back in the days when I said I was going to implement TFTD I made some notes about the bubble trails after bullets. I think it was just observations, I guess I share my notes, you never know if it's helpful.
- double animation speed, 8 frames long, particles gradually fade out each frame (transparency)
- 2x2px and 1x1px white/blueish particles emitted on the bullet trail at more or less random positions, but evenly spread out. (each voxel bullet travels through has 50% change to emit particle?)
- particles rise in voxel space, smaller particles move faster than bigger particles with a bit of randomness in the speed (not all particles of same size move at the same speed)
I don't know if it's correct or not or useful, so use it at own risk :p I was almost about to implement it, I'm sorry I never got that far... I think I got stuck in how to get transparency work. (and how to spell the word correctly :p)