The lighting process doesn't actually read texture data at all, and texel size isn't relevant here.
What I'd like to see however is a comparison made using the ID1 start map. I'm suggesting that because otherwise there are too many variables in the equation - new maps, new textures, new lighting, etc. The ID1 start map is a known constant and takes all of those variables out. We're left with engine and replacement textures as factors that must be considered, and nothing else to distort the analysis.
There are 4 things that must be compared to make the comparison complete and scientifically valid:
- ID1 start map, native textures, RMQ engine
- ID1 start map, external textures, RMQ engine
- ID1 start map, native textures, other engine
- ID1 start map, external textures, other engine
That's the only way you're going to get anything out that's not tainted by subjective opinion.
I haven't done a direct comparison yet, but I believe that a histogram of standard external textures (Rygel's or QRP) with native ID1 textures will show that the externals are darker overall. They do look that way to my eyes anyway.
On overbrights.
Seems to be some confusion here about what overbright lighting is and what overbright lighting does, which may be leading to the statement that the engine doesn't do overbrights.
It's quite clear that the engine does do overbrights - you can check some of the
well-known trouble spots to confirm.
Overbright lighting doesn't make things brighter; what it does is increase the dynamic range of lighting. Any light in the standard range won't and shouldn't appear any different, any light above the standard range will be clamped without overbright lighting but won't be clamped (or - at worst - will be clamped at a higher level, although that doesn't apply to RMQ) with overbright lighting.
There are a few different ways of obtaining overbright lighting, with the standard being to shift down by an extra bit when calculating the lightmap then double the result after you multiply texture by lightmap. Q2 and Q3A double the texture and leave the source lighting fairly dark (they weren't written to target the required GL extension). DirectQ encodes a scaling factor into the alpha channel of the lightmap texture which is then decoded in the pixel shader. RMQ blends multiple lightstyles entirely on the GPU so it gets this as a natural result.
They're all just mathematical operations on numbers though, and the operation is the same whether the texture is external or not. In that case, if a different result is achieved, then the inputs must be different.
There is actually a second source of lightmap clamping going on too, and that's in the light tool. We have a solution worked out to address that, but nothing coded yet.