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11-04-2017, 03:17 PM   #1
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Question for Darktable (under Linux) users

So, after trialling Darktable 2.2.5 under Linux Mint for a few days, I'm completely sold. I love it, and (for me) it's easily capable of replacing Lightroom 6 - at least so far as RAW development is concerned.

However, there's one processing function I just can't seem to find an equivalent for, and it's making my workflow very clunky...

I should explain, I shoot a *lot* of different lenses - mostly vintage Soviet glass, but plenty of other stuff too. Some of these lenses exhibit barrel or pin-cushion distortion... not to a huge degree, but sometimes enough that I want to correct it. Darktable has the "Lens Correction" (Lensfun) tool, but that relies on already-existing profiles. I know that, for as-yet-unprofiled lenses, I can provide sample photos and request that a new one is created - but, that's time consuming. In any case, sometimes I don't want perfect correction, but a bespoke adjustment for the photo in question.

In Lightroom 6, I can manually adjust for simple barrel and pin-cushion distortion without having to select a lens profile, and with prime lenses this is more than enough to get the desired result. But I can't find such a tool in Darktable... The only way I can achieve this right now is to export my image to a TIFF file, load it into GIMP, and use the lens distortion tool there. This works well enough, but it adds what I feel are unnecessary steps to what should really be a simple "darkroom requirement".

Am I missing something, or is Darktable missing something? I'm hoping it's the former, but beginning to suspect it may be the latter!

This won't prevent me from continuing my move to Darktable, but I feel it's a fundamental flaw in what is otherwise an outstanding RAW development tool...

11-04-2017, 03:39 PM   #2
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Hello BMC,
I don't see any lens correction tools available in the current development source, other than the 'Perspective Correction' tool, which probably isn't much help.
Send the darktable devs a feature request via their bug report, (see here development | darktable), as your requirement seems like it could be useful for many users.

Cheers,
Terry
11-04-2017, 03:49 PM - 1 Like   #3
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If you just want most of the distortion removed, maybe you can find a lens with an already existing profile that has a similar amount of distortion. You could try to duplicate an existing profile and rename it.

If you are feeling adventurous (and you seem like the sort to give this a try!), you might even be able to open and edit a lens profile to hand-tweak the parameters.

P.S. Have you seen Lensfun or Calibration images upload
11-04-2017, 04:08 PM   #4
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QuoteOriginally posted by photoptimist Quote
If you just want most of the distortion removed, maybe you can find a lens with an already existing profile that has a similar amount of distortion. You could try to duplicate an existing profile and rename it.
I had thought of that too However, there's a huge list of lenses - and the problem is, there's no way to tweak the adjustments for a selected lens; so, it's very hit-and-miss. Great idea, though

QuoteOriginally posted by photoptimist Quote
If you are feeling adventurous (and you seem like the sort to give this a try!), you might even be able to open and edit a lens profile to hand-tweak the parameters.

P.S. Have you seen Lensfun or Calibration images upload
I'll definitely create lens profiles for my most-commonly-used glass - you're quite right, that's the way to do it. But, I do have a large number of lenses that are primarily collectible pieces. I probably shoot with them once every few months at best. For those shots, I don't care about getting the correction spot-on. Rather than invest time in creating a profile for each lens (especially the primes - which is most of them), I'd really like a simple slider (as in Lightroom 6) that allows me to counteract (or emphasise, for that matter) simple barrel or pin-cushion distortion.

Thanks for the input - much appreciated!

11-04-2017, 04:13 PM   #5
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QuoteOriginally posted by tduell Quote
Hello BMC,
I don't see any lens correction tools available in the current development source, other than the 'Perspective Correction' tool, which probably isn't much help.
Send the darktable devs a feature request via their bug report, (see here development | darktable), as your requirement seems like it could be useful for many users.

Cheers,
Terry
Thanks, Terry - much appreciated! I've subscribed to the development list, and if no solutions are forthcoming here, I'll post a request in the next day or two. I'm sure others would find this extremely useful too
11-04-2017, 05:10 PM   #6
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In the lens correction module you can adjust a bunch of things.
11-04-2017, 05:18 PM   #7
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QuoteOriginally posted by derekkite Quote
In the lens correction module you can adjust a bunch of things.
Thanks, Derek. Could you elaborate a little for my benefit, please?

As an example, I see the options shown below. I can't see any option that allows me to adjust the distortion to counteract (or incur) barrel or pin-cushion distortion.

If you're aware of any way to do this, I'd be eternally grateful for your guidance!

Thanks in advance...
Attached Images
 
11-04-2017, 07:53 PM - 1 Like   #8
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Hi, Mike. You aren't the first to ask this question. In fact, there is a feature request for a manual lens correction module in the darktable redmine system already (#9087).

https://redmine.darktable.org/issues/9087

Unfortunately, redmine is down at the moment for some reason so I am unable to check the status of this request. I remember about 5 years ago, darktable did have a manual lens correction module. But they decided to borrow code for automatic lens correction (from the rawtherapee project, I believe). This was easier for the developers since the new module depended on outside 3rd party libraries to provide the internals (specifically, exiv2 for lens ID recognition and liblensfun for the corrections) and they were freed from its maintenance costs.

However, I think even the developers know that the ongoing delay between getting new or old lenses into the outside libraries makes users grouchy for knobs they can twist themselves. So I wouldn't be surprised if such a module either reappeared or the current automatic module sprouted some better manual overrides.

Jack

11-05-2017, 07:25 AM   #9
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QuoteOriginally posted by jbinpg Quote
Hi, Mike. You aren't the first to ask this question. In fact, there is a feature request for a manual lens correction module in the darktable redmine system already (#9087).
Thanks, Jack - that's very helpful

I'm glad the issue has been raised before, and I'll certainly want to raise it again if it has become an inactive request.

Can I ask, what's the best way to submit feature requests - via the darktable-dev mailing list, redmine, github or ... ?
11-05-2017, 08:50 AM - 1 Like   #10
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Hi Mack,

There are a huge range of operators for distortion on the command line using convert utility

The one you might need is -distort barrel "A, B, C" where the 3 coeffs are for 3rd, 2nd and 1st order radial distortions
If the radial coeffs are not avialble for your lens , you can find the coeffs by trials ( usually 3rd order is the main one)

Here are some examples of crazy overcorrections
original img: (XR Rikenon 1:3.5 28mm uncorrected
https://app.box.com/s/ievjf7xtl8y5ik7uglpjiwthdnqidd36

positive 3rd order radial overcorrection:
ZZ@localhost Barrel_Example]$ convert IMGP0206.JPG -distort Barrel "+0.5 +0.5 -0.5" My_barreledCrazy1.JPG
https://app.box.com/s/dk2wvaqsrx2j2bknsfh3tpa8w98swgis

negative 2rd order radial overcorrection:
ZZ@localhost Barrel_Example]$ convert IMGP0206.JPG -distort Barrel "-0.0 -0.5 -0.5" My_barreledCrazy2.JPG
https://app.box.com/s/91ghuz6hmg6mvnqtwsuanx9qnwp9fmfz
11-05-2017, 12:52 PM - 1 Like   #11
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Mike - I find the best way to approach an issue is to bring it up on the mailing lists first, especially now since the tracker website seems to be having problems. There are two pertinent mailing lists for darktable: darktable-dev (more for discussing internals) and darktable-user (for everything else). If you haven't subscribed to both yet, I suggest you do so. Gives you a good feel for dt's progress and where the developers' focus is at the moment. Details on how to subscribe can be found by either a quick google or at the project website.

Jack
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