Happy New Year!
I rendered this turntable video to test how Terragen 4's new light scattering model for clouds responds to different camera and lighting angles.https://youtu.be/87TkK1_avO0
There are some really cool effects I want to talk about here! 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. Terragen 4 can produce these results by simulating multiple scattering much more precisely than Terragen 3. The clouds are also lit by the surrounding atmosphere, which in this video shows up as blue in the shadows. Terragen 3 could approximate this too with GI, but the new model in Terragen 4 shows real detail and self-shadowing of the environment lighting in the areas that the sunlight doesn't reach, and these areas would appear more flat in Terragen 3. The "silver lining" effect which is strongest when the clouds are back-lit is also much better than before.
You might be wondering about the shape of the cloud itself. This is a localised cloud with a few layered fractals to control the shape/density. Terragen 3 can also produce shapes like this, but it would be very difficult to make them look realistic because of the lighting. Until now, lighting has limited the kinds of clouds we can render photo-realistically. One of the goals of Terragen 4 is to remove that barrier. When Terragen 4 ships, we'll include presets or projects to show how to render clouds like this. Another goal of Terragen 4 is to make it easier to use, not just more capable. I hope that it might be possible to create cumulus clouds like these with just a few sliders, not a network of nodes, but that idea is still on the drawing board. Even so, the new lighting will make it easier to get good looking results from simple cloud setups.
When I compared render times between the new and the old lighting model, they were almost exactly the same. That's not always going to be true, but it was in this case. You'll still be able to use the old clouds whenever you want.
The alpha testers know that there are a few kinks that I need to work out with the new clouds, but as I do, we'll be able to render lots more examples to show you what's possible with Terragen 4.