Sharpen and Blur Per LAB Channel

Software darktable
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1

I’m reading Photoshop LAB Color by Dan Margulis, which outlines several powerful postprocessing techniques in LAB. Many of these can be performed using the “tone curve” module, however in Chapter 5 he talks about sharpening and blurring. In particular, he notes these basic rules for reducing noise effectively and also sharpening the image:

  • sharpen the L channel
  • blur the A and B channels separately depending on the noise in each (using a Gaussian blur)

How can I do this in darktable? I can sort of achieve this using the “lowpass” module and using “Lab color” as the blend mode (I think this is the same), however I think it only allows me to apply the “lowpass” blur to both A and B uniformly, not individually. How can I blur just A or just B?

Regarding sharpening, I think I can sharpen the L channel by itself by using the “sharpen” module and the “Lab lightness” blend mode - is this correct? Moreover, does darktable have an equivalent to the Photoshop “Surface Blur” filter? He recommends this over Gaussian if available

Related to this, it would be really helpful if I could see each channel individually, so I can assess how much noise is present in each channel. Is there a way for me to show just the L, A, or B channels by themselves in darktable?

1 Like
2

I’d try a separate instances with an accent on color of noise. Maybe there are better techniques, though.

3

How do I do that exactly? I tried a parametric mask and selected either a
or b, but I couldn’t seem to get the selection to apply to just that channel

4

So, here’s material you can use as an example or, at least to figure out what exactly you need to do with a mask
https://youtu.be/_Yq60o5yQsg?t=6m7s

I understand you want to grab the entire channel, but I’m not sure if it is possible with single instance/ Hence how he uses the mask and if one is not enough - use multiple instances
I suggest you use the Equalizer module too - it can blur and sharpen an image with wavelets algorythm so it might match your needs

5

Thanks, that video demonstrates that technique well. It seems like you could use two copies of the module as follows:

  • copy 1: drag left sliders in channel to 0; apply settings
  • copy 2: drag right sliders in channel to 0; apply settings

Would this effectively apply the effect to the whole channel? One thing I don’t understand i when I did this, I could not use the “normal” blend mode; I had to use “Lab color”. Why is this? If I’m making a mask only on the B channel, why would it apply to the other two channels?

6

Yes, that’s what I meant.
Sorry mate, I can’t tell whether this solution will be effective, there is only one way - try it on your image. I suggest trying those instances with LAb color blending, consequently switching a/b channels in each instance, while keeping the L-channel off.
What exactly is different with Normal an Lab color mode?

7

With the new blend mode you can now blur the A and B channels separately in the low pass module. Create 2 instances and set one to blend mode Lab a-channel and the other to Lab b-channel and adjust the blur radius as needed. It works quite well.