Rotomatic is a physics-based MIDI sequencer plugin that turns chaos into musical ideas.

The first version, Rot-O-Matic, was a weekend Unity experiment in February 2024, after I got inspired by the Tombola sequencer on the OP-1. Up to five marbles bounced inside a spinning polygon with 3 to 10 sides and every hit played a random note from the chosen scale: major, minor, pentatonic, phrygian or lydian. Each marble had its own tiny synth voice, a square wave coded straight into the audio callback, with a bit of low-pass filter on top. I decided it was fun and quirky enough to post on the r/Unity3D subreddit.

I made a random melody generator/sequencer, that is not only fun to look at but also sounds like absolute garbage.
by u/m_brudi in Unity3D

I was overwhelmed by the response: the post got close to 900 upvotes and almost 90 comments. The most requested feature was a way to sync the notes to BPM to make it more musical, so I got back to work. A week or so later I posted a new version with working sync, an octave selector and a low-pass filter. It also had toggles for marble-to-marble collisions (more collisions, more notes), for velocity-based volume (harder hits play louder) and for reverb (with a cool trail effect). I added Zen Mode too, which makes the polygon fullscreen and hides the controls. You can download this version (Windows and Android) here: itch.io

I posted this version on r/synthesizers.
The great feedback from people who actually make music is what pushed me to turn it into a proper thing. After some research I found JUCE, a C++ framework for audio applications. It let me build a real VST that works in any DAW and outputs MIDI, which was the goal all along. It wasn’t easy, though. I’m very comfortable with C# and Unity because that’s my job, but C++ and JUCE were a completely different thing. I used Box2D for the physics, lost a lot of evenings figuring out sync inside the DAW and found a million ways to crash Ableton, but eventually it started to work. I played with the UI a lot to make it look cool (which I hope it does), and a year later I posted a short video of the VST on r/synthesizers.

I got more great feedback, so I added a gravity slider and MIDI input, which lets you spawn, remove and reset marbles from your MIDI keyboard (thank you Roger C). I also made most of the parameters automatable. The main thing I learned from that post was that people were actually willing to pay for this plugin. Woah! After some quick research I went with Payhip. I bought the domain, set up a store, wrote the documentation and stressed a lot. I found new ways to break the plugin and crash the DAW, and fixed them. Sync worried me the most, so I made a free demo version that lets you test it before buying. Then it turned out the version that worked in Ableton acted weird in FL Studio, so I fixed that too. I also messaged a couple of redditors and gave them keys to test it (thank you again Roger C). The worst part of the whole “proper release” process was recording a nice video. The plugin is random by nature, so I spent countless hours recording my screen while the marbles played different notes, until I got something that sounded nice enough to get people interested.