[request] 3-way/single crossfader feature

This could be accomplished with a controller mapping by mapping each fader to increase the EQ knob for one deck and decrease the corresponding EQ knob for the other deck at the same time. You'll probably need to write a custom JavaScript mapping for this. Refer to the controller mapping documentation:
https://mixxx.org/wiki/doku.php/start#controller_mapping_documentation

You do not need to learn C++ or modify the Mixxx source code.

Thanks Be (be.ing) for your help!
Yes, your solution is something I already considered, but it has some drawbacks in my opinion.

The crossfader gets useless this way and has to stay in the middle, doesn't it?
And what if I need to adjust the frequencies of one track via the knobs? Is it still possible, without unpredicted/unwanted behaviour, at the latest when fading? Mode switching between 1 and 3 crossfaders would also not be possible this way.
I would rather prefer a clean solution.

The crossfader would still be valuable for manipulating the deck volumes with that setup.

If you want frequency crossfaders with that setup, you cannot also have detailed control of the frequencies per deck. This is not so much a limitation of Mixxx as it is a limitation of finite range potentiometers. Alternatively, you could get a controller with infinite encoders surrounded by LED rings such as a MIDI Fighter Twister and map some encoders to act like crossfaders and others to be normal deck EQ knobs. The tradeoff is that you would not be able to feel the position of the EQs without looking at the controller or the computer screen.

@Be

Thank you for your explanation! I just can't help myself... the mapping solution wouldn't feel right to me... more like a 'workaround'. Hope you understand that!
I would rather prefer the C++ solution.

@Daniel Schürmann

Thank you, too! The Photobucket link took a while to open.... and yes, exactly this 'Tri Faders' feature is what I want to model!

I have some (little) C++ knowledge .... adapted a board game in the past (~75% complete) and recently tried to enhance Clementines EQ with some additional bands for Tinnitus therapy.... nothing too serious so far, but I'm willing to learn!

Sounds like you already have a good idea how it could work. That leaves me optimistic.

Will check the link to the source code now and see if I can get an overview.

Where would be the right place to 'just ask'?

Here or at our mixxx-devel mailing list https://sourceforge.net/projects/mixxx/lists/mixxx-devel

Ok, noted - thank you both again for your efforts!

Just finished building a preliminary version of the MIDI controller with 3 faders and gave Be's suggestion from post #1 a try.
Although controlling the EQ knobs on both decks works fine , there is a major flaw in this solution.
When using the EQ knobs (instead of the fader) to fade to deck 1, it is impossible to listen (preview) deck 2 via headphones any more. Deck2 is silent, naturally, because all EQ knobs are down.
So unless there is a way to bypass the EQ output for the headphones, this solution will not work for me.

And I'm not so familiar with the Bus effects, but I got the impression that Daniel's suggestion in post #4, adding a separate EQ Rack effect per Deck, would involve the very same problem?

No, it works because the R/L effect hooks are post fader.

These effects are used in Shade skin. Using group_[BusRight]_enable, so you may be able to adopt your skript to this.

After https://github.com/mixxxdj/mixxx/pull/1254 is merged we have also an other hidden dedicated effect rack for such EQ Fader.
[MasterEffectRack_EffectUnit_Effect1]

So we may consider a combobox below the xFader, which routes the xfader position in a special way to this effect rack.

It would also be real cool if we could skript this routing.
If we now have also transport effects, we can really make cool transitions using the xfader sliders.

Also auto DJ will benefit from this a lot.

Yes! Finally! The scripting took me a while, but now I have a solution which works like a charm!
Thank you soooo much!!!

I can use either Bessel4 / Bessel8 LV-Mix EQ, or LinkwitzRiley8 EQ.
All of them do the job.
For now I focused on Mixxx 2.0.0, but think I will adapt the scripts later, when the beta arrives.

Also like your combobox idea for routing!
Wouldn't it be even possible to do this graphically, via bezier-curves in a graph or similar?
I mean, in the way like some VST EQs do this.
Then we would be able to 'paint' transitions 'on the fly', either with mouse, or with touchpad controllers..
Coloured curves could represent different transitions for faders, or effects... and so on...
This also would make it easier for people, who have little to no programming skills, to create their own routing.

Anyway, I'm so happy ( at least for now :D ) and want to thank the whole team, for making such a nice piece of software available on linux!

solved

Thanks Daniel Schürmann, that solved my question.

There is no blocking reason for other than this is a LOT of work.

I can think of extend the solution in the xfader preferences pane. Here the real gain calculation code is used to paint the visualisation.

So the user can write his transition script first and will see than the results.