What format does opengl uses to compress textures? I'm making a map for www.planetquake.com/qbism but I got a question. It supports both JPG and TGA textures, what is the difference between them? For regular textures with no materials, is there much loss by using 8bits TGA? How can I know how much ram textures are needing?

OpenGL uses whatever the card supports, in the case of most computer thats going to be the DXT compression the same as DX uses.

JPG are lossy compressed, TGA arent and have an alpha channel.
Try the 8bit TGAs and see, however chances are you'll want to keep as much colour infomation as you can, a 32bit DXT compressed texture is going to look better than a 16bit texture and probably take less room.

You can estimate at how much space you'll require by adding the size of the textures together, however if you upload them and ask for them to be compressed by the card this wont be very accuate, so its often best not to worry about it.

There an article out there comparing 3dfx proprietary texture compression with DXT and it says that 8bits textures should look as good as 24bits 99% of the time. I think it's true, for concrete, grass, glass, bricks, blocks and many other textures with no alpha channel, both 8bits and 24bits versions looks nearly, if not exactly, the same for me.

Then I got a problem, rhe engine still has no support for dxt.