nice sense of directionality and self-shadowing in the scattered light. The clouds are also lit by the surrounding atmosphere, which in this video shows up as blue in the shadows, and that environment light is also multiply-scattered through the cloud. Terragen 3 could approximate this too with GI, but the new model shows real detail and self-shadowing of the environment lighting in the areas that the sunlight doesn’t reach. The “silver lining” effect which is strongest when the clouds are back-lit is also much better than before.

We still have a few kinks to work out, but in a future post we’ll show the new clouds working in the ray-traced realtime preview!


Matt Fairclough

All stories by: Matt Fairclough

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