LAB Support in Darktable

Software darktable
1

I’m currently reading through Photoshop LAB Color - The Canyon Conundrum by Dan Margulis, which was recommended to me as being the definitive guide to processing in LAB. I’m really excited for how much support Darktable has for LAB, but I am unsure about how to configure Darktable as per some of the instructions, for example:

  • he recommends inverting the orientation of the “tone curve” module so that darkness is to the right and lightness is to the left
  • he recommends setting the curves default for highlights and shadows to not quite pure white or pure black (you can do this in Photoshop via Image - Adjustments - Curves - Auto - Highlights or Shadows - and setting the L channel to slightly lower than 100 or slightly above 0 respectively)
  • be able to pick a pixel and see the LAB values (e.g. 90L 10A 20B)

Are any of these things possible in Darktable? Thanks!

2
  • Reversing the curves is not possible.
  • You can try to tweak the basecurve and store it as an auto-apply preset. The defaults are trying to mimic what in camera processing does but more often than not that crushes blacks and whites.
  • The color picker is on the left side of darkroom. Set it to Lab and you get what you want.
3

Thanks for the quick reply! The base curve module operates in RGB, but I
want to modify the L channel without affecting the colors, so I don’t think
it will work for this purpose. I tried dragging the endpoints on the tone
curve module (which I know does operate in LAB), but I can’t seem to just
slide the endpoints in without affecting the curve. Ideally I’d like to
slide them in a bit (leaving the curve as a now less-wide straight line),
and then make adjustments to the curve after that. Is that possible?

4

Right, basecurve isn’t in Lab.
Moving the endpoints of the tonecurve works fine for me. But I am not sure what you mean with “less-wide straight line”. Could you sketch that quickly and show it as an image? I guess that’s easier than describing it.

5

This is an example of adjusting it in Photoshop:
https://goo.gl/images/qr4w1k

6

Do mean something like that?

7

@pk5dark was quicker, I basically took the same screenshot. Just drag the ends of the line in.

8

What would be the advantage or benefit of this change?

9

@houz @pk5dark - that’s exactly what I’m looking for - for some reason when
I tried last night, dragging the curve endpoints in resulted in the curve
bending as well. I’ll give it try again later. Is it possible to set
specific values for the endpoints, or not really?

As far as inverting the orientation of the “tone curve”, I think the author
just finds looking at the curve in that orientation to be more intuitive

10

I find dark on the left intuitive.

Very sinister :stuck_out_tongue:

1 Like
11

I looked it up and it sounds like the reason he prefers lightness to the left is because this is how Photoshop orients the curves for CMYK and grayscale images. Perhaps this helps when converting to CYMK for printing?

Also I can confirm that I was able to adjust the “tone curve” tool as expected now too. Is it possible to set specific values for the endpoints?

12

No.