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The making of Cosmosis: Remix
by merlucin | Chapter 2: The earth
Cosmosis: Remix begins with planet earth. The original Cosmosis demo was also beginning with the planet earth. The difference is that the remix uses a 3D earth instead of a graphic with alpha. in fact it uses four earths, composed, in order to get the final effect of a planet with lighs that is being partially illuminated by a moving sun. 

The original Cosmosis scene Our remix, based in the original scene
Our earth was modeled using Modo and Lightwave. We had lots of maps for the earth: diffuse, reflection, bump, normals, etc... but we only used the diffuse ones since the result was almost the same. Using all the maps was also impacting the graphics performance a lot.

At the left image you can see the difference between a raytracing rendering with bump mapping and our realtime rendering at a preliminar stage of the demo. 
By rendering spheres with different textures four times and then combining the output, we can reach an earth model with clouds move independently of the ground and lights that dissapear when the sun light incides the surface, It's also important the fact of rendering the clouds layer in an sphere slightly bigger than the rest of layers. This is because this way we can avoid aberrations in the earth's borders caused by the superposition of spheres rotating at different speeds.
A final retouch consists in manually adding a sun halo just in the moment the sun comes out behind the earth, Playing with the OpenGL blending modes and this lens flare, you can achieve great results, similar to those HDR renderings you can watch in lots of games.

The only requirement for this fake consists in knowing the camera position before the scene is rendered.

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