Fix volumetric clouds

[codeonwort]

My volumetric clouds, implemented several years ago, but it never worked as I wanted in various aspects:

• Weather map's cloud coverage and cloud type
• Contribution of base noises and erosion noises to cloud shapes
• Density gradient for cloud coverage
• Cloud lighting
• Raymarching

After some rework:

Fixed things so far:

• Cloud coverage and cloud type works as intended.
• Base noise determines overall shapes of clouds and erosion noises only contribute to small carving.
• Density gradient works as intended.
• Cloud lighting does not result in random number.
• Adaptive raymarching.
• Banding artifacts (thanks to NVidia STBN).

TODO:

• Implement temporal reprojection. But it's a different situation than TAA. I can't derive exact world positions of prev/current cloud particles that first-hit camera rays.
  • Fast temporal reprojection without motion vectors looked promising at first, but it seems it uses too much memory. I'll need 25 textures of the scene size for 25 different offsets. Can't alias them because the merge kernel accesses all of them.
• Improve raymarching quality. Commented below.

[codeonwort]

Note about raymarching quality

Uniform raymarching (min=54, max=96)

Adaptive raymarching (min=54, max=96)

Adaptive raymarching, reference quality (min=200, max=400)

Raymarching strategy:

1. Coarse march until hit a cloud particle.
2. If hit, switch to fine march.
3. If fine march hits nothing 10 times, switch to coarse march.

But once a ray exits fine march, it never reaches to the second cloud. There is just not enough iterations left once fine march is done. This is largely due to erosion noises which carve the cloud shapes and cause long-stay fine march, but they are also essential for visual.

I don't wanna increase the iteration count but I'm out of idea for now.