Thank you @PhotoPhysicsGuy!
I wrote a more detailed motivation for the use of these sigmoids over at Github, see the first post. But in short, they model at least the base curve presets surprisingly well and offer a straight forward parameterization that does pretty much exactly what you would expect.
Both work on the separate RGB components of the chosen working profile. The difference is in their shape which is easiest spotted in highlights where Weibull gives more punch. I recommend going back and forth on an image where you expect some highlights to hit pure white.
@Jade_NL and @priort I’m aware of the sensitivity, I left out the soft bounds on purpose, for now, to make it clear how wide range it handles without breaking down. Softbounds will improve the step size and make it less sensitive. I have also been thinking about defining the slope at 0.18 instead of the exponent/base, might give a different feeling to the how sensitive it is.
About black point and crushing shadows, could you post an example? My gut feeling is that you should work some more with the tone equalizer, graduated filter, or masked exposure changes to get that to work
And those are some nice test images, especially the Noctilucent clouds proved challenging!
Noctilucent clouds
sigmoid, log-logistic, contrast = 1.61
graduated density (for lifting the lower part of the image)
tone equalizer, custom
exposure
denoise (profiled), chroma only
white balance, 5413 K, bluer!
[Play Raw] Amulree Kirk
sigmoid, contrast = 2.65
Kingfisher: Softproof failure
sigmoid, contrast = 2.44
The last picture seems to have input color issues with the blue channel, those issues remain sadly so no point in posting the result from that.