That's the one, thanks. Spritz is where most people discovered it and the reason why the highlighted pivot letter is the norm.

However, RSVP itself has a long history that dates back to reading research in the seventies done in the context of how quickly one can read words without eye movements. Spritz's main innovation was the optimal position of the recognition point, allowing to keep it fixed while the word around it moves to form a new word, plus the framing elements at the top and bottom. I'm using them, so no need to patent something that others have already pioneered.

What I didn't like about theirs is that it was only available as an SDK inside other apps, and not as a standalone reader. It came up on a Samsung watch and a couple of other readers, but you couldn't really submit your own epub. This is what gave me the idea to make this one.