darktable ep 053 - The Tone Curve module

[@YuryKhrolBY]

Thanks Bruce for yet another great video on Darktable! Though it seems like there is a bit of confusion in this one with Lab independent channels colorspace. Color channels (a and b) don't work on lightness scale, but colors instead: b channel x axis goes from the most saturated blues on the left, through 0 saturated center, towards the most saturated yellows. And it has negative values for blue, positive for yellow with a 0 value neutral grey in a center. So when you drag the left part of the line down you're adding negative values to negative values (sorry if my English is confusing), i.e. make blues even more saturated and not adding blue tint to the shadows as you suggested in video. If you drag the left part of the line up, you're adding positive values to negative ones - adding yellow to blue, i.e. desaturating the blue. Central point is fixed as it represents neutral grey colors and shouldn't be affected. When you move the central point up or down you're adding the color tint to the whole image (positive values in b-channel give yellow tint, and negative blue tint) It is probably confusing that color channels in Lab colorspace has negative values and scales goes from -128 to 128, while L channel as well as combined color spaces use 0 to 100 scale.

[@audio2u]

Thanks for the clarification!

[@philosynth]

Just discovered this channel and subscribed immediately. Very clear explanation! Thanks!

[@audio2u]

Thanks, glad you found it helpful!

[@wido123123]

As an amateur guitar player and amateur photographer, I agree with your initial statement: I. LOVE. TONEEEEEEE

[@markdurston4658]

Very well explained Bruce. Thanks for taking the time. Would be great also to see a demonstration of the practical uses of the various options and functions. For me personally, that would help me decide when to use this module, and which images would benefit most from it. Perhaps at some point a "tone curve part 2" video tutorial? I'm going to study the manual as well, to see what it has to say.

[@audio2u]

Like I said, I use this module on 99% of the images I post-process. And generally, for the three things I described... Set a black point, set a white point, and apply some mid-tone contrast.

[@FairMiles]

Hi Bruce. Just FYI, you did not explain in this video how to set the white and black points with this module by moving the corner anchors (maybe you can add it in your next one if that was your intention)

[@BillWohler]

I was thinking the same thing. I think Bruce covered the white and black points in another video, but it would be easier to find in a Tone Curve video. I'm away from my computer so i can't test, but I'm wondering if dragging those corner points would help explain the color swatches in the lab space.

[@BillWohler]

Spoke too soon. Discussed in a video for Patron subscribers...

[@JulesMoyaert_photo]

Thank you, Bruce! Nice!

[@audio2u]

My pleasure!

[@emrg777]

As always nice video Bruce...one thing I often do for an over or underexposed image is apply a linear tone curve...so don't change anything...then use screen or multiply and tweak with opacity...its amazing how this one small adjustment can really impact overall tone without messing with contrast sliders and the like....you can tweak the curve but often I leave it and go on to a new instance of the tone curve if I want to further tweak things.....just a further tweak that I thought I would share...I have also found the presets handy ....as quick starting points for edits...

[@audio2u]

Cool. Thanks Todd!

[@DenisThibeault_inci]

Great explanation of the module - your video clarifies a lot for me, thank you.

[@pearcem66]

So glad I found you. Thanks Bruce👍🏼

[@audio2u]

Thanks Mark!

[@charleshacker765]

I'm glad you mentioned what you did about the background color swatch in the Lab. Not all light pixels are magenta, for example! I think this is probably your most thorough explanation video. No doubt because it is one of your favorite tools. It would be nice to see an episode that focuses on your preferred modules.

[@audio2u]

Thanks!

[@JillesvanGurp]

One use of the AB channels is to simply steepen the curves to increase saturation. Remove the center node and drag the extremes to the middle ensuring the line crosses through the center so you don't introduce color casts. This is a really neat way of increasing the saturation because you get a lot of control. If you combine it with a parametric mask on the saturation channel, you can easily add some vibrancy to dull image without over saturating already saturated colors. I have a preset for this that I use all the time.

[@audio2u]

Nice one!

[@vs9400]

Thank you so much for those videos. They are the best available.

[@audio2u]

Glad you like them!

[@joaovtaveira]

There's also a eye dropper that is very handy. I use this module quite a lot. Congratulations for another great video.

[@audio2u]

Thanks!

[@dirkosswald3712]

apropos eye dropper: When the eye dropper is enabled, then you see a single point as a straight line in the tone curve histogram only. Good. But when you then use the the color picker module (the one on the left side in darkroom) in area mode then you get the span of all the colors / luminosities shown in the tone curve histograms. Sweet.

[@wido123123]

Hi Bruce, I think I know why the LAB graph behaves that way. While it may not be intuitive, you have explained that when you move the curve down, you're moving the input values into the output (darker) values. When you do that in the A or B channel, you are doing the same (for example, moving the green values into the magenta ones). To me, that makes sense, even if it doesn't match the colors below. Also, I tend to apply a bunch of contrast curves, so I tend to use the independant LAB in order to have less saturation, or to do a slight white balance (the B channel works great to do just that, moving so slightly the mid point up or down)

[@liberdovat]

Yes. I was thinking this also. You have to separate the rectangle into two triangles; the triangle below the 45 degree white line adjusts greens in the shadows/midtones/highlights, and the upper triangle adjust the magenta in the same areas. Once you think it in this way it becomes a bit clearer.

[@audio2u]

Interesting. I'll have to experiment some more.

[@WayneRiesterer]

Great tutorial as always. This is a great module and very similar to DaVinci Resolve's Custom Curves so, for me at least, it provides a similar tool for colour grading. Thanks for posting this up :)

[@audio2u]

Glad it helped! 😃

[@alphabeta448]

Hey Bruce, thanks as always for your super useful videos. Did you notice that, if you hover with the mouse over one node in your curve, you can move it by hitting the arrows in your keyboard? That's very useful to me and kind of compensate for the fact of not being able to input a numeric value like you do in all other modules.

[@audio2u]

No, I did not know that! Thanks!

[@dirkosswald3712]

And you can move the node up or down with the mousewheel when you are close to a node. Super practical since this restricts to only move up or down (and not left and right).

[@cosmo0080]

greetings from Ontario very interesting when u tell us you do not know about something u make me want to look for the answer Merry Christmas

[@audio2u]

I try to be honest about the bits that I DON'T know! 😃

[@cosmo0080]

@@audio2u the good think about that is it can be our homework now we have a more interactive situation and maybe someone can post the answer making it better for all Blessing

[@alangaughran]

Thanks for the Tone Curve description. On this and a number of other modules, there seems quite a lot of duplicated functionality in Darktable. Have I missed a point here - and if so could you set me straight. The key similarity here is to the Base Curve, but the latter is applied by default on my RAW imports. You're having a torrid time over there at the moment with the fires. I hope things settle soon.

[@audio2u]

Tone curve and base curve are similar. Automatic base curve can be disabled in preferences. And yes, as I've said multiple times across my videos, there are multiple ways to skin a cat in darktable! 😃 A lot of modules can do similar things to other modules.

[@alangaughran]

​@@audio2u - Thanks. This is what KZbin should be like across the board.

[@finyo83]

What’s the difference between the base curve and the tone curve? Why would someone use one rather than the other? Can they be used in conjunction with each other? Thanks!

[@audio2u]

I'm not 100% sure, but I think they do similar jobs. It's just that the base curve module has presets named for different camera brands, and darktable will automatically apply the relevant base curve based on the EXIF metadata. Can you use both? Sure. But you most likely will end up with something that looks over processed. If you shoot raw, you might like to disable the base curve and process the image manually. In one of my videos, I explained why I don't like the base curve.

[@folmonty]

Since installing V3.0 I noticed the tone curve module has changed a bit. Do you like the changes?

[@audio2u]

You mean Tone Curve RGB? I haven't been shooting much lately, so I haven't really had a chance to delve into it. But there'll be a video coming on that topic.

[@fotografie-joerglang]

Super good explanation for this module, thank you very much and Greetings from Germany :-)

[@audio2u]

Thanks Jörg!

[@autoauto2000]

Ford Falcon...nice. Yours?

[@audio2u]

Nah, just a prop for a photo shoot.

[@EstebanFonsecaAR]

While on the LAB separated channels mode, you can try LAB saturation by stretching both A and B channels, both channels needs to have same values top and bottom. :) Got that from a LAB saturation tutorial in Gimp here: kzbin.info/www/bejne/foDXiaysmK1qfc0 Of course in darktable it is much faster :) but if you watch the video, you will get the idea. :)

[@kr0rjsh]

Hello Bruce, Tone Curve and RGB Curve. Are those doing the same job?

[@audio2u]

Yes. I think the original Tone Curve module works in the LAB colour space, while RGB Tone Curve, as the name suggests, works in the RGB colour space.

[@kr0rjsh]

@@audio2u Thank you. One thing I noticed is that you can adjust the luminosity and colors using Lab in a single instance of Tone Curve module, but with RGB, one need to create multiple instances, one each for total Luminocity and the independent colors. Have you got a chance to look at that?

[@audio2u]

I hadn't thought about it, to be honest, but I guess that makes sense. I will confess to not really understanding Aurélien's argument for RGB over LAB workflows.

[@JohnWilmot1179]

I've just installed Darktable and in the tone curve section I don't see any line, the gray square is empty...

[@audio2u]

Hmm, that's weird. Not sure what to suggest there.

[@JohnWilmot1179]

@@audio2u I'm so annoyed cause it's such an important module... And I don't know what to do... And I don't even see the histogram... the rectangle is all gray

[@audio2u]

You're on v3.4 though, right? So you can still access it through the "deprecated modules" subset in the module manager.

[@JohnWilmot1179]

@@audio2u Yeah. It's the 3.4.1 version, the latest one. In the "deprecated modules" I don't find the "Tone curve"; There i find only these modules: channel mixer, fill light, zone system, global tonemap, tone mapping and invert... Thanks for your reply

[@audio2u]

Oh ok. So, if you still want to use it, find an old photo you have processed with the zone system. When you edit that image in the darkroom, you will still have access to the ZS module. Create a style, and choose only the ZS module. Now you can apply that style to any new images, and you will have the ZS module.