pad_nebula

The nebula front: a slab of soft, churning, additively-blended puffs. The mesh is a lattice of degenerate quads inside the slab; each cell hashes into one puff that rolls on three slow cycles of the film's clock and billboards at the camera. The astral slice flies the whole entity along X by film.frontX(t), so the front crosses the archipelago and its leading edge arrives on camera in shot 5.

half_x shapes the leading and trailing faces, floor_y / ceil_y keep it off the cream sea and under the starfield, and near_fade dissolves a puff as it reaches the lens so the front stays a volume the camera can be inside of.