I'm loving the possibilities with the new Karma atmosphere, but wondering the best approaches to having a sun in the scene. The old Karma sky included settings for the appearance of the sun, but it seems in the new atmosphere we are supposed to provide the sun ourselves, but I'm not sure what the intended approach would be.

If I add a distant light, it appears the light the atmosphere correctly, but there is no actual sun in the sky. I have tried creating a spherical light and messing around with making it very large and very distant and very intense, and it sort of works, but feels like a lot of guesswork to get something which feels correct. It also doesn't seem to interact with the atmosphere to create the look of a sunset on the sun (making it redder as it sets).

I am interested in getting something which is physically correct both on the ground level / within the atmosphere, as well as from the perspective of being in space / low Earth orbit.

Does anyone have any suggestions for how to set this up? There is not much mention in the Karma Sky Atmosphere documentation about the sun, which seems like a crucial oversight.

Perhaps you're using the wrong node? The Karma Physical Sky node creates the sun and sky, and can use either a domelight skymodel or an atmospheric sky model, either of which have the sun visible by default.

No, I’m talking about the new atmosphere model: https://www.sidefx.com/docs/houdini/nodes/lop/karmaskyatmosphere.html [www.sidefx.com]

Which is a much better atmospheric/sky model than the older Karma Physical Sky. And while the images on the documentation show it with a sun in the sky, there’s actually no sun by default.

I’m fairly sure you’re not supposed to just combine the two approaches. The nice thing about the new atmosphere is that it works from space as well. Much like the atmosphere in Unreal Engine it’s actually simulating a planetary atmosphere.

I’m just asking about the best approach to adding a sun to this very nicely physical atmosphere model.

localstarlight
No, I’m talking about the new atmosphere model: https://www.sidefx.com/docs/houdini/nodes/lop/karmaskyatmosphere.html [www.sidefx.com]

Which is a much better atmospheric/sky model than the older Karma Physical Sky. And while the images on the documentation show it with a sun in the sky, there’s actually no sun by default.

I’m fairly sure you’re not supposed to just combine the two approaches. The nice thing about the new atmosphere is that it works from space as well. Much like the atmosphere in Unreal Engine it’s actually simulating a planetary atmosphere.

I’m just asking about the best approach to adding a sun to this very nicely physical atmosphere model.

That is built into the physical sky node now; the sky mode just needs to be changed from dome light to atmospheric and it will use the new volumetric approach.

Ah, OK, thanks for letting me know about that, I didn't realise that was an option.

That said, it's still tricky to get the sun looking correct when working with the default settings.

Here is an example of the kind of 'orbital sunrise' I am looking for:



Here is the default sun geometry from the Karma Physical Sky as seen from orbit:



I can push and pull the atmosphere parameters around to try and get something working for this particular instance, but it breaks the look of everything else, and I assume the default values for the atmosphere are based on real values for an Earth-like atmosphere.

Previously I have been doing this in Redshift, where I use an actual VDB with values ramped according to real-world values, and it works. It's slow and cubmersome because of the size of the volume, but it works. I'm hoping to replace Redshift with Karma, but we do a lot of space stuff, so it would be great if this worked as expected.

You can get pretty far with just the default settings, but please keep in mind that the default sun intensity of 1 is *very* dim so you need to increase it considerably. Also don't forget to increase camera clip range to ensure that the atmosphere isn't cut off. There are several factors things to keep in mind:
A) When it comes to the look of atmospherics, viewer's position matters just as much as the atmospheric properties. If the photo reference was taken from the ground (the photo you posted almost looks like it), then trying to emulate its look from high in the orbit may be challenging.
B) With photos, a lot of the glare you see from the sun isn't just scattering, but also lens flare, which karma does not simulate (nor does it any tone adjustment of the camera, unless viewing through some sort of output transform in OCIO).
C) In reality atmosphere would act as a lens and bend light, but in karma it doesn't - which means you won't get distorted shape of the sun near horizon (and possibly dimmer twilight). To work around it I would suggest disabling sunlight visibility and use a geometry (or add it in comp).
D) Also important is the presense (or lackthereof) of cloud coverage which not only gives texture and additional scattering but casts shadows over atmosphere which can alter the look significantly.