Tim Hockin wrote:
It comes down to who owns the authoritative value for a parameter. In order to implement automation and undo, the host must keep an accurate value for all parameters (unless every plugin implements these functions themselves, which is completely impossible). So the host must have an authoritative value for each parameter. But if the plugin is the ultimate authority on the value of parameters (like in the VST model), then you have a competition. At best, you have a painful time keeping the values in sync. At worst, you end up with irreconcilable errors in the stored values in the host.
Playing devil's advocate:
Many hosts have reasonably good undo and automation of VSTs already. In fact, just about everything that I do with a VST is undoable today. Using this mechanism.
What if we had something like the following (again, devil's advocate):
The plugin is the owner of parameters. Whenever a parameter is changed, the plugin notifies the host. Notifications are timestamped and include the old value and the new value.
The host can build it's undo stack that way, and can keep UIs in sync. The host can arbitrate GMPI event sources, but not things like MIDI input (should we choose to allow it). The host *can* undo MIDI (or whatever) input because it got notification.
Plugins that do not do notification are "broken", just like they are in VST-land.
How does that sound?
It's just a few steps away from "host manages everything, plugin tells host about MIDI and sends events (actor)". Both involve the plugin notifying the host of changes via MIDI.
What I *don't* get is why anyone would do this. If you already wrote GMPI parameter support, why would you want MIDI input directly? It's more work. I can understand the argument of only wanting MIDI input, and not wanting to deal with GMPI parameters, but I really don't want plugins doing that :)
-- Mike Berry Adobe Systems
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