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December 23rd, 2008

Since most of the gameplay in Overgrowth takes place outdoors, it is very important to have a sky that looks good and renders fast. The fastest possible way to render a sky is to just draw an image to the screen, and the best source of realistic sky images is reality, so all of the skies in Overgrowth start out as simple panoramic photographs like this one:

sky panorama photo

Once it is loaded in the engine, it is converted into a skybox, then blurred slightly for specular reflections, and finally blurred 90 degrees for diffuse lighting and fog. To make the sky and the terrain look like they belong together, we use the blurred sky cubemap as a lookup texture for ambient lighting and for distance haze.

fog terrain

fog terrain composite

This sky rendering technique is very powerful because just by changing the sky texture, we automatically change all of the ambient lighting and distance haze, so the whole scene still fits together. Using traditional techniques we would have to painstakingly tune each of these parameters, and still end up with inferior results! Here is an example of replacing the sky texture with an entirely different one, changing the whole feel of the scene:

alternate sky