Widget display contradicting value in a drag: value set by UI vs value known to the DSP, that bubbles up there

[p0nce]

Sometimes incredibly annoying, not sure if someone else has it.

[p0nce]

Reproduced with Distort + vsthost64 on ~master

[p0nce]

Is it conflict between those two?

Source: https://forums.steinberg.net/t/clarification-of-parameter-handling-in-vst-3/201914/2

[p0nce]

Fix need backport to v12, when the fix exists

[p0nce]

• Distort + VST2 + debug mode => no bug
• Distort + VST3 + debug mode => yes bug
• Distort + VST3 + release mode => yes bug, less often

[p0nce]

Is it buffer splitting? from Sep 2021

• Distort + VST3 + debug mode + Dplug v11.0.0 => no bug

[p0nce]

Found an (unrelated?) pretty bad performance issue, parameter tracks are too large

[p0nce]

What happens is that double y1 = getParamNormalized(id); fails and return zero, it gets reinjected then
The ParamID seems bad

[p0nce]

The bad ParamID is VSTHost-only, there is nothing we can do unless they fix that bug.
On Ableton Live, it's a different thing.

[p0nce]

REAPER trace when dragging a slider

[9672] setFromGUI( 0.816920 )    // onMouseDrag setting the value
[9672] Param value = 0.816920   // drawing
[9672] setFromGUI( 0.820766 )    // onMouseDrag setting the value
[9672] Param value = 0.820766   // drawing
[9672] Parameter changes for ID = 3 // audio process
[9672] setFromGUI( 0.824612 )     // onMouseDrag setting the value
[9672] track 3  value[0] 0.820766   // process
[9672] setFromGUI( 0.828458 )     // onMouseDrag setting the value
[9672] track 3  value[1] 0.820766   // process
[9672] Param value = 0.828458      // drawing
[9672] track 3  value[2] 0.820766
[9672] setFromHost( 0.820766 ) with tag 0
[9672] setFromHost( 0.820766 ) with tag 0
[9672] setFromGUI( 0.824612 )
[9672] setFromHost( 0.820766 ) with tag 1

So even in REAPER, things are a bit unsynchronized because the UI change happens before the audio process, then process happens with former values.

[p0nce]

Ableton Trace:

[17680] setFromGUI( 0.353846 )    // UI moving slider in onMouseDrag
[17680] Param value = 0.353846    // draw
[17680] setFromGUI( 0.346154 )    // UI moving slider in onMouseDrag
[17680] setFromHost( 0.369230 ) with tag 17   // setParamNormalized
[17680] Param value = 0.369230  // draw slider
[17680] setFromGUI( 0.361538 )   // UI
[17680] setFromGUI( 0.261538 )   // UI
[17680] Parameter changes for ID = 3
[17680] setFromGUI( 0.253846 )
[17680] track 3  value[0] 0.361538   
[17680] setFromHost( 0.361538 ) with tag 17 // setParamNormalized
[17680] track 3  value[1] 0.361538
[17680] Param value = 0.361538
[17680] setFromHost( 0.361538 ) with tag 0  // audio callback
[17680] setFromHost( 0.361538 ) with tag 1   // audio callback
[17680] setFromGUI( 0.346154 )
[17680] setFromGUI( 0.353846 )
[17680] Param value = 0.353846
[17680] setFromGUI( 0.330769 )
[17680] Param value = 0.330769
[17680] Parameter changes for ID = 3
[17680] track 3  value[0] 0.353846

Ableton just have a little bit more of interplay, since the host is also sending setParamNormalized.
So why does VST2 doesn't have that in Ableton?

[p0nce]

In Reaper, dragging a slider in VST2 doesn't lead to a call to setFromHost.
In Ableton, dragging a slider in VST2 send a single setFromHost, but at the point where the Edit has finished.
Hence, slider movement is much more fluid in VST2.

[p0nce]

• Does the problem also exist in LV2? => yeah
  REAPER trace:

[11540] Param value = 0.286893  // draw
[11540] setFromGUI( 0.282507 )
[11540] Param value = 0.282507 // draw
[11540] setFromGUI( 0.278121 )
[11540] Param value = 0.278121 // draw
[11540] setFromHost( 0.278121 ) with tag 15
[1896] DOLBY notifier : Receive NewSession Event
[11540] setFromGUI( 0.273735 )
[1896] DOLBY notifier: Process Name: reaper.exe
[11540] Param value = 0.273735 // draw
[11540] setFromGUI( 0.269349 )
[11540] setFromHost( 0.273735 ) with tag 15   // THIS COULD MAKE A MISDRAW
[1896] DOLBY notifier : Receive NewSession Event
[11540] setFromGUI( 0.264964 )
[11540] setFromHost( 0.269349 ) with tag 15
[11540] setFromGUI( 0.260578 )
[11540] Param value = 0.269349 // draw
[1896] DOLBY notifier: Process Name: reaper.exe
[1896] DOLBY notifier : Receive NewSession Event
[1896] DOLBY notifier: Process Name: reaper.exe
[11540] setFromHost( 0.260578 ) with tag 15
[1896] DOLBY notifier : Receive NewSession Event
[11540] Param value = 0.260578
[1896] DOLBY notifier: Process Name: reaper.exe
[11540] Param value = 0.260578
[11540] Param value = 0.260578
[1896] DOLBY notifier : Receive NewSession Event
[1896] DOLBY notifier: Process Name: reaper.exe
[11540] Param value = 0.260578
[11540] Param value = 0.260578
[1896] DOLBY notifier : Receive NewSession Event
[11540] Param value = 0.260578
[1896] DOLBY notifier: Process Name: reaper.exe
[11540] Param value = 0.260578
[11540] Param value = 0.260578
[1896] DOLBY notifier : Receive NewSession Event
[1896] DOLBY notifier: Process Name: reaper.exe
[11540] Param value = 0.260578
[11540] Param value = 0.260578
[1896] DOLBY notifier : Receive NewSession Event
[1896] DOLBY notifier: Process Name: reaper.exe
[11540] Param value = 0.260578
[11540] Param value = 0.260578

So there is some kind of need need for UI to ignore parameter values when set by the host, if the widget is dragging...

[p0nce]

The fundamental problem is that parameter value as seen by the UI/user is different than parameter value as seen by the DSP/automation, and sometimes the state need to be separated for a while. On the plus side, the bug isn't terribly unworkable.

Having two IParameter interfaces for those could help.

[p0nce]

Summing up the issue:

Only a problem if the same widget sets and listen to one Parameter

[p0nce]

Ideas to fix that:

• in onDraw, if isDragged, use last submitted value as ground truth instead of current parameter value
  => all widgets need to change, but only those where the problem shows. This can be a new Parameter accessor.
• there is a valueForDSP and valueForUI storage for each parameter. setFromHost doesn't modify valueForUI during an edit. All widgets get parameter value with a special accessor. value() deprecated.
  => all widgets need to change

Thought: if we know the parameter is going to change to whatever the UI said, why not give the value given by the UI a higher priority? And refuse the host changes even for processing audio.

[p0nce]

What calls are used by Widget to get parameter values?

• draw: getNormalized(), value(), valueAtomic()
• animation: valueAtomic()
• clic: value(), getNormalized()

[p0nce]

So, there is a simple solution to this, probably need much testing:

    /// From a normalized double [0..1], set the parameter value.
    void setFromHost(double hostValue)
    {
        if (isEdited())
            return; // If edited by a widget, REFUSE host changes, since they could be 
                    // "in the past" and we know we want newer values anyway.
        setNormalized(hostValue);
        notifyListeners();
    }

[p0nce]

• REAPER and Live + VST3 => OK, now fluid
• VST2 + REAPER and Live => OK

[p0nce]

Review all SetFromHost calls and see if that's sustenable => likely yes.

• test on macOS
• Backport eventually => no, too dangerous. In the new fix, calls to beginParamEdit/endParamEdit need to be perfectly balanced, this was only checked in debug before.