The head of every particle program. It creates the particles and gives each one a starting position, a velocity, and a colour taken from the incoming picture.
All of that is held in textures, one pixel per particle. A 256 by 256 state holds 65,536 particles; a 2048 by 2048 one holds over four million.
Keeping the state in pictures is what lets the whole simulation run as ordinary image passes. Each frame reads last frame's state as a picture and writes this frame's out as another one, with one pixel doing the work for one particle. No special machinery, just the same texture-in texture-out arrangement everything else here uses.
Six starting layouts are available. Attrition gives every particle a small chance of being reborn each frame, and because the timing is drawn from a continuous clock rather than a frame count, the respawns spread out evenly instead of arriving in visible waves.