I've never really thought about whether the game looks the same using its Windows vs Linux renderer. I installed HL2 to check out the anniversary update and decided to try the Windows renderer on Proton. Would you look at this difference... Linux is the top one, Windows is the bottom:
Is this is a thing? Have you tried other games and seen the Windows version looking better?
Oh yeah. Trying to get Black Mesa - which is a HL2 mod that became a game in its own right - to look right on a modern Linux PC has been less than fun. Textures, lighting and shadows are messed up in many places, basically breaking the flashlight most of the time. The game is 99% there, but that other 1% is impossible to ignore. The game plays fine... when you can see.
Kind of glad I got the game on sale.
But anyway, yes I'm kind of surprised that the textures seem to be different in the two screenshots. The lighting is necessarily different because HDR isn't well supported (if it is at all) on Linux, but I would have thought that keeping the textures in line would be something Valve would be able to do.
Old games don't use "HDR" the same way we use it today. In old games, enabling HDR makes the lighting calculations in the game engine have infinite range which will then be mapped onto SDR colorspace, which is all software and very much supported in Linux.
If anything the screenshots show a gamma calibration issue. From my experience on Linux native Team Fortress 2, the in-game gamma slider does not do anything.
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A great, short explanation of the different meanings of HDR is up to minute 1:40 in the video below.
https://youtu.be/frBNnNWEbyk