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exposure: rename the black setting #1972
Users have taken the poor habit to use the black parameter to add more density in blacks. This is not the place to do it (use levels instead) and can seriously harm colors by pushing near-black RGB values into negatives, thus out of gamut: https://discuss.pixls.us/t/darktable-filmic-and-saturation/10622/11
We rename the "black" parameter "unclip negative values" to make its technical purpose clear and add a warning tooltip.
This setting should be let alone most of the time, except if the raw black level is wrong.
But that is exactly the purpose: obfuscate the name so people will need to read at least the tooltip, if not the doc, before they start messing up things without knowing it.
"black" suggests it's you average friendly slider, that can help you get more/less density, and in that sense, it's redundant with the levels module, except this variant can backfire badly.
"unclip negative values" suggests "don't use that unless it's to fix an issue and you know what you are doing".
The goal is actually to prevent most users to use it, because there are safer way to do the same thing in the software, and the fact that most modules deal silently with negative values makes it super difficult to track down the clipping issues to the exposure module when you don't know the guts of dt.
I have had multiple users reporting that either filmic or colorbalance clipped lowlights, and when they sent me the XMP, they all had their black level cranked like crazy, outputting negative values that nor log or gamma functions know to handle. So the problem they created before in exposure showed later in other modules. And now that this bad habit has been taken, it needs harsh measures to be reversed.
@aurelienpierre : I would see a warning in the tooltip in this case as it is done for the sliders in the "target/display" section of filmic. We could also rename "black" to "black level" if this helps? With is an indication that we are talking a level and not a simple adjustment of the dark tones. Hope this compromise will be ok to you.