Mark wrote
>> No math here – I wouldn’t know where to start.<<
If of interest:
One simplified approach is to look at
RGB curves is in terms of adjacent tangents – means to approximate a curve section by section by a straight line which represents the “local” slope and offset. As common with linear equations, the term Offset refers to the crossing point of a straight line with the vertical “y” axis:
Slope > 1: increase of contrast
Slope < 1: decrease of contrast
Offset > 0: decrease of HSB-Saturation
Offset < 0: increase of HSB-Saturation
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RGB-Levels’-whitepoint settingor a respective "curve" which is already a straight line,
or +EV with the Exposure slider in ACR (from an empirical point of view):
Slope > 1: increase of contrast (difference between lighter and darker pixel)
Offset = 0: Color integrity is perfectly maintained. Linear scaling of RGB data does not change the intensity ratios of R:G:B per pixel, which is synonymous to unchanged HSB hue & saturation.
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RGB-Levels’-blackpoint settingor a respective "curve" which is already a straight line,
or the Shadows slider in ACR:
Slope > 1: increase of contrast
Offset < 0: increase of HSB-Saturation, mainly for the shadows with decreasing significance towards the highlights.
Note that the saturation boost and damage of color integrity is the more severe the higher the underlying gamma of the working space is:
http://www.c-f-systems.com/Docs/ColorIntegrityCFS-243.pdf---
Increasing brightness w/the RGB-Levels’-midtone slider,or a respective solely right-curved curve
or the Brightness slider in ACR:
From the shadows to the mids:
Slope > 1: increase of contrast
Offset > 0: decrease of HSB-Saturation
From the mids to the highlights:
Slope < 1: decrease of contrast
Offset > 0: decrease of HSB-Saturation
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Reducing brightness w/the RGB-Levels’-midtone slider,or a respective solely left-curved curve:
From the shadows to the mids:
Slope < 1: decrease of contrast
Offset < 0: increase of HSB-Saturation
From the mids to the highlights:
Slope > 1: increase of contrast
Offset < 0: increase of HSB-Saturation
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Before addressing more complex sigmoidal curves, it should be mentioned that analogous Lab lightness settings or changing to Luminosity blend mode are by far NOT always superior. It depends.
Peter
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