Hi all,

I have a question. I'm working on a space engine at the moment, and I have successfully created geodesic grid sphere's for planets and have all the appropriate physics for space ships (using a physics engine). So far I have a very rough draft/clone of Kerbal Space Program (but years behind their progress, not intending to compete with their product. But rather as a hobbyist just playing around with graphics API/physics APIs and trying to bind them together.)

Now that I have the basics down, I've realized that my planet graphics are really bad and I want to improve it. The first thing I want to do is create an atmosphere.

I have a technique down and I just want to bump it off you guys, as I have no real idea how to implement it. I am guessing I'd do it in HLSL.

So what I'm thinking is to create another geodesic grid sphere around the planet's surface with a larger radius. But rather than just make it a blue color, which of course won't give a good gradient or depth to the atmosphere, to do something else with it:

What I'm thinking of doing is make this atmosphere surface 2 sided and also transparent. Then calculate the amount of distance between the start of the atmosphere and either the planet itself or the backside of this new atmosphere sphere. The more distance or pixels between the start and the backside, the more atmosphere is present and color the pixel according (blue for earth).

Now the question is this: How do I implement that in a shader? I assume I may have to render like a shadow map, and write the depth of the start of the atmosphere and compare that to the object behind it, much like you do with shadow mapping. Would this be the best approach? Before I waste days trying to implement this, can you guys maybe point me in a better direction if you may have one?

Thanks.

Along with the above, try looking at Celestia. It's open source so you can look at how they do it.

Thanks for the comments guys. There was a few approaches here I didn't think of. Phil's approach would probably work great for my scenario, but I'm thinking it might not work well if I was landing on the planet itself? I want to try to create a hybrid engine where I can fly around in space and then also land on the planet. So sort of like a flight sim + planet orbiting. But I'm sure I could get it to work if I played around with it.

I like Sean O'Neil's and Eric Bruneston's methods (which look great, but I have a feeling I could make an entire career out of rendering an atmosphere if I wanted to after seeing those sites)... I'm going to see if I can absorb what they are doing. Some of their diagrams looked similar to what I wanted to do (I'd probably simplify it though).

I did a little tutorial on planets a while back that included atmospheres

http://stainlessbeer.weebly.com/planets-1-mercators.html

The HLSL for the atmosphere shader is here

http://stainlessbeer.weebly.com/planets-9-atmospheric-scattering.html

Not how I would do it these days, but maybe it will give you some ideas.