Per, What about adjusting the blue half of the B channel in LAB using the "tone curve" module? This will operate on only the blue parts of the image.
Andrew On 28 March 2017 at 09:22, Per Östlund <[email protected]> wrote: > On 2017-03-28 16:01, Remco Viëtor wrote: > > On mardi 28 mars 2017 14:49:09 CEST Per Östlund wrote: > > > Hi, > > > > > > I've digitized my father's collection of old slides with the camera + > > > macro lens + light table method. Most of the images look fine, but some > > > images with certain blue hues are giving me a headache. I'm guessing the > > > issue is that the light table I built used LEDs with a poor color > > > spectrum for the task, causing the blue channel to blow out. > > > > > > As soon as I touch e.g. the levels module I get the usual issue that > > > very saturated blues turn black. Using the gamut clipping option in the > > > input color profile module fixes that particular issue, but doesn't do > > > anything for the colors otherwise. > > > > > > The best solution I've managed to come up with so far has been to set > > > the input color profile to "linear Rec709 RGB" which heavily desaturates > > > the image, and then increasing the saturation a lot with the color > > > correction module. Hardly an optimal solution, but with some tweaking I > > > can usually get the colors to look approximately the same except that > > > the blues don't clip anymore. > > > > > > This method can require quite a lot of tweaking to look good though, so > > > my question is if someone knows of a better way to handle this issue > > > (besides reshooting with a better light source). Here's an example > > > RAW-file (don't mind the blur, that's from the original slide): > > > https://www.dropbox.com/s/6mshjycwm4n10xa/img_014.ARW?dl=0 > > > > First the good news: as far as I can see you have no blown channels in the > raw file, even the blues stay within range of the sensor. The bad news is > that with the default settings, a large part of the image gives > 'out-of-gamut' warnings. And to be honest, the image looks over-saturated. > > > > So what I tried was: > > - set base curve to 'leica' (less aggressive than the Sony default, esp. > in the highlights). > > - set gamut clipping to 'adobeRGB' (or sRGB). > > > > then I used two copies of 'contrast brightness saturation', > > - the first for a global saturation correction of -0.2, > > - the second was at saturation -0.35, with a parametric mask limiting the > effect to the blue colors: 'b' channel from -128/-128 (full left) to -29/-3. > > Thanks, I tried your settings, but to my eyes it makes the image too dark > without really fixing the issue with the blues. > > If all slides are similar, you could set this up for one slide and the > create a style to apply those settings as a base to others. > > Unfortunately not, I already have a style which fixes the issue somewhat, > but usually have to spend a lot of time fixing individual images. > > But would it be possible that the original slides have faded a bit over > time, and especially the red dye? > > Very possible, the example slide is about 40 years old. Unfortunately I > don't have access to the slides at the moment. > > > > Good luck, > > > > Remco > > > > > > ____________________________________________________________________________ > darktable user mailing list to unsubscribe send a mail to > [email protected] > > > ____________________________________________________________________________ > darktable user mailing list to unsubscribe send a mail to > [email protected] > ____________________________________________________________________________ darktable user mailing list to unsubscribe send a mail to [email protected]
