New lighting pipeline
One very big thing is that this RC starts using the new linear rendering pipeline meant to allow mappers and other artists to rely on physically correct lighting and blending equations. This makes color management more compatible with the “what you see is what you mean” principle, especially when you write shaders.
This new “linear pipeline” feature is only used and can only be used when the map is especially built for it. This new rendering technique isn't used on previously released maps. No Tremulous maps, no previously Unvanquished maps will be affected, unless you explicitly rebuild such map for the linear pipeline on purpose. Previously released maps can also be rebuilt to be rendered the old way, the new feature is only used when explicitly requested.
This feature was already shipped in the 0.55.5 release, but since no played map were rebuilt for it, it was not used. This release candidate brings rebuilt maps so we can evaluate them. It is expected that the 0.56 starts shipping linearly rebuilt maps, but it is not decided yet which ones will be distributed the new way.
For evaluation purpose, all stock maps have been rebuilt the linear way in that RC, so we can evaluate them, gather feedback and improve them. Some of those maps already look good out of the box, some may benefit some tweaks.
We already implemented some tweaks but more may be done. For example we may rework a bit the lighting in Vega and in Chasm.
We may also rework a bit the trak5 glass texture that is used in both Parpax, Spacetracks and Station15. In fact we also implemented a workaround for a bug that affected that glass texture and made it sometime invisible (while it should not), so the rebuild of those maps making use of them are actually less buggy than before. Actually this texture doesn't look that good even without the new linear pipeline, but it happened that it wasn't drawn at all before, and when being partially drawn, some people actually believed that the bug was in the part the texture was partially drawn, not the part the texture wasn't drawn. So actually, while we can improve a bit that texture, there is more chance that you find it ugly because it is ugly and it wasn't drawn before than because of it being rendered the linear way.
In general, I want to remind that the human brain is wired to consider as uncomfortable or even threatening something that is familiar but looks different, and that being totally independent of the characteristics of the object considered. So it can be possible that your brain feels that something is broken the first time you look at those rebuilt maps, a sentiment that would not affect someone who would never looked at them before.
In fact, like what happened when we fixed the too-much short light falloff, and when we fixed the overbright bug, once the brain starts to be fed with scenes rendered with physically correct equations and be accustomed to it, the brain will start spotting the inconsistencies and weirdness of older maps. We looked at buggy renders for decades, so it takes time to deconstruct the habituation and the various compensations we developed.
So, please be easy and give those rebuilds a chance. We can improve those rebuilds, but (unless we introduced a bug without knowing it 
) we have to acknowledge those aren't buggy by definition. Our brain will likely cry because we never seen maps built that way in our game yet. Until now we played all our games in the Platon's Cave.
The textures that most of the time need to be tweaked are the translucent textures. For example the Plat23 forcefield was tweaked to make sure the grid is visible, but the foggy background can be improved. The priority is to tweak the most obvious things. We are an actively developed game so we may postpone some tweaks when the most obvious issues are addressed first, and we may improve things progressively over time.
The Vega map has suffered a lot from three bugs in the past, bugs we fixed over the years if not decades: the short light falloff (shadows were literally painted on the opposite close wall a light was), the overbright clamping bug, and the non-physical light computation and blending. It's probably not a bad thing to assume that because of all these bugs, the light intensity couldn't be properly evaluated before and then have to be tweaked.
So, I encourage you to give those rebuilds a chance. We're fixing bugs, but sometime fixing bugs means we have to revert workarounds. So if you see something perfectible, it means we can improve it, not that something is wrong.
Tonemapping tweak
The Tone mapper is a nice feature, but when it was calibrated, we were still experiencing some overbright bugs (at the time the bug was the opposite of the old one, it was too much brighter with legacy maps). In some ways the current Tone mapper configuration was working around one bug that is now fixed.
And now that we have a working linear pipeline, we are able to render high lights we could not render before, and the current Tone mapper configuration may erase those high lights we can now produce in our maps.
So we reduced the default r_toneMappingHDRMax value from 8 to 2.
If you test the new linear pipeline with a HDR Max value of 8, your appreciation may not be about the linear pipeline but about that HDR Max value. So make sure you test it with a value of 2.
As a general rule if you spot something you believe is wrong, please also test with tone mapping disabled, so we can know from where the weirdness can come and properly diagnose it.
Also, please test with r_gamma 1, this gamma feature was never good and washed colors. Prefer the r_toneMappingExposure option instead, we made sure the exposure option can be used even when tone mapping is disabled. So I deeply encourage those who used a r_gamma value different than 1 to reset it to 1 and use r_toneMappingExposure. Any r_gamma not being 1 is a bug and was in fact a partial workaround for maps not being built the linear way. The exposure option is not buggy and should be preferred.
The r_toneMappingExposure may be renamed in a close future as it's independent from tone mapping.
) we have to acknowledge those aren't buggy by definition. Our brain will likely cry because we never seen maps built that way in our game yet. Until now we played all our games in the Platon's Cave.
