Celestia Forums

Is it possible to implement a cloud shadow/bump map and a cloud colouring feature?

I mean like this:

Very nice picture - I will try to reproduce this type of view with my all-sky model as a demonstration. I've been working in the context of doing volume rendering of 3-D hydrometeor and other fields, straddling the fence with radiative transfer.

That would be really fantastic! Celestia would need some kind of cloud bump map to achieve this effect.

Here is a start. Some cloud shadows are visible on the right side, though not yet the crepuscular rays (where the shadows reach the terminator).



It's worth noting that at the terminator (and in the zone of the reddish clouds) the sun is already a bit below the horizon at ground level.



I'm also unsure if the shadows being cast in the air are showing up as well as the shadows being cast onto the ground. This works when I render from ground level, though I'll have to see if this is working from space. Things also look different since my example is over land (where I may be overdoing the sunset land illumination) and it appears the photo is over the ocean.

Yes, very nice work! These are real shadows! Is it possible to implement these functions in Celestia?

Thanks john71. Still a number of things to tweak - this is such a great example that you found to work with. It's worth considering both the cloud shadows in the air and the shadows on the ground as two separate functions. Also, dense clouds act more like solid objects, and thin clouds scatter light more diffusely and volume rendering helps with this. So there are lots of variables and we can start by describing more how cloud illumination, clear air illumination, and ground illumination are all calculated in Celestia. Then these elements can each be modified if we're in a cloud shadow.

Perhaps a bump map (as you suggest) can be a simple first step as well, treating clouds like white mountains. This works best for dense clouds having low cloud bases. Does Celestia presently handle mountain shadows (on the ground and in the air)?

Bump maps and shadows work like this:

Thanks for the demonstration. In the middle panel, would the mountain shadows get longer over the landscape when the sun gets low? If so one could experiment with a bump map and modified terrain albedo (texture) made from cloud data. Even some dummy clouds at first might be a good experiment.

Yes, the shadows get longer, but the problem is that a bump map only works with surface textures. The cloud texture is above the surface texture. So we need a cloud bump map on the level of the clouds and we need a cloud shadow texture on the surface texture.

Added after 8 minutes 34 seconds:
Cloud shadows should also work but they seem to be not present at HD multi-layered ctx cloud textures.

Sounds like a good plan for the cloud on ground shadows in Celestia. I can also report some improvements in adding the cloud shadows in the air, and setting the ground illumination to be more accurate. Here we see additional cloud shadows up top in the neighborhood of the terminator, where the shadows are in the air rather than on the ground.



Next I'm starting to clean up various map projection issues. As a result this version shows some long cloud shadows more like what we see in the space station image.

Nice work, they seem quite real despite of the low resolution! Celestia has another issue, which can be a more serious problem. When using 32k multi-layered cloud textures you cannot move the textures, so they must be motionless. One-layered cloud textures on the other hand can't really have higher resolutions than 8k...

Thanks - I'll have to try the hi-res cloud texture in Celestia now that this works on my Mac (running Mavericks). Below is a slightly higher resolution version and is equivalent to a 12K global view (3km on the Earth's surface). I can also work at higher resolution with limited domains if I can find one handy with clouds of sufficient vertical relief.



Maybe there would be more flexibility with techniques for working with clouds if Celestia were to merge with Proland - as suggested in another recent thread. Even so Proland would need some improvements to do the 3-D volume rendering or other 3-D cloud rendering functions. Can the multi-layered textures in Celestia each have a shadow? The thing about a multi-layer is that I assume each layer is thin though if they have bump maps it would allow for more structure. When we look at a cloud from a sideways perspective the vertical thickness of the clouds becomes important.

Another item for me is the darkness of some of the clouds that should be accompanied by the correct amount of scattered light from the air lit by twilight on top of them. The clouds should also reflect more of this light. We can see how this looks now in the full view below.