It will be raw bytes, format depending on what mode you're in. Usually 32bpp. Expect RGBA, but it might be ABGR or anything else. But you'll be able to figure it out quickly enough. Older hardware or weird modes might be more or less (16/24 bpp modes from the past, or maybe 10/10/10/2 for HDR out? I've never looked into that), but as it will be uncompressed it won't be much work to figure out.