[LV2] Looking for a way to implement parameter name/range updates in JUCE+LV2+Ardour

[Albert Graef]

Hi all,

This discussion started in
https://github.com/plugdata-team/plugdata/issues/1034, but David suggested
that I bring it up on this mailing list so that we can hash out the details
and maybe craft an implementation that will make this kind of dynamic
plugin changes work in LV2.

The dynamic changes in question are the ability to adjust automation
parameters on the fly, specifically their names and ranges. This
functionality is being used in plugdata, which lets you run Pd patches as a
plugin in a DAW, and supports various plugin formats (including LV2) via
JUCE. pludata is like Ableton's Max for Live
<https://www.ableton.com/en/live/max-for-live/>, but entirely open-source
and based on Miller Puckette's Pd <https://en.wikipedia.org/wiki/Pure_Data>.
It's a highly dynamic environment which lets you change the name and ranges
of automation parameters exposed to the plugin hosts. This is really a
necessity, as Pd patches keep changing all the time (in fact, plugdata is a
full-blown Pd-like environment which lets you edit the patches right there
in the plugin editor), and so frequent changes of the automation parameters
(which are represented as as special param Pd objects in the patch) are
expected and should be doable without much ado.

This already works with the AU, CLAP, and VST3 plugin formats, since the
corresponding host APIs support these kinds of changes, and they're
supported in the corresponding JUCE wrappers as well. (I haven't tested all
of these myself, but I did test the plugdata VST3 on Linux and Mac and it
properly supports these.)

LV2 doesn't at present, which is a noticeable limitation of plugdata's LV2
variant, so we'd like to change that. I have to admit right up front that
this is not something that the vast majority of plugins needs. It
specifically caters to the needs of highly dynamic plugins like plugdata,
of which there aren't many. But it's rather important for plugdata, so
there's sufficient incentive to make it work; LV2 should be at least up to
par with VST3 on that IMHO.

David suggested that the easiest and most practical way to go about this
would be to utilize LV2's patch messages (http://lv2plug.in/ns/ext/patch).
This should be flexible enough to send messages to the effect of
plugin->rename_param(char
*current_name, char *new_name) and plugin->change_param_range(char
*param_name, float minval, float maxval) to the host, which should do the
trick. Of course this would have to be implemented in the corresponding
JUCE method (presumably that's ParameterStorage::audioProcessorChanged() in
juce_audio_plugin_client_LV2.cpp:210
<https://github.com/juce-framework/JUCE/blob/d24c2729268e322f3ba1b5070eb96ab232d7f6ba/modules/juce_audio_plugin_client/juce_audio_plugin_client_LV2.cpp#L210>,
which currently doesn't do anything), and on the Ardour side in the LV2
host code which needs to listen for those messages and then do whatever is
necessary to implement those changes (we should hopefully be able to snitch
this from the corresponding VST3 host code in Ardour).

I'm willing to do the necessary coding myself and embark on crafting
corresponding pull requests for JUCE, Ardour, and plugdata, as necessary.
But I need help on how to implement these changes using
http://lv2plug.in/ns/ext/patch. If anyone has any code snippets
demonstrating how to utilize these patch messages on both the plugin and
the host side, please toss them my way, so that I can study them. Thanks. :)

Robin, I'll probably need your help as well in order to figure out where I
need to make the relevant changes in the Ardour source which still looks
like a vast abyss to me. ;-) Or if you'd be willing to add the necessary
changes to Ardour yourself, that would be much much appreciated!

Alright, I think that this should be enough to kick off the discussion.
Please check https://github.com/plugdata-team/plugdata/issues/1034 if
you're interested in further details on how this relates to plugdata. I'm
on vacation for the next three weeks, but I'll try to respond to your
replies as soon as I can.

Best,
Albert

-- 
Dr. Albert Gr"af
Computer Music Research Group, JGU Mainz, Germany
Email: aggraef at gmail.com, web: https://agraef.github.io/
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