You might be able to encase your 3D objects in a slightly larger object, which is rendered counter-clockwise instead of clockwise. That way, all the polygon faces to be rendered face in, not out, and when you rotate around the object, you only see the overlap from the slightly larger image to the slightly smaller image. Then it's just a matter of getting the color, lighting, and shading correct.

Totally untested, unfounded solution, so take with a grain of salt if it doesn't work. If it does, send money. LOL

Hi,

You creat a texture with a filled circle and then use say a gaussian blur or other blur in a paint package to get the falloff as you want it.

The create a quad that always faces the camera. If you ensure that it drawn first or simply position its axis at the planet objects centre and then scale the quad until the texture you made gets drawn outside the radius of the planet object.

The texture doesn't need to be very hi-rez as you can get the bilinear filtering to smooth it out.

That should work.

Hope that helps.

take a look into atmospheric scattering

http://www.gamedev.net/reference/articles/article2093.asp
http://sponeil.org/

http://www.ati.com/developer/dx9/ATI-LightScattering.pdf#search=%22atmospheric%20scattering%20site%3Aati.com%22

It all depends on how much realism you need and how much free time you have on your hands. If the answer to both is "lots" then go look up atmospheric scattering. This will quite likely involve shaders too for performance reasons (of course you can do anything on the CPU but it may not be wise).

If you just need something that looks reasonably good (or unrealistic but flashy in the arcade-ish sense), the cone method is adequate. I've used a cone with its point at the center of the planet, and the opening angle set so that the cone intersects the sphere at the horizon from the camera's point of view. Only the part above the surface is rendered, of course (producing a quad or triangle strip like a skirt along the horizon, and all of these polygons face the camera).

The reason I wouldn't go with the big quad is that it's not trivial to get it to look right with perspective, in particular when the camera is close to the planet.

why a cone ???
a circle will suffice, just make it rotate in de same angles as the camera does (a billboarding effect )