Hi, I have no real first-hand experience - but thanks for sharing this; one day, it may come useful! Recently, there was a related discussion on the 'Darktable (Unofficial)' Facebook group; you may want to check that out: https://www.facebook.com/groups/darktable/permalink/1194802980685278/
I'll post there a link to this thread via the mail-archive. Kofa On Wed, 20 Feb 2019 at 17:40, Stéphane Gourichon <[email protected]> wrote: > > Hello everyone, > > Has anyone experience about restoring faded color photographs from > positive print, using darktable? > > There is high-expertise information here: > https://photo.stackexchange.com/a/22727/34215 ... which also implies lot > of work and manual adjustments. > > Knowing that prints are based on subtractive color synthesis, it seems > natural to model the problem as if some of the inks were linearly (or > even non-linearly) faded. But RGB being additive, common "multiply" > operation (e.g. "channel mixer" module) doesn't fit this model. With an > "add" technically it would do, but it would be practically too awkward > to adjust both "multiply" and "add" sliders on each change (see below > for what I've found). > > What does darktable provide as options? > > I have tried the following darktable modules : > > * Color balance (interesting how a negative "lift" answer the "multiply" > + "add" issue mentioned above, see effect on histogram). > * Color correction (cf. > https://www.darktable.org/2012/03/color-correction/). I tried it and it > is IMHO unsuitable. > * Color zones (unsuitable IMHO). > * Color mapping. > > > The best result I had so far involves approximate manual steps to get to > a point where the "color mapping" module can pick up and nicely do the > bulk of the work. > > Method > > * In dartkable open the faded image. > * Open "levels" module, click "auto" then adjust the black point to > avoid too black shadows. > * In "color balance" perform manual approximate correction, mostly using > negative lift sometimes combined with gamma. > * The important criteria of the previous two steps are (1) get an > overall correct luminance (2) correct the strongest aspects of the color > bias, even if the result is not fully satisfying. For example, get skin > tones that mostly look like skin, sky that mostly looks blue, etc. Show > image side-by-side with a modern photograph to get a comparison point. > * The important step: follow > https://www.darktable.org/2013/04/color-mapping/ using as source a > modern photograph with good color balance that features the same > elements (in my case: blue sky, wood, skin, foliage). Worry that it > won't work. Adjust "color dominance" and suddenly get a decent (possibly > good) result. > > Results > > This method worked rather well, with moderate manual work and produces > results resembling the modern photograph used as color reference (which > is the point of the "color mapping" module). Obviously it's not the > intended use case for the "color mapping" module. > > Interestingly, the "color dominance" parameter appears to have some > "threshold effects": from 0 to 100%, result starts awful (color casts > applied to wrong areas), does not change at all for some ranges, and > suddenly changes to something rather nice, then back to something > unacceptable and dramatically different near 100%, staying at that when > reaching 100%. Based on https://www.darktable.org/2013/04/color-mapping/ > I would expect monotonous change and the more relevant results happening > with low values. Perhaps this behavior comes from a too approximate use > of the "color balance" module and would theoretically disappear if that > step was done "perfectly"? > > Limits > > The limits of the approach are that it has some "apply whatever color" > (from modern photograph) rather than "restore original image colors" > flavor. > > Also, many faded photos have areas less faded. Most of them near the > edges, sometimes spots anywhere in the picture. These remain visible on > end result as blobs of strong color change. > > Others directions to explore? > > Yes, doing once, applying on many. > > It may be interesting to use the original vs processed image to > calibrate a simple (non-linear but monotone) model of ink fade. Results > may be expressed as parameters for a "tone curve" module, or even as 3 > instances of "base curve" operating on R G and B (as shown fully > manually on https://www.youtube.com/watch?v=RQXbmC_v2d4 , you should > probably mute the sound). The resulting module parameters could be > applied to other photos from the same batch and get decent rather good > color correction without more work. Obviously it would get as biased as > the color difference between the photograph used for calibration and the > modern source photograph used as color reference, which may or not be a > problem, depending on the goal. > > In the meantime, I could get some correct results by performing the > method on one photo and copy-pasting the "color mapping" module on other > photos with mostly the same subject. On others photos, it still kind of > works on some images. At least it can be tried to instantly get > something on n photos at the cost of one copy-paste, letting me > disabling it only on those photographs where it is not satisfying, which > is still a benefit from working on all the photos manually. > > > Has anyone also tried to restore color from old scanned faded paper photos? > > Any comment welcome on any part of this endeavor. Thanks! > > -- > Stéphane Gourichon > > ____________________________________________________________________________ > darktable user mailing list > to unsubscribe send a mail to [email protected] > ____________________________________________________________________________ darktable user mailing list to unsubscribe send a mail to [email protected]
