Hi,
I'm trying to create a skybox for my level which features planets and nebulas, kind of like in this image: http://4walled.org/src/ea509f2d98c6e9038603a06683b46234.png
I managed to get it looking good using 2048x2048 textures, but I am worried about the performance hit I will encounter from using such high resolution images.
Is there a better way of doing what I am trying to achieve or is high res textures the only way to go?
Unless you are targeting low end graphic cards it should be fine, though will eat up a fair amount of memory (16mb*6 = 96mb + mipmapping) unless they are compressed. So in this case i'd be more wary of memory useage/requirements than performance. Can you not drop down to 1024x1024? That could reduce memory by 75% (4mb*6 = 24mb+mipmaps)
There are other solutions such as using a different mapping such as spherical (on a sphere obviously), which from memory can potentially half the amount of memory you use on a cubemap with little loss in quality. HDR Shop can be useful for converting different mapping schemes if its still available.
Alternatively you could take the approach of mapping a bounding sphere or skybox. That is you create a mesh that allows you to place elements from a texture atlas or simply assign vertex colours or both. This is what HomeWorld did many years ago and there used to be a good webpage about it, but google is coming up empty.
You might also want to check out the inspirational Journal of Ysaneya for the Infinity game/engine he did some great stuff with nebulas and skyboxes. At one point these were just 1024x1024
Edit:
Found a page about Homeworld backgrounds here
Another solution could be to write a custom shader that draws procedurally pixel-perfect stars on a large sphere, a star dome basically.
You then could either use neatly placed billboards for features like planets and nebulas in space that are textured with much smaller but still large-ish textures. Or, probably even better, you can customize the star-shader in a way that it draws given textures to certain positions on the sphere. That would require a little math and shader-knowledge, but will probably deliver the best looking solution even on high screen resolutions.
Make sure you turn of “generate mip maps” under advanced texture import settings for each image… otherwise it looks totally crappy.