Geforce Experience captured it somewhat washed out, but the changes should still be noticeable:
https://www.youtube.com/watch?v=LhhOy7BPtzM

I used QeffectsGL:
https://github.com/crystice-softworks/QeffectsGL/releases
...which gives horrible video quality to SiN at their default levels, so I added the following new section to its config file QeffectsGL.ini:

[sin.exe]
ZTrickFix=1
RenderFrame=1
Multisample=16
MultisampleHint=16
BloomDarken=18
BloomScale=1.25
SSAOMinZ=0.0001
SSAOMaxZ=0.75
EmbossScale=0.05
SSAOScale=4.0
Hue=3
Saturation=0
Lightness=2
ExtensionLimit=4000

The last line is important, since it even allows people with crashes to run the game because it fixes the length of OpenGL extensions.
The only problem is that some characters still light up like Xmas trees in day levels (as can be seen in the third demo), but at least its less bothersome than with the default settings.

Additionally, I used the following config in my autoexec.cfg:

// set fps "0"
set fov "106.3"
set gl_modulate "1.2"
set intensity "1.1"
set con_playerskin "0"
set con_driver "1"
set con_brightness "8"
set con_texmode "0"
set con_fullscreen "1"
set con_playermodel "0"
set cl_forcecinematic "1"
set cl_oldladders "0"
set cl_run "1"
set gl_playermip "0"
set gl_flashblend "0"
set gl_cull "1"
set gl_clear "0"
set gl_showtris "0"
set gl_skymip "0"
set gl_picmip "0"
set gl_round_down "0"
set gl_nobind "0"
set gl_lightmap "0"
set gl_bitdepth "0"
set gl_swapinterval "1"
set gl_ext_palettedtexture "0"
set gl_ext_compiled_vertex_array "1"
set gl_ext_pointparameters "1"
set gl_ext_multitexture "1"
set gl_ext_swapinterval "1"
set gl_texturesolidmode "default"
set gl_texturealphamode "default"
set gl_ztrick "0"
set gl_particle_att_c "0.01"
set gl_particle_att_b "0.0"
set gl_particle_att_a "0.01"
set gl_particle_size "30"
set gl_particle_max_size "40"
set gl_particle_min_size "2"
set gl_skintexturemode "GL_LINEAR_MIPMAP_LINEAR"
set gl_texturemode "GL_LINEAR_MIPMAP_LINEAR"
set gl_polyblend "1"
set gl_shadows "2"
set gl_finish "1"
set gl_driver "opengl32"
set lookstrafe "0"
set lookspring "1"
set maxentities "8192"
set maxsurfaces "8192"
set maxconsoles "32"
set m_filter "0"
set rate "5000"
set r_dynamic "1"
set r_detailthresh "200000"
set r_detaildist "4000000"
set r_conthresh "200000"
set r_condist "2000000"
set r_conoffset "0.1"
set r_lowdetail "0"
set sv_showdamagelocation "0"
set sv_showdetaildamage "0"
set sv_gibs "1"
set sv_gore "1"
set sv_maxbloodspats "4096"
set sv_maxbulletholes "8192"
set s_primary "0"
set s_mixahead "0.2"
set s_loadas8bit "0"
set s_khz "22"
set vid_gamma "0.7"
set vid_ypos "22"
set vid_xpos "3"
set vid_ref "gl"
set vid_fullscreen "1"

If you have any idea on how to improve any of this, please share.

Thanks man.

This solution made my day.

Game runs great now.

Thanks a lot for this!

One suggestion:

set gl_round_down "0"

would be better.

You're completely right, thank you!

Also, distance where objects dissappear can be increased (some primitive form of "LOD" this game has):
set r_detaildist "4000000"
set r_condist "2000000"

All corrections have been updated in the original post.

Something else you can do in your autoexec.cfg is alter aliases to your own needs:

alias d1 "menu_main"
alias q1 "demomap demo1.dm2 ; set nextserver q2"
alias q2 "demomap demo2.dm2 ; set nextserver q3"
alias q3 "demomap demo3.dm2 ; set nextserver q1"

"d1" is what the game executes at launch. Instead of going through the intro videos and the "login" screen, with this you go to the main menu directly. If you miss the login screen, change "menu_main" with "menu_logon".

"q1" through "q5" are the demos that play when you select "Game" -> "View demos". Here, intro videos are skipped again (not a fan of those :) ).

Again, if you think something is missing, please post it :)

I'm happy to say that the "Xmas trees people" (the excessive shine of human characters) that happened when using QeffectsGL, no longer happens after the new update. And muzzle flashes still light up properly, both in bright and dark areas. I don't know what Nightdive did, but between this and the proper widescreen support, I'm grateful.

Still, I tried ReShade (https://reshade.me/) and, after messing around for a good while, ended up with the following config:

PreprocessorDefinitions=
Techniques=PPFXBloom,Colourfulness,EyeAdaption
TechniqueSorting=PPFXBloom,Colourfulness,EyeAdaption

[Colourfulness.fx]
backbuffer_bits=8.000000
colourfulness=0.350000
col_noise=1
enable_dither=4294967295
lim_luma=0.700000

[EyeAdaption.fx]
fAdp_BrightenHighlights=0.100000
fAdp_BrightenMidtones=0.200000
fAdp_BrightenShadows=0.100000
fAdp_DarkenHighlights=0.100000
fAdp_DarkenMidtones=0.200000
fAdp_DarkenShadows=0.100000
fAdp_Delay=1.600000
fAdp_Equilibrium=0.500000
fAdp_Strength=1.000000
fAdp_TriggerRadius=6.000000
fAdp_YAxisFocalPoint=0.500000

[PPFX_Bloom.fx]
pBloomBlendMode=0
pBloomCurve=1.250000
pBloomIntensity=0.500000
pBloomRadius=64.000000
pBloomSaturation=1.250000
pBloomThreshold=0.450000
pEnableHDR=0
pEnableLensdirt=4294967295
pLensdirtCurve=1.200000
pLensdirtIntensity=0.120000
pTonemapContrast=1.020000
pTonemapCurve=3.000000
pTonemapExposure=1.200000
pTonemapMode=0
pTonemapSaturateBlacks=0.000000

If you want to try it, copy all of this and save it in its own .ini file (i.e. "Sin_Reshade.ini") in the game's folder. Just don't name it "Reshade.ini", or worse, "Qeffectsgl.ini" :)

Install ReShade pointing to sin.exe, with API = OpenGL, and you can select only the needed effects (Colourfulness, EyeAdaption, PPFXBloom) or all of them if you want to experiment later.
Start the game, press the Home key to show the ReShade UI, select the Settings tab and set Style as either "Light" or "Dark" ("Default" will make some things invisible). Get back to the Home tab and select the preset .ini file you just created. Press the Home key again to hide the ReShade UI, and play the game!

You should see almost immediately that the colors resemble more of a Sega game than a Quake2-powered one (for a laugh, use the default Colourfulness settings), and the bloom is more nuanced. There's a better contrast in dark areas, while QeffectsGL depends on the increased gamma to see in the dark. Also, there's a nice "dirty lens" effect, only visible in bright areas.

The problem with ReShade is that there's no good solution for antialiasing, since neither SMAA nor FXAA effect seems to work. The only solution is to force FXAA in the video card's options, which will make the default console font of the game hard to read (I hope you're not using subtitles). You can also force SMAA in your video card, but fonts in the computers inside the game will break even more than with QeffectsGL's "Multisample" option, so you'll have to pick your poison.

Talking about video card's options, you should force anisotropic filtering with either wrapper; the benefit is visible. By the way, you can swap between wrappers by backing up the "opengl32.dll" file for both and using in the game's folder the one you want. You can freely leave the .ini files of both wrappers in the game's folder.

In summary...

QeffectsGL:
+ Proper (or at least, decent) AA
+ Better bloom
- No other visual effects

ReShade:
+ More intense colors
+ More variety of effects
- No ideal solution to apply AA

I'll stick with ReShade, but YMMV. I can upload a video if you want, but as with the last one, I don't think YouTube video compression will make justice.

Note that I haven't used any ReShade effects outside the default ones, and even on those I haven't fully explored the options provided, so, if you have played with ReShade and SiN, or any other Quake2 engine game, I'll be glad to read any suggestions. And if you want me to try any other OpenGL wrapper, leave a link to download it :)

Unno bout OP or anyone else in this post but I just use ReShade.

much easier and as you may see from my attached photos makes it look much MUCH better without impacting performance much

if your curious what shaders I use:

I basically added both FXAA and SMAA for smoother edges
along with 2 diff types of bloom
and 2 diff types of HDR
and sum tonemapping
and colorfulness

and no I didnt have to install anything but the game and ReShade. technically you shouldnt have to.

unno why but gog staff for every game that uses 3dfx or opengl, well anything nglide, feel they need to include the .dll files in each individual game but that does nothing but break it honestly.

id recommend deleting all the opengl.dll files that are installed in thee default game along with the 3dfx.dll files.

those files basically are just nglide files

but how gog includes them messes up the latest version of nglide which you may get here: http://www.zeus-software.com/downloads/nglide

just install that then delete the .dll files i mentioned in your games installed folder and enjoy.

oh and yeah get ReShade.

lemme know and i can share my ReShade configuration file

Thanks for the info! It has been changed in the original post :)

Looks pretty nice!
Wish I could remember the console commands that I used to make darker environments a bit more visible.
I can only remember of setting vid_gamma to 0.8 (the lower the brighter) and something like gl_intensity but I don't remember the value exactly, this particular setting was used for making buttons and other special parts more visible on the distance

I backed up all my Sin config files here, included is the original OpenGL renderer tuned to work in modern systems.
If I recall correctly, these config files of mine already have the console commands in place for making environments a bit more visible and there's even a keyboard shortcut for turning FullBright On/Off (I think it's comma and period keys)

https://www.dropbox.com/sh/cy4b3dvu7k7clzp/AAA5Aju7NeDa6vZoVruhXDpJa?dl=0

(no game assets on this link, only config files, no piracy here...)

Objects draw distance issue
I just checked the SiN Gold (GOG latest version) and realized that default config.cfg file in ..\base\players\blade have duplicate lines for some settings, mainly it had the line *set r_condist "100000"* (the default minimal draw distance), so I just copied all the lines from your tweaked autoexec.cfg right to my current config.cfg and it solved the draw distance for objects and probably other issues as well. We probably need some kind of notification for that for other users to make sure config.cfg don't have priority duplicates with lower quality settings. Texture filtering mode
Well some users might think that it is the best video quality so far, but I beg to differ for me nothing beats crispy texture look with good old pixels, so I strongly suggest to use or at least to try those instead:
set gl_skintexturemode "GL_NEAREST_MIPMAP_LINEAR"
set gl_texturemode "GL_NEAREST_MIPMAP_LINEAR"
...
Disappearing objects (including corpses)
Does anyone know if there is a cure to turn it off, I can't stand how dead bodies and other objects are melting away within seconds, this kills the joy of playing the game.

I use the sin patcher

https://www.moddb.com/games/sin/downloads/sin-ultimate-patch