APIC affine matrix C: 3 vec4 columns per particle (xyz used).
Deformation gradient F: 3 vec4 columns per particle (xyz used).
Grid after the last substep: xyz velocity (m/s), w mass (normalised units), one vec4 per node.
Terrain height per node column (x + z·nx), metres above origin[1]; then (from nx·nz) the friction multiplier per column.
xyz position (local to origin), w render value (0…1, the colour ramp key). The pool's position.
x temperature (K), y size scale, z random 0…1, w free. The pool's props.
x material index, y Jp, z and w free for materials.
xyz velocity (m/s), w 1 alive / 0 empty slot. The pool's velocity.
GPU buffers of a solver (the particle pool's own plus the MPM state), for custom passes.