Such kind gentleman!
Bart, it was easier than you're imagining. I did screen shots of every measure in every track of the midi sequencer. Here's just a snippet of a measure in one of the tracks:
In Photoshop, I lifted all the notes (orange rectangles), placing them on their own layer and deleting the original. Instant masks! For any one measure, there were up to 4 tracks, and so for each slide, I used up to 4 such masks. Luckily, this piece was composed with only 6 (very long) measures, and so only 6 slides were required for effect.
Setup for one slide assuming 3 sequencer masks:
Mask
...Gradient with x/y unlocked, the width narrowed down to 2 pixels
Mask
...Gradient as above
Mask
---Gradient as above
Background Mask
...Gradient
Each slide was the exact duration of the measure it represented, so starting a gradient bar on the left and panning it all the way to the right meant that each midi note would light up at just the right time.
Mikey, if bubbles grow in fractal patterns, then, by Jove, you've got it! The background masks are closeup photos of bubbles.
Barbara