I really like the color calibration module in 3.6.0 and I use the CAT16 option to try and do an automatic colour correction of scanned images. It often does an excellent job, but sometimes I would like to just tweak the color using some RGB sliders after the CAT16 adaptation. This option would seem an improvement to a humble user like myself or am I misunderstanding something about using this tool.
This video by Aurélien is a useful brief intro:
You can use colorbalance or colorbalancergb for that
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Actually, I don’t think color balance rgb has RGB sliders. Rather, a second instance of color calibration with adaptation set to “bypass” would provide simple gain controls for the RGB channels (and even permits cross mixing like the old channel mixer module).
Thanks for that suggestion. Your suggestion would work, but makes for a clunky workflow. I feel if Aurélien provided RGB sliders in the CAT adaptation option it would be a more intuitive and easier tool to use. However, maybe Aurélien has some very good reasons for not doing this.
So the issue is that when you are using colour calibration for CAT purposes, it is not working in Lin.Rec2020, but instead in a special CIE colourspace. So, if you manipulate the 3 channels in that space, the results will not be what you expect (there was a long thread about this elsewhere in the forum). This is why it is best to split off a new instance, and you can set the colour space of the second instance to use your working profile space.
I would argue though, that direct manipulation of RGB channels is very hard for a human to control, and are always relative to a particular colour space, whereas the hue/chroma/lightness controls provide a much more human-friendly interface for fine adjustments.
Even though I would regard myself as a very experienced person editing photographs I have yet to really master fast and quick colour corrections in colour balance. I presume this is because I have allowed the look of the module where colours are divided into shadows, highlights and midtones to overwhelm me. I will give this more of a try and see how I go. But I guess what I was suggesting is that three extra sliders in the color calibration module might be more intuitive for many people than the 22 potential sliders in the color balance module. Thanks for your suggestion about the color balance module. As for the Color Balance RGB module, I am going to have to do some reading and watching videos before I can make sense of that module.
So I am very familiar with using curves to adjust colours. So would the RGB curves be a good option for me since years of working with RGB has wired my brain to prefer RGB over HCL?
If you want to make a global colour correction, then then color calibration is the right module for that. Set the illuminant to custom, then you can use the hue and chroma sliders to set whatever tint you want to the overall image. Here, it is useful to understand the concept of “complementary colours”, where a complementary colours are on opposite sides of the colour wheel, offset by 180° in hue.
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So this is a display-referred way of thinking (and I guess you are also used to working in sRGB space which has different primary colours to Rec2020). With RGB Curves, as soon as you make a global exposure adjustment, or otherwise try to scale the DR for a different display, you need to throw away your curve and start again. This is the reason why filmic replaces base curve in the scene-referred workflow, and RGB curve suffers from the same problem.
That said, you can do what you want, just be aware of the limitations of the approach you choose.
Scene referred workflow seems a very new way of editing images and as far as I am aware is limited to DT. I have actually embraced filmic and love the ease of using it once I got my head around it. However, I am doing some very challenging negative scans and there seems to be no nice scanning software out there to give good colors from the start. So I bring the unsharpened scans into DT and use multiple instances of contrast equalizer to sharpen, denoise and apply local contrast and clarity. But then I am left with the challenges of colour. The scans are in Adobe RGB which is the best colour spaced offered by the scanning software. I only ever use sRGB as an output for JPGs on the internet except and not during editing. I am finding most of the time the colour is easiest to handle in GIMP so I export the image from DT and open it in GIMP. Auto levels will often, but not always, produce nice colours. However, I need to be careful of harsh contrast changes made by auto levels in GIMP, but this is easily resolved as I can then further edit these auto level settings with the curves tool before applying the destructive edit. If auto levels doesn’t work in GIMP on the image I instead go to curves and individually adjust the color channels to get the colour balance right. One of the reasons for using GIMP is the ease of spot and scratch repairs when restoring photographs. I am just finding the best options in both DT and GIMP for my image work. I must admit that DT has taken 99% of the work away from GIMP with my camera photography, but scanning and photo restoration still heavily benefits from GIMP.
Have you tried the negadoctor module??
Right, if you are dealing with scans of negatives, then negadoctor is the right module to use instead of filmic, and it has features to deal with colour casts. Film negatives are already non-linear in nature, and so a scene-referred workflow doesn’t really make sense.
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I have tried the negadoctor module and it is a fantastic addition to DT. It would definitely be a great way to use a digital camera to photograph negatives and process them. I have an Epson perfection 850 scanner. I have tried the Epson Scan software and the Silverfast software that was supplied with it. I have also tried scanning a negative as a positive and then using negadoctor to turn it into a positive. This last method using negadoctor had a great ability to avoid the clipping of highlight details which I was struggling with in the other programs. I have since communicated with Silverfast and come up with options to retain the highlights. I must say I really hate with a passion both Silverfast and Epson scan software. Previously I found Canon scan software much nicer but that scanner died. There is no FOSS options suitable for scanners. If there was I am sure the passion of the developers would produce a better product than the half baked commercial solutions that are being offered for scanning. But negadoctor is another great example of FOOS solutions. I believe Aurélien developed this?
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For your flatbed scanner you should try VueScan. Not FOSS, but delivers good results and is relatively cheap at around $80. It also supports a ton of different scanners.
Tried vuescan. No happier with that. Canon software was not too bad. Gave better colour for most megs and images