Fix walls being too bright in OpenGL

[jackrjli]

This reverts some of the changes that were made to the indexed lightmode shader when it was refactored for smooth fade mode (#409). In my testing these end up making the indexed lightmode less accurate to software.

Note that this does not address floors and ceilings being different between software and OpenGL, but walls should be pretty close.

Comparison screenshots:
Software

OpenGL, without this change

OpenGL, with this change

[Kappa971]

If you look at this #588, you'll notice that I'm getting a different result than shown here.
My screenshots are in 1920x1080, maybe the lighting changes based on the resolution?

[jackrjli]

@Kappa971 Are you autoloading the WAD shared in Discord? That's an older version of what I have here.

[Kappa971]

@Kappa971 Are you autoloading the WAD shared in Discord? That's an older version of what I have here.

I'm not autoloading any WADs, except widescreen STBAR.

[jackrjli]

This is what I get at 1920x1080 resolution:

Software

OpenGL (no fix)

OpenGL (with fix)

Could you test this WAD (or build dsda-doom.wad with this change) to see how it looks?
gllightfix_new.zip

[Kappa971]

As you can see by comparing your OpenGL (no fix) image with the one in the issue I opened, they are equally different (as before), so I think the resolution has nothing to do with it.
I am using DSDA-Doom compiled on January 27th and not version 0.28.3. I'm on Windows 11 and I'm using an AMD GPU.

With "gllightfix_new.wad" the result seems the same instead:

but still different from software rendering, so it's not really a fix but just a slight improvement. Someone with OpenGL knowledge would be needed to dig into this (In my opinion).

[Pedro-Beirao]

@Kappa971 so your concern is that it isnt a 1:1 match with software? Well it never will be. OpenGL is just a completely different way to render.
But this is a good change that shortens the difference.

[Kappa971]

Just to know, why are @jackrjli and I experiencing different behavior?
@jackrjli OpenGL (no fix)

@Kappa971 OpenGL (no fix)

[Pedro-Beirao]

I dont know how to answer that. The no-fix behaviour is also different for me, but the fixed one is the same as the screenshots you guys sent.
Maybe in the old code there was something that was calculated differently in each GPU?

Interestingly, the no-fix looks very similar, just slightly different, to the new one in my machine (M1 Mac).

OpenGL (no-fix)

OpenGL (fix)

[Pedro-Beirao]

Seems right, and definitely gives better results

[jackrjli]

No idea why we all get different "no fix" results, but at least the "with fix" results are the same 😅
For the record, I'm on Linux (Fedora 41), with an Nvidia GPU.
Just to throw one more into the mix, I gave it a try on my laptop with Intel integrated graphics:

OpenGL (no fix)

OpenGL (with fix)