ab_ash_mantle

# ab_ash Thirty centimetres of dead volcanic ash, as its own sheet of geometry standing off the stone — and the way it leaves. One number takes it away: `coverage`, driven from 1 to 0 as the rain crosses the crater. The mantle does not fade out. Each fragment carries its own threshold, made of a low-frequency patch field plus how high up the crater it lies, so a falling coverage strips the high terraces first and leaves the garden floor last, in ragged patches. Two things happen together and they are what make it read as a THICKNESS leaving rather than an image dissolving. In the vertex stage the sheet SINKS into the stone where it is going, so its retreating edge keeps its depth all the way out; in the fragment stage the same field cuts the sheet away entirely once it has sunk, so what is left behind is the stone and not a grey film over it. The edge between the two is soaked slurry, not dry ash. | property | | |---|---| | `coverage` | 1 → 0, the retreat | | `wetness` | how wet what is still lying is | | `ash_depth` | how far the sheet sinks — matches the offset the mesh was cut with | | `ripple_scale` / `ripple_strength` | the wind-laid relief | No textures. Four bands of relief — dunes a wide shot reads, wind ripples for the mid ground, and two grains that only exist close to the lens — each faded out as its features reach the size of a pixel, so a 250 m wide of the crater floor is clean while the same material still carries millimetre grain at 8 cm.