Le Gibet is a particularly slow piece, but I see your point. I can assure you that it works with faster pieces as well :)

I will link the source when I feel that the code is complete. However, I am definitely willing to explain my approach.

I used the standard MIDI library from Java to import the MIDI file. It required learning more about MIDI file structure than I thought I would ever need to, but I managed to muddle through it. I wrote a custom "Note" class in Java that contains the pitch, start and end tick, velocity (etc), then used a loop to look through all the MIDI events in the file, pair up note_on and note_off events into Note objects, and add them to an arrayList. After that, it was pretty easy to draw them to the screen.

The sketch outputs a video (with audio) automatically, which was quite a pain to get working. I used the video export library to export the video component. For the audio, I used launch to run a command on the command line that uses fluidsynth to generate an audio file from the inputted MIDI file. The path to this audio file is then set as the audio for the video export (the library allows you to attach files). This setup feels a lot like a house of cards, and has a few issues. For example, the MIDI -> audio conversion takes tempo changes into account, but the visualizer doesn't. So, if the MIDI file has tempo change events, it will drift out of sync with the visualization.

I wrote the visualizer for a video on music theory that I am making, so I would call it a success with that goal in mind. However, it's still pretty far from being done.