Allow transformation of widget connected controls.

[Mark Pitt]

When writing a custom skin for Mixxx in the <connection> tabs have the ability to reverse a knob or sliders value i.e. rather than gain being increased by turning the knob clockwise instead it decreases.

[RJ Skerry-Ryan (rryan)]

We should probably support a general transformation syntax like this:

<Connection>
  <ConfigKey>[Channel1],volume</ConfigKey>
  <Transform>
    <Invert/> <!-- unary minus operator -->
    <Add>5</Add> <!-- add 5 -->
    <Multiply>10</Multiply> <!-- multiply by 10 -->
    <Divide>20</Divide> <!-- divide by 20 -->
  </Transform>
</Connection>

This encodes the operation (-parameter + 5) * 10 / 20

This is a much more restricted expression language than it could be. For example, we could do it like this:
  <Transform>
    <Multiply>
      <Add>
        <ControlParameter/>
        <Constant>5</Constant>
      </Add>
      <Divide>
        <Multiply>
          <ControlParameter/>
          <ControlParameter/>
        </Multiply>
        <Constant>127</Constant>
      </Divide>
    </Multiply>
  </Transform>

Which encodes (parameter + 5) * (parameter^2 / 127).

But this wouldn't be trivially reversible without building a generic equation solver into Mixxx ;). We have to be able to translate from and to the transformed value space.

[Daniel Schürmann (daschuer)]

Hi RJ,

I think widget control connection is complicated enough.
For me the value skaling is a core part of Mixxx, not Skins.

The original request is asking jut to reverse the orientation of a control.

So what about this:

<KnobInverse>
<SliderComposedInverse>

Or this:

<Connection>
  <ConfigKey>[Channel1],volume</ConfigKey>
  <Invert>true</Invert>
</Connection>

Do you heave valid use case for other scalings?

Kind regards,

Daniel

[RJ Skerry-Ryan (rryan)]

The main use case is inversion, so yes an <Invert> would work. Not all controls are normalized from [0, 1] so it may be useful in some cases to do arbitrary functions to them. This would be purely for skin-based customizations.

Of course, if the transformation makes sense at the control level then we should e.g. create a new CO behavior that does the transformation there (so that MIDI will have the same transformation).

Even if <Invert> is the only supported option at first then I'd leave room for future expansion by putting it in a <Transform> block.

[Daniel Schürmann (daschuer)]

A <Transform> block is OK to me.

[RJ Skerry-Ryan (rryan)]

https://github.com/mixxxdj/mixxx/pull/157

[Swiftb0y (swiftb0y)]

Mixxx now uses GitHub for bug tracking. This bug has been migrated to:
https://github.com/mixxxdj/mixxx/issues/5724