ART basic question

I’ve proven to myself (more than a few times) that I have the ability to (very) occasionally understand relatively complex concepts yet completely stumble over simple ideas. :no_mouth: So I’m going to ask this very basic question Just To Confirm ™

Regarding the usage of two tone curves RawPedia says the usual practice is to “lower values using the first curve, and to raise values using the second one”. Visually that would generally look something like the first curve tends to sag downward while the second curve tends to bulge upward. More or less, right?

So are the two curves evaluated and then a resulting composite curve applied to the pipeline? Otherwise one would be applied serially after the other (and to the first curve’s result, not to the yet unaltered pipeline). Or is that the advantage of using two curves?

I understand – and don’t doubt – what RawPedia says, I’m just trying to grok why / how it’s better to use two curves. Is there some fundamental principal I’m missing? FYI, if you start pushing math my way I’ll probably try to politely disappear. :grin:

~2+ years ago I read (but very incompletely understood!) RawPedia pretty much in its entirety while playing with RawTherapee. I then pushed out of my brain whatever I learned by reading and playing with darktable. Now I’d like to get better with ART, so I’m trying to resurrect some of my lost RT / ART “knowledge”.


P.S. - Maybe I should just save and use this topic for additional Dumb Questions…


Rawpedia gets less and less accurate as time goes by, unfortunately. Not because of rawpedia, but because ART “Ain’t RT:stuck_out_tongue_winking_eye:
The two curves get combined together (and also the contrast slider is another curve that gets thrown in the mix) to produce by standard composition the function that gets then applied to the individual pixels of the image.
Why do you need two then? Just because of convenience: sometimes it’s easier to express a function as a composition of simpler ones rather than a single complicated formula… Nothing mysterious or deep I’m afraid :slight_smile:

Hello, do you mean “convenient for developers” here? As an end user / amateur photographer I’ve never understood why or when to use two tone curves when you can get the same results with one (at least in my simple eyes). And yes, I read RawPedia several times on this topic.

So I pose a general question to other users of ART and RawTherapee: do you use the two tone curves and if yes, why or when? In what circumstances give the two a better result? Curious for your answers.

Well, I meant for the users. What is convenient or not for the developer(s) is not very important :wink:

It’s not a matter of quality, just of convenience of the UI. Say you have your default curve applied, and you want to pull back the highlights a little bit. Which is easier, mode1:

or mode2:



Yes (in RT). If I’m happy with the automatched curve as a starting point then I’ll use the second tone curve to carry out any minor tweaks. Otherwise I’ll use one tone curve to set the black and white points and the other to carry out any further tonal adjustments. Just habit I guess.

I don’t use ART regularly but if I did, I could imagine being happy with just one curve and doing the fine tonal adjustment with the tone equaliser.


2 is easier. So it’s indeed a question of convenience as you said, and the two curves are not there to produce “better quality”. OK, that’s clear, thanks.

Thanks for your comment. I think it comes down to that word: habit.

In ART and RT I’ve the habit to start with the neutral profile, then tune with some exposure tools of choice. In ART I often start in the local editing tab. If that doesn’t work (rare), I restart from neutral and see what the auto-expo gives, and fine-tune again.

When still dissatisfied, I’ll mix some exposure tools. But it’s only seldom that I (need to) touch the second tone curve, in my “workflow” the thing comes in the last place. Just habit!

When still dissatisfied, it’s time to delete that photo! :wink:

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Reminds me of the old IBM unix joke: What does AIX stand for? “Ain’t Unix”…

I don’t suppose there’s any kind of reference on which RawPedia parts to ignore, eh? :slight_smile: I would actually be willing to help with ART documentation (as my time allows) but it’s difficult to document something one doesn’t yet know. At any rate I now understand about using two curves - Thanks!

I wasn’t sure if I was correctly interpreting what I read in RawPedia: “While you are free to use only one tone curve to make your adjustments, you can gain much finer tonal control if you use two curves at once.” I took that to mean using two curves is more granular (in a UI sense) than just one. For example, if in a screen / pixel / UI sense a move of X pixels on a single curve produces Y result, you’d effectively have X*n pixels available between two curves for the same result, basically making two curves n times easier to use (in a sense).

Yes, that’s also true, you have finer control indeed. But in principle if you enter the coordinates via right click you can probably achieve the same precision – actually I’ve never tried this though

As an example (applies, I think, equally to RT and ART):

If I’m postprocessing some Sony S-Log2 footage, I’ve found that it’s a lot easier to have a second tone curve that brings up the midtones and rolls off highlights, while using the “standard” S-shaped tone curve for the first, instead of trying to manually create a single curve that has a small “foot”, significant midtone gain, and a lot of highlight rolloff.

(In general, if I’m doing S-Log2 input, I’m creating a HALD CLUT to process video, and thus must avoid any local tonemapping operators such as the DRC tool that I would normally use in these situations)

That fits with what I heard in one of Andy Astbury’s videos (I think?) – Use the first curve for the fundamentals then tweak with the second.

I guess I’m mildly surprised / pleased that I actually understood something correctly, on my own!! :stuck_out_tongue_winking_eye: