Help developing a colour negative with darktable 2.7 Windows version

I’m struggling to convert colour negatives with darktable. I’ve managed to get really good results in just a few clicks with both RawTherapee (new version 5.7 with Film negative processing) and Photoshop.
With darktable, I’m getting poor results with both the invert module method and reversing a tone curve method.
My process is

  • Import raw file
  • Reset default base curve (or leave it, but it usually ends up overexposed)
  • Invert module using a sample from the unexposed negative film
  • White balance module picking a neutral grey from image
  • Mess around with curves, rgb levels, colour balance and find that it takes ages to get anything half decent.

For reference, in Photoshop, I do the following:

  • Import (just open image from Camera Raw)
  • Crop and rotate
  • Invert
  • Levels > Options > Find Dark & Light Colors > Snap Neutral Midtones

In RawTherapee, I do:

  • Import
  • Crop and rotate
  • Film negative process
  • Adjust white balance with spot sample
  • Create new tone curve for proper contrast

With Photoshop producing the best results, I’ve tried to mimic the process in darktable by using the auto option in RGB levels module, but the colours are still way off.

Attached is a sample raw file and a jpg from what I can get in just a few clicks with RT and PS. The image is not a great one to begin with, but it’s a good example because it has lots of different colours and light contrasts.
Am I doing something wrong, can anyone find a good workflow that doesn’t involve lots of colour/tone adjustments in darktable? I’m cancelling my Adobe subscription soon. Thanks

SF - Mexico-25.CR2 (22.5 MB)

I edited your post so the link would work (just FYI).

Thank you! What did I do wrong?

Not sure, the link looked like it was trying to link an image? Usually you can just drag-and-drop into the editor to create an upload link. Probably just something silly.

Have you tried lut 3D module in darktable :
https://www.darktable.org/2019/05/New%20module-lut3d/

I used negative.png found from here:
http://rawtherapee.com/shared/HaldCLUT.zip
kuva

@yteaot the HaldCLUT titled “negative” is a simple linear negative, so not ideal for film which is non-linear. That’s why RawTherapee has the dedicated tool for film negatives.

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Yup. The new negative inversion tool is one of the primary reasons I gave RT a try initially.

I tried to use darktable a year or so ago for negative inversion, and was never able to get good results without a LOT of manual fiddling, to the point where I put all of the relevant capture equipment on the shelf until I sorted out the processing side of things. I was considering making some UI improvements to the darktable negative inversion tool (no sliders makes it impossible to finetune), RT’s tool leads to a MUCH more efficient workflow and its approach means I don’t have to do nearly the level of finetuning I found myself having to do in darktable.

Thanks for the tip. I hadn’t tried it, but I have now. I do get better results after applying the lut and then white balancing, but there are still a few issues with it. Firstly, certain colours show strange behaviour, such as the yellow in the Corona bottle and the reds at the bottom of the tree. Also, the lut 3D module is applied later in the pixelpipe so certain modules are then reversed, such as exposure, color blance, etc. This makes further editing a real pain because colours and tones are back to front.
I’m actually wondering if there’s a bug with the Invert module and/or a colour management issue here. I was experimenting with different input profiles and I was getting better results with sRGB and Adobe RGB instead of the default enhanced colour matrix. There are still big tonal problems with this route, but I’m not getting the strange hypersaturation problems.

Yup, I’m having the same issues. It looks like it is possible to get a good image with darktable eventually, but it requires a lot of manual work to get there. The purpose of the invert module should be to get you 80-90% there, just like in RT, but it’s only getting me about 50% there at best. I’m also getting some strange colour/tone behaviour with it, which might be something in the workflow I’m not understanding, but it’s been frustrating me for several days now.

Here are two examples of what I think is strange behaviour.

The first screenshot is when I first apply the invert module using a sample of the unexposed negative. This is before anything else except for crop/rotation and with no base curve. Even after WB, the colours and tones are way off, a lot of processing is needed after this. Is this the way invert is supposed to work?

The second and third screenshots are from reversing a tone curve instead of using the invert module. Some WB and basic levels adjustment has been done. Certain colours become hypersaturated and take on an unusual hue. I have not seen this with regular raw files, nor have I seen it when processing colour negatives with RT and PS. I’m not sure if this is related to my problems with the invert module, but I’m wondering if it’s pointing to a larger colour management issue?
If anyone could shed light, would be appreciated.


Capture%202
Capture%203

Have someone used RawTherapees Film negative module to this photo?

DCP(automatic), film negative, crop, white balance, tone curves.
First time I use the film negative, 3 minutes work.

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Yep, RawTherapee does a great job. Have you tried with darktable?

Regrettably no, as learning DT after RT and GIMP is too much for my brain :flushed:

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The issues you describe sounds like not getting the inversion film color EXACTLY right.

If you don’t get it EXACTLY right, you wind up with really strange color artifacts in the shadows - kind of similar to what happens if a raw converter botches the black level. It’s nearly impossible to get the level exactly right with a colorpicker like darktable’s invert module.

This is part of the reason why RT’s inversion tool does NOT clip at the “black” point, and you must manually bring that down to zero. I usually use the LAB tool - really any tool that clips luminance instead of individual R/G/B channels should be OK.

Edit: I originally didn’t like the fact that the inversion tool didn’t clip, but I adjusted pretty quickly. A good trick: Put your cursor over some of the unexposed negative. Look at the L* value - then go to the L* curve of the LAB tool and right click the lower leftmost point. Type in the value you saw for the unexposed film as the input value and leave output at 0.

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Thanks for the tips. Just to clarify, when you talk about the LAB tool, are you referring to darktable or RawTherapee? I’m getting really good results with RT, but not with darktable.
I also think something else is going on other than just problems with the invert module in darktable.
As an experiment, I opened up the exact same RAW file with GIMP, one using RT as the raw converter and one using darktable. I then did an invert in GIMP. You’ll see from the screenshots that the darktable one has those weird yellow artifacts in the Corona bottle.
I didn’t change anything in RT or darktable after opening the files. I just closed them and they went straight into GIMP with the default conversion settings.
Is there something wrong with my color management settings? Everything is default.


I think the key is to set input color profile to a log-encoded profile such as sRGB or AdobeRGB (the second one gives a better result perhaps because of its larger color gamut).

So, here is my workflow for negatives in darktable:

  1. Set input color profile to AdobeRGB
  2. Enable invert iop and select a part of unexposed film material (here I selected the bottom-right part after rotation). The bigger the selected area is, the more accurate the white balance would be.
  3. Do white balancing (here I selected the white part of the bottle cap in the bottom-left part of the photo)
  4. Fix exposure

The result of the above workflow on this photo will be something like this, which is a good starting point in my opinion:


darktable 2.7-git: SF - Mexico-25.CR2.xmp (3.3 KB)

Here is my final result after some more fine-tuning:


darktable 2.7-git: SF - Mexico-25_02.CR2.xmp (5.5 KB)

Hope this helps.

Thanks a lot msd. Yes, I had got similar results by changing the input profile, but I was still wondering if this was the best way. Having come from Lightroom, I’m not used to changing input profiles, so wasn’t sure if this was something I should be doing. I also find that there’s still a fair amount of manual tuning needed with this route, which makes me wonder how useful the Invert module is.
So should I be using something like the Adobe RGB input profile for all my shots? Regular RAWs seem ok with the default enhanced colour matrix, but my experience with these colour negatives makes me wonder how it’s processing colours. The exposure is also too dark if changing input profiles.

My pleasure :slight_smile:

Actually, it’s not the best way IMHO since it might affect color accuracy. If “unbreak input profile” supported gamma values more than 1, it would be better to keep “input profile” as standard/enhanced color matrix (just a guess. I’m not sure about this). One may also build a reverse gamma curve using base curve but it’s a tedious task for me!

You shouldn’t change the input color profile for your regular shots. The need for changing it for negatives comes from the fact that the film negatives are inherently gamma-encoded in opposed to the linearity of “regular” shots.

Ok makes sense.
So I guess my best bet is to keep using RawTherapee for colour negatives. It produces good results in a very short space of time. It’s a shame that darktable doesn’t have the same colour neg support because it has so many more features than RT for other processing tasks. I was hoping to just use the one program, but it’s no big deal.