@Tilly, you've mentioned using CamTwist with success quite a few times! I don't want to derail the thread, but how do you deal with audio? I love CamTwist's abilities but the few times I've used it my audio gets wayyyy off from the video and I'm not sure how to remedy the situation. It's not a processing power issue on my end, I've got a top-of-the-line 2015 MacBook Pro.
Sorry to barge in on the thread, OP most people do use ManyCam but it's kind of Windows-only—there is a Mac version of ManyCam but it doesn't run on El Capitan so for anyone who's up to date, it's useless.