Hi! Never tried with old photos in dt, imo new versions of The Gimp will do better job here. But I do occasional film processing in dt (both reversal and negative). Generally I don't experience problems with well processed modern film, but digitizing old film is just pain ... Lack of RGB-curves is my biggest complain — https://redmine.darktable.org/issue s/9559 , so one can adjust color shift, fix color cast in dark or light areas, but crafting custom curves is impossible. Retouch is quite primitive, no inpainting, just cloning, working with wacom makes me cry (buttons are very small). But wavelet decompose/masking works great.
"color balance" — I just don't get it's logic, will be thankful if someone will explain it to me. I use "channel mixer" + "color correction" + "tone curve". That's all folks ))) On Wed, 2019-02-20 at 17:27 +0100, Stéphane Gourichon 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-mappi > ng/ > 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! > ____________________________________________________________________________ darktable user mailing list to unsubscribe send a mail to [email protected]
