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02-13-2022, 12:29 PM - 19 Likes   #1
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"Perfect" colour negative film conversions using GIMP and RawTherapee

PLEASE NOTE!

This thread has been superseded by a later, updated and more-detailed tutorial in the Post-Processing Articles forum, here:

https://www.pentaxforums.com/forums/61-post-processing-articles/438373-colou...pee-5-8-a.html

I have asked the moderators to close this older, out-dated thread, and cordially direct you to the new one linked above for further discussion :)

Kind regds,

Mike


---

IMPORTANT UPDATE!

Since writing this post and publishing the "how-to" video on YouTube, I've run into a couple of inaccurate conversions with some of my older negatives. Further investigation has revealed the cause (I'm a little annoyed with myself for missing it) and, as a result, I've refined the process very slightly:

When the raw file is loaded into GIMP - which in turn loads it into RawTherapee automatically - it's essential to un-check "Clip out-of-gamut colours" (the very first item in the Exposure section). If this box is checked (which is the default), it's possible (though not always the case) that the colours in the negative won't be true to what was captured by the camera, and the inverted image in GIMP may have a colour cast (the very thing we're trying to avoid!). So, please uncheck the box as directed, and it will work correctly!

I also recommend setting Saturation to +10 in RawTherapee before the Colour Toning step, as this ultimately results in better saturated positives - but you can always add saturation when conversion is complete...


---

By now, quite a few of you will know that I'm shooting film and digitising my negatives at home with a DSLR. To convert them into positives, I've been using a commercial Lightroom plug-in called Negative Lab Pro. It generally does an excellent job - but it's not perfect, and can occasionally require a fair bit of tweaking or re-running to produce the desired output. It also hides what it does behind automation and presets (the whole point, to some extent), and this can make it difficult to predict results with absolute confidence.

I've tried Darktable's "Negadoctor" module, and RawTherapee's "Film Negative" module, and whilst they can work pretty well, the results are extremely variable and highly dependent on the content of the digitised negative. I've also tried entirely manual conversion in GIMP, inverting the tone curve then adjusting R, G and B curves in a bid to achieve realistic colours and tones without any colour cast, but it can be very time consuming and rarely produces 100% satisfactory results.

For the past week or so, I've been working on a method of conversion using a combination of GIMP and RawTherapee, and I'm confident I've cracked it. I've tested the approach with numerous negatives of my own - plus a few belonging to others - and I'm getting exactly the results I want and expect. It's more consistent and predictable than Negative Lab Pro, in my limited experience, and runs rings around Darktable and RawTherapee's integrated conversion modules.

The essential workflow is as follows:

- When digitising negatives, set the camera's custom white balance against the light source used
- Set correct camera and lens exposure parameters to ensure no clipping
- Capture negatives with at least some of the border visible in the frame

... then:

- Open a negative raw file in GIMP, which loads it into RawTherapee via plug-in
- In RT, uncheck the Clip out-of-gamut colours box (highlighted, because this is a new step as per the update at the top of the post )
- Optionally, set Saturation to +20 for more vibrant positives (or saturation can be added when conversion is complete, if required)
- Set a Linear / Standard tone curve
- Set White Balance to Camera
- Set the Input Profile to Camera Standard
- Use the Lockable Colour Picker tool to sample a point on the border
- Use the Colour Toning tool to adjust until the picker's RGB values match closely (within +/- 1%) - this effectively removes the film base colour at source
- Exit RT and the image is transferred to GIMP
- In GIMP, invert the image
- Select the exposed area of the photo and include a little of the border (but no sprocket holes or other elements)
- Open the Levels tool and click Auto Input Levels
- Adjust brightness using the right-hand pointer of the Output Levels control (critical if the photo contains no highlights)
- Adjust gamma using the middle pointer of the Input Levels control (this varies photo to photo, and largely depends on the quality of the original exposure)
- Click OK to apply (don't click Cancel, or the levels won't be saved as a preset, which is needed for the next steps)
- Undo the adjustments (as they currently apply to an incomplete portion of the image)
- Select All
- Re-open the Levels tool and from the Presets drop-down, select the most recent "Last used" adjustments
- Click OK, and the conversion is complete
- Export the image as a positive TIFF master, with .TIF extension

This may seem like quite a bit of work, but having practiced variations of it countless times while researching this, it now takes me around 2 minutes from opening the file to exporting the completed positive master. I don't think that's bad for a manual conversion process where the user retains complete control, and one that produces consistently good results.

The great thing is, since this is based around GIMP and RawTherapee, it's completely free and will work for those with Windows, Mac and Linux systems.

The "secret sauce" in this approach is a recipe of three ingredients - (1) use of a relatively neutral camera profile, (2) a flat tone curve to begin with, and (3) use of RT's Colour Toning tool to cancel out the film base colour cast - much like acetate filters or a dichroic head in traditional wet-printing. It seems pretty obvious to me now, but removal of the film base colour before conversion is absolutely key to achieving good colours in the final image, with no colour cast or shift to deal with.

I've put together a short YouTube video to demonstrate the process. I don't run a monetised channel, and I've no interest in building a community of followers. I may never produce another video I'm not trying to pimp my own content and take attention away from my beloved PentaxForums... I simply made the video to assist others with quality colour negative conversions.

I'll admit, it was kind of fun putting together my first (and probably last) YouTube video... and I guess I'd get a kick if anyone clicks the like button on it One in the eye for Jared Polin and Tony Northrup, eh?




Last edited by BigMackCam; 02-18-2022 at 04:03 PM.
02-13-2022, 02:36 PM - 2 Likes   #2
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Well done, Mike! Never used RT or GIMP meself, but I can follow the workflow. Who are these people you mention anyway?
02-13-2022, 03:02 PM - 1 Like   #3
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QuoteOriginally posted by StiffLegged Quote
Well done, Mike! Never used RT or GIMP meself, but I can follow the workflow. Who are these people you mention anyway?
Thank you

I've used RawTherapee a good deal, but only dabbled with GIMP until quite recently (previously, I used it for sharpening, cropping, cloning etc.). I've learned a lot during this last week of playing around I'm sure the GIMP aspects of my approach can be replicated in Photoshop, since GIMP imitates it in many ways... but RawTherapee's Colour Toning control is amazing, and I've not found an easy way to replicate that in Lightroom 6 (the version I own) or other raw converters. Maybe Darktable has something similar... I'll have to take a look.

As for the people I mentioned, rumour has it they were planted on YouTube to disrupt peace and harmony in Western society using something called "clickbait titles"...? it's beyond my understanding as a new YouTuber

02-13-2022, 03:02 PM - 1 Like   #4
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You made it look pretty easy. Nice video tutorial.

02-13-2022, 03:11 PM - 4 Likes   #5
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Mike - the module you are looking for in darktable is Color Correction. It is the same module as you used in RT. Some may not know but darktable and RT are friendly to each other and often use each other's open source code. - Jack
02-13-2022, 03:19 PM - 1 Like   #6
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QuoteOriginally posted by 740K70 Quote
You made it look pretty easy. Nice video tutorial.
Thank you - you're very kind I learned some basics of Kdenlive while creating the video - though I realise it's pretty clear I'm a rank beginner

The conversion process itself really is easy. It was "getting there" that was the hard part... so many options and permutations in the approach, and trying to simplify it. I think it turned out well, though. Negative Lab Pro still wins hands down on workflow efficiency with generally good results, but for amateurs like me who can spend a few minutes dedicated to each photo, the method I've shown is arguably better - more consistent and predictable.

QuoteOriginally posted by jbinpg Quote
Mike - the module you are looking for in darktable is Color Correction. It is the same module as you used in RT. Some may not know but darktable and RT are friendly to each other and often use each other's open source code. - Jack
Ah, thank you, Jack - that's great to know I'll take a look at DT's Color Correction module in the next day or two...
02-13-2022, 03:41 PM - 1 Like   #7
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QuoteOriginally posted by BigMackCam Quote
As for the people I mentioned, rumour has it they were planted on YouTube to disrupt peace and harmony in Western society using something called "clickbait titles"...?
Nobody of any importance then!

02-13-2022, 04:55 PM - 1 Like   #8
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For scanning/digitising film I've been using Vuescan VueScan Scanner Software for macOS, Windows 10, and Linux for years.
Admittedly it's geared towards dedicated scanners rather than camera output, but you may be able to adapt it's workflow to your needs.
There's a demo download, which watermarks the output but is otherwise fully functional, as far as I am aware, so if you've got an hour to fiddle about it might be worth your while having a look at least.
02-13-2022, 04:58 PM - 1 Like   #9
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Well done! Thank you for this, keep it coming!
02-13-2022, 09:51 PM - 1 Like   #10
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Perfect and free . . . there's gotta be a catch . . .

Good job and will have to try it out.
02-14-2022, 02:21 AM - 1 Like   #11
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QuoteOriginally posted by BigMackCam Quote
- Use the Colour Toning tool to adjust until the picker's RGB values match closely (within +/- 1%) - this effectively removes the film base colour at source

That's a fantastic tip, Mike. Just using the eyedropper to correct white balance from a sample of the negative's border never really seems to get it completely white, so being able to tweak the RGB values has got to help. I'll be trying out your method this evening.
02-14-2022, 02:40 AM   #12
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QuoteOriginally posted by Dartmoor Dave Quote
That's a fantastic tip, Mike. Just using the eyedropper to correct white balance from a sample of the negative's border never really seems to get it completely white, so being able to tweak the RGB values has got to help. I'll be trying out your method this evening.
Thanks, David

It took me more than a week of experimentation to arrive at that specific action

White balancing, as you point out, doesn't really work... If you set it using the border alone - so you get a white border on the negative - it results in a tint to all the colours after inversion. If, instead, you set an average white balance based on the entire exposed image, the border will have a tint

I tried R, G, B curves correction in GIMP... achieving a better result, but still not quite right, and requiring very different manual corrections per image. My next step was to create and merge a layer copy of the image rendered only in the inverse of the border colour. I was getting closer, but still no cigar After various other very-nearly-but-not-quite derivative attempts, I had to look outside of GIMP, started to play with the colour tools in RawTherapee, and found that the Colour Toning tool - when used in Colour correcting regions mode - seems to work just like acetates, especially when using a flat, linear tone curve and basic colour matrix profile. It's remarkably effective... I just wish it had the ability to automatically calculate the necessary colour tone to arrive at white for the sampled area!

Adjusting the colour toning is really the only tricky part (to begin with, at least)... It takes a little bit of practice, a steady-ish hand and a half-decent mouse to get the accuracy spot-on - but the results are definitely worth it, IMHO. As I mention in the video, +/- 1% variance in the RGB values is what I aim for. In reality, I suspect a slightly bigger variance wouldn't be a problem. The Auto Input Levels function in GIMP's Levels tool essentially performs a white balance by equalising and setting black & white points for the R, G and B channels, and discarding a very small percentage of outlying pixels from each channel - so it should more-or-less correct for any slight inaccuracy in the RawTherapee Colour Toning stage...

Last edited by BigMackCam; 02-14-2022 at 10:22 AM.
02-14-2022, 05:24 AM   #13
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When you photograph the negative including a border, the histogram is 'polluted' with more white (highlights) than is in the actual photo, and you get a different and incorrect representation in the histogram. This has repercussions on everything you do in post, starting when the histogram is stretched over the full width and fine-tuning contrast with the ratios between shadows, midtones and highlights. I imagine the histogram is more compressed, notably in the highligts, this can only have a negative effect on tonality.
So I always take care nothing of the light outside the frame is coming through to get a correct representation of the densities from the frame into the histogram.
02-14-2022, 05:46 AM   #14
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QuoteOriginally posted by Kobayashi.K Quote
When you photograph the negative including a border, the histogram is 'polluted' with more white (highlights) than is in the actual photo, and you get a different and incorrect representation in the histogram. This has repercussions on everything you do in post, starting when the histogram is stretched over the full width and fine-tuning contrast with the ratios between shadows, midtones and highlights. I imagine the histogram is more compressed, notably in the highligts, this can only have a negative effect on tonality.
So I always take care nothing of the light outside the frame is coming through to get a correct representation of the densities from the frame into the histogram.
I like to include the border, but - unlike the example in my video - I usually exclude the sprocket holes... though there are occasions where I want those too, along with the film information (I recently converted some XP2 Super negatives that look great with the sprocket holes and info ). Including the border (but no sprocket holes) has zero impact on the captured DNG, as the dynamic range of the negative with or without the border is less than the camera sensor is capable of capturing at base ISO. So long as exposure is set such that there's no clipping at either end of the histogram (in Live View, I usually aim for a relatively central distribution), the border has no detrimental impact (if someone can show me two raw negative files - one with border, one without - that prove me wrong, I'll gladly revise my stance on this)... and because I use a linear tone curve during most of the processing, there's no compression of tonal information. The compelling benefits of including the border - the main reasons for doing so - are that in post-processing we can (a) sample the border's colour tint and correct for it properly (i.e. NOT just using white balance), and (2) once the image is inverted, we have a known black point to work with. The border is unexposed, so by definition it's the darkest point on the film...

I'm not trying to convince anyone to do things my way... I'm just sharing what's working very well for me

Last edited by BigMackCam; 02-15-2022 at 02:01 AM.
02-14-2022, 05:52 AM   #15
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By adding a border the original histogram is compressed with more white than is in the negative. Why don't you take a copy from the leader of the film as a reference instead of adding extra white for each frame.
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