On this page:
- VST 2 and VST 3 version of your Plug-in have the same UID
- VST 2 and VST 3 version of your Plug-in have not the same UID
- Preset/Project compatibility VST 2 <-> VST 3
- Parameters compatibility
Here, you can find some informations about how to write a VST 3 version of a plug-in which already exists as VST 2 version. How to guaranty that a Host will correctly replace the old VST 2 by the new VST 3 version of your plug-in.
For a VST 3 Host the important information is the UID of your (processor) plug-in, with this information it will find the correct VST 3 version to use.
2 scenarios are possible: The VST 3 and the VST 2 version of your plug-in have the same UID or not.
It is possible to reuse for a VST 3 plug-in the same generated UID of a VST 2 version (based on 4 characters + its name), this should work with most VST 3 Hosts. Check the function which generates a VST 3 UID from a VST 2 UID here and reuse this value as UID for the VST 3 plug-in (processor component UID)
For this use case since VST 3 SDK version 3.7.5 we have 2 possibilities for a VST 3 plug-in to inform the Host about which VST 2 plug-in it could replace:
Using the moduleinfo.json file
The Compatibility json array in the moduleinfo.json allows to list the UIDs which could be replaced by the VST 3 plug-in.
Check the FAQ here which shows how to use the helper function VST3::tryVst2StateLoad and read a VST 2 state into a VST 3 plug-in.
It is possible to write back for VST 2 backward compatibility by using the helper function VST3::writeVst2State (public.sdk/source/vst/utility/vst2persistence.h).
VST 2 was using index to identify parameter, VST 3 uses an ID. It means that to be compatible the VST 3 parameter ID should be kept to same index value as the VST 2 one (starting from 0).
If the parameter range or behavior is not changed, previously parameter automation in host should be played back correctly.
If you change the meaning of a parameter (range, conversion normalized to plain value), it is recommanded to create a new ID for this parameter.
You could add in the VST 3 plug-in a structure to your parameters by using Unit.