“define atmospheric scattering inside the planet volume” I do not remember what I said but I do not promise anything here, just discuss things and possibilities.
The SkyAtmosphere component simulates a terrestrial planet, not a gaz planet. There is a virtual ground. Just put your terrain or your planet mesh on top of it. Looking at Fortnite, UE5 tech demo, Hourence SkyAtmosphere demo, Ryan planets and other projects I saw here an there: it looks very usable to me. It can even be translated and rebased now. But yes it is true I do not know what you are doing exactly…
That being said, to render a gaz planet yourself, you should be able to:
- reduce the planet radius, increase the atmosphere radius
- tweak the atmosphere density to make it look like a gaz planet
- then put you level at the position that suites you. The virtual planet can now be moved around in 4.25 if you want your level to still be at the origin. Just put it where you want.
Oh and you can also use a sky dome and a shader to do anything you want in it with any custom colorization and whatnot.
Very good question about the sun illuminance! I think this is explained in tool tips but not in the documentation? (I will check and we will update the documentation if needed). The sun illuminance is specified as illuminance reaching the ground when the sun is at zenith. So if you put 1000Lux, the ground will always receive 1000Lux, even if you make the atmosphere thicker. It is done the same way as this, section 4.1. The outer space sun illuminance is simply deduced from the ground illuminance and atmosphere properties. This is more practical for artist to specify this way because usually this is where things happens and they do not know/want to guess the outer space luminance to setup the sun light relatively to other lights. Then when you move the sun around, everything is coherent and ratio between sun and sky are still respected because we recover the original outer space sun illuminance (and this is the input to the sky system). Example of sun/sky measurement have been done in this document, section 4.6. I did some measurement in unreal and it matches (you have to be very careful and disable anything that can influence measurements such as AO). More faithful result are obtained by using r.SkyAtmosphere.MultiScatteringLUT.HighQuality 1.