Something that's been bugging me for the longest time... how does generating HDR lighting take the same amount of compile time as LDR? I used to assume that VRAD ran two lighting passes, one for the minimum exposure level and one for the maximum, but then I realized that it would need to run a third for people who have HDR off. Which it clearly doesn't do. So then I thought, maybe it takes the LDR lighting data and converts the HDR into some sort of modifier, where the channels actually represent how much brighter and darker the range gets relative to that. After all, if you've ever seen the HDR skybox textures, they clearly don't look anything like what you actually see on the screen. It's more akin to a bump map in how it uses its RGB values. But then I realized that you can compile the game with only HDR lighting. If the HDR lightmaps worked that way, there'd be no base levels to refer to. So I'm completely stumped. Has anyone out there ever dug into what lightmaps actually look like and/or some in-depth documentation (perhaps one of Valve's SIGGRAPH papers) of how VRAD actually works?