hm... is it something like IPC? By 'message passing', would it be ok to use a java image-processing/video-processing library?

(I'm on the Flutter team.)

IPC is "inter-process communication". This message passing is same-process.

We haven't yet proved that it's sufficiently performant for extremely latency-sensitive work, and we currently copy the bytes for the messages when switching so it's not very suitable for high-bandwidth work like video where you're sharing the bytes between Dart and Java.