I've seen a few plugins, most notably
+++ RealiDrums by Realitone +++ that have slide controls to change the density or complexity of the midi output.
Lets say for a moment each track had a complexity fader. The logical place for the fader is to add a fader to each track in the mixer.
Lot's of track settings could be put shown in the mixer but are not. For me only the fader metaphor makes the mixer a choice for putting complexity settings.
But I assume that the method of setting the initial complexity value in your model doesn't matter.
I hadn't had a song setting for complexity in mind. The simpler approach is IMHO that any bar setting overrides the song setting for a track (which in turn could inherit from a global song setting).
The option to use relative value has its merits as you can play with the initial value to figure out the best overall complexity. Percentages will make only sense if the initial value is fine grained (as a fader implies). If we get "only" a scale from 1 to 9 (5 being "normal") an amount will do fine.
If the bar setting is implemented as a plain text field all three options could be available:
* number without sign => fixed amount
* number with sign => relative value
* number with a % suffix => relative value in percent
That's probably what be done in Reaper, king of tweaks.
The drawback is that the user may enter invalid data (eg letters). What's more he has to remember all the possible formats. Great for power users, not newbie friendly.
As an alternative two drop-downs may be used. One shows the measure (amount, relative, relative in %) the other the possible values. For % the dropdown will show only values in steps of 10 (or replace thevalue drop down with a text field).
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