[LV2] Feature request: DSP write_function() ?

[Filipe Coelho]

The idea is that input ports can never be modified by the plugin.

If you want your plugin to be able to change its own parameters, you can 
already use the patch and parameter APIs together to achieve this.
It was a bit painful before, but host support for this is improving.
Qtractor, Carla, mod-host, jalv and synthpod support this. While Ardour 
has some level of support but not complete afaik.

Now, if you really want to keep using control ports and have a way to 
change parameters, see discussion at https://github.com/lv2/lv2/pull/42

Rui started a proposal already implemented in Qtractor and his v1 
instruments.
I dislike his approach because it requires the plugin to use an Atom 
port for notifications, which is overkill for simple plugins that do not 
need it for anything else.

I made my own extension which is just a feature with a function, but it 
is only a request that the host is allowed to say no to (this is quite 
important, as there might be parameter automation lines for the 
parameter in question, or being modified by some external source, or 
locked by the host for some reason)
See the spec of it 
https://github.com/KXStudio/LV2-Extensions/tree/master/kx-control-input-port-change-request.lv2
Currently only implemented in mod-host, but I will add it in Carla 
before the next v2.3 release

For surge, the best is very likely to continue the work on the 
lv2-parameters target branch. We started some little work, but it is not 
finished.


On 20/12/20 10:49, Sven Jaehnichen wrote:
>
> Hi,
>
> the old problem: control ports are READ_ONLY in LV2. Me and other 
> plugin programmers struggle about it. I know it has been discussed 
> here before and I know the workarounds (like parameters) and their 
> limitations.
>
> Actually I read in unfa's software chat that the Surge programmers 
> team are struggling about this problem too:
>
>> |We made this choice because parts of the current LV2 spec and Surge 
>> are incompatible in a couple of ways, most importantly in the LV2 
>> assumption that plugins never modify their control input port. 
>> Actions like ‘changing parameter types when an effect changes’ or 
>> ‘patch changes’ are not compatible with this design constraint. As a 
>> result, the LV2 - especially in Ardour - unreliably saves and 
>> restores state. The solution - which we are working on - is to use 
>> new LV2 APIs that allow current Surge behavior (and match what other 
>> plugin specs like AU and VST do).|||
>
>
> My request: LV2 UI can call the host to change control port via the 
> write_function(). I don't see such an option for the DSP instance. 
> Why? It would be nice to have it. Or are there restrictions why it's 
> impossible or problematic to implement?
>
> Best wishes
> Sven
>
>
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