Skybox are made to improve performance. When you stay on a small region of a galaxy, there is no point on rendering the galaxy each frame, so the game save the galaxy wieved from your point in a cubemap texture. This is why you can saw lines appearing sometimes in the sky : they are the junctures between the skybox tiles.
Imposter: renders a single-sprite image of distant galaxies and nebulae to improve performance; usually has little to no impact on visual quality FB = Framebuffer: dramatically improves the performance of rendering galaxy, aurora, comet, and nebula sprites at the cost of image quality (only readily noticeable on galaxy and nebula sprites).
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