Blend modes explained

Hi - darktable newbie here but very impressed with the program so far. Great resources all over for learning. Some challenges I have, such as avoiding halos in some modules will just take time to figure out. But one area I’m interesting in understanding more is the blend modes in the modules. I’ve seen complex explanations of some that goes into the math and theory behind them, but I’m really looking for the layman’s explanation of what each mode is intended to do and when it might be used. Does anyone know of a good resource I can check out. I know the manual does give some descriptions but not all are covered and sometimes its not clear when the blend mode is really about.

I appreciate your suggestions.

I did come across https://www.youtube.com/watch?v=F7_kaTP7_W4 recently. Not specific for darktable, but i think it suits most software that uses blendmodes.

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Thanks - its a great video, still a bit more technical than what I was hoping for.

Two modes that I think that are most commonly used both by myself as well as in examples from othert are the ‘overlay’ and the ‘softlight’.
I often explore the different modes the get ideas on usage by simply scrolling through the list and looking at the different results. You might find some interesting/unusual results.

I know this one https://docs.gimp.org/2.10/en/gimp-concepts-layer-modes.html

It is from GIMP but the concepts should be similar.

Yes, I’ve seen a number of videos where they use those two. Also saw one where HSV lightness used in Channel Mixer but to make it work had to use gray destination, then crank red and slightly reduce blue then apply the blend mode to get it back to colour from B&W - not sure I’d even figure that out on my own!

Thanks!

It’s easy to figure out : don’t use modules that produce halos, they are inherently broken (masking and blurring in Lab cannot work). Not every bit of darktable is clean, I’m afraid, but you don’t have to use all the soft to get good pictures.

The layman’s explanation is “use this to do that” in this context, because it’s really all about maths and the link between what you do on RGB code values and what it means for photons in real-life. Remember most of image processing is just simulating numerically what use to be done with glass filters and light sources back then. Then, it depends whether you blend scene-linear data, or display-linear/gamma encoded data.

For example, the overlay blending mode expects display-linear data (where 0.5 = 50 % means middle grey) to work properly, whereas the multiply mode does not make much sense in display-linear (overlay is actually a tweaked multiply mode with a threshold at 0.5 and a sigmoid above). Soft-light is another variation on the same theme. They are often used to blend frequency-separated layers back :

  • blur(image) = low-frequency,
  • image - low-frequency = high-frequency,

then work separately on both (increase contrast on HF to get more perceptual sharpness, or decrease it on LF to smoothen skins without loosing texture), and merge them back with overlay when you are done (or soft-light for a more subtle effect). That’s exactly how the sharpen or high-pass modules work in darktable, and in most other softs.

Another example, multiplying a (scene-linear) image by a solid color layer simulates a real-life color filter (in the days of film, they liked to use yellow filters on their lenses while shooting B&W to get darker skies). On the opposite, dividing by a solid color allows to remove unwanted color cast (pick the color you want to remove).

So, I don’t think you can avoid the maths overhead… Sorry.

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Thanks! Not sure I quite understand all of this but I’ll keep working at it.

It took me a couple of years to figure that out, so give it some time to process and persevere.

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An interesting and useful way to use a blend mode is here: Darktable Tricks
It helps a lot to unterstand which part of the image is the module going to affect and how much.

PS: Also a lot more interesting tips and tricks about darktable in that thread.

Thank you - good advice. Also, just want to pass on my thanks for your work creating and updating the Filmic module - its outstanding! Also really appreciate you already having plans for V2 and V3 of it as described to Bruce Williams’ youtube video on the module.

That’s a great thread - thanks for pointing it out.