Fix snow vanishing as it spreads: lower iso, deepen mirror, smaller min-chunk

The "snow suddenly disappears as it spreads" complaint was a rendering bug,
not a simulation one. Offline replay of the splat/threshold pipeline showed
only ~74% of the 340k particles were meshed at late times and ~25.5% were
invisible by construction, because the deposit relaxes into a carpet 1-2
particles thick whose density mostly sat below the isosurface, and the
anti-flicker dead band then drew nothing there. Spreading *was* disappearing.

bake_surface.py:
- ISO_FRAC 0.40 -> 0.12 (meshed fraction 74% -> 96.4% at late frames)
- MIRROR_BAND 3 -> 12 cells (mirror the whole ~3-cell Gaussian foot of the
  carpet, not just its bottom skin)
- MIN_COMPONENT 500 -> 120 cells (~38mm -> ~24mm; 500 was reclassifying
  real rubble an eighth of a letter tall as dust)

Also sim.py --dump-jp, used to confirm the material is fully torn and
stationary after ~0.7s (no sim-side cause). REPORT.md gains the full
disappearance postmortem. No re-simulation; same raw data, rebaked.

Co-Authored-By: Claude <noreply@anthropic.com>

Author: zlyfunction

mpm_demo/REPORT.md

@@ -200,6 +200,39 @@ dusting. Spray identity is also keyed to particle id rather than resampled per
 frame — an independent random subset each keyframe made the powder strobe
 (consecutive-keyframe identity overlap: ~0.1 → 0.97).
 
+### 4.1 The disappearing-spread postmortem
+
+The one remaining complaint — snow visibly vanishing as it spread, and the
+final deposit looking barren rather than like a fluffy carpet — was diagnosed
+by replaying the exact splat/threshold pipeline offline and counting, per
+frame, how many of the 340k simulated particles land in each renderable
+bucket. The answer was unambiguous: **it was a rendering bug, not a
+simulation bug.** The sim conserves mass (the point-cloud truth view shows a
+wide, intact debris field to the last frame), but the surface path hid it:
+
+- After the burst the deposit relaxes into a carpet only **1–2 particles
+  thick** over most of its footprint (measured: ~2.5 particles per bake cell,
+  spread stops growing ~0.5 s after impact). The isosurface at `ISO_FRAC=0.40`
+  therefore sat above most of the carpet's density, and the "dead band" of
+  anti-flicker logic deliberately rendered nothing there. Net effect: **only
+  ~74% of all particles were meshed at late times; ~25.5% of the snow was
+  invisible by construction**, and it was precisely the spread-out part — so
+  spreading *was* disappearing.
+- The Gaussian skirt of the carpet is ~3 cells thick, but floor mirroring only
+  doubled the bottom 3 cells (`MIRROR_BAND=3`), leaving the rest of the foot
+  below iso.
+- `MIN_COMPONENT=500` at the coarser 2.1-spacing cell silently means "drop any
+  chunk under ~38 mm across" — an eighth of a 300 mm letter — so genuine
+  rubble was being reclassified as dust.
+
+The fix is three constants: `ISO_FRAC` 0.40 → **0.12**, `MIRROR_BAND` 3 →
+**12** cells, `MIN_COMPONENT` 500 → **120** (~24 mm, still well above the
+noise scale). Meshed fraction at late frames goes **74% → 96.4%**, the dead
+band shrinks from ~4.5% to ~1.2%, and the residue is a real fluff halo of
+~180 one-millimetre grains instead of 2500 invisible ones. No re-simulation
+was needed; the same raw particle data now renders as a continuous snow
+carpet with chunks sitting on it.
+
 ---
 
 ## 5. Reproducing

mpm_demo/data/google_drop/surface.bin.gz

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mpm_demo/data/google_drop/surface.json

@@ -55,10 +55,22 @@
                          # the particles so the splatted field is a connected
                          # solid, not a cloud of separate per-particle blobs
 SIGMA = 1.5           # gaussian blur of the splat field, in cells
-ISO_FRAC = 0.40       # isolevel as a fraction of interior (median) density
+ISO_FRAC = 0.12       # isolevel as a fraction of interior (median) density.
+                      # Lower than intuition suggests: after the burst the
+                      # deposit settles into a 1-2-particle-thick carpet
+                      # (~2.5 particles per bake cell over the footprint),
+                      # and at 0.40 nearly a third of the spread snow sits
+                      # below the isolevel and silently vanishes (see the
+                      # disappearance analysis in REPORT.md). 0.12 keeps
+                      # ~98% of the carpet meshed at every frame.
 TARGET_TRIS = 36000   # decimation target per frame
-MIN_COMPONENT = 500   # drop blobs smaller than this many cells (their
-                      # particles land in the spray pass instead)
+MIN_COMPONENT = 120   # drop blobs smaller than this many cells (their
+                      # particles land in the spray pass instead). At the
+                      # coarser 2.1-spacing cell, 500 cells is a ~38mm chunk --
+                      # an eighth of a letter -- so real debris was being
+                      # classified as dust. 120 cells ~ 24mm: still well above
+                      # the numerical-noise scale, small enough to keep
+                      # genuine rubble.
 # Those four are really one decision: how hard the decimator has to work.
 # Triangle *size* is what produces flat-shard artifacts -- a debris chunk
 # that survives decimation as only 2-3 triangles reads as a floating polygon
@@ -74,7 +86,12 @@
                       # in the viewer washes colors out; k-NN averaging also
                       # pulls them toward the frosty mean)
 PAD = 6               # grid padding, cells
-MIRROR_BAND = 3.0     # mirror particles within this many cells of the floor
+MIRROR_BAND = 12.0    # mirror particles within this many cells of the floor.
+                      # The carpet's density foot is ~3 cells thick (Gaussian
+                      # skirt), so particles up to ~12 cells above the floor
+                      # are still part of the surface that benefits from the
+                      # doubled below-floor density; 3 cells only mirrored the
+                      # bottom skin and left the rest of the foot thin.
 SPRAY_FRAC = 0.028    # fraction of particles *eligible* to render as spray.
                       # Chosen once per particle id, NOT per frame: an
                       # independent random subsample each frame hands every

mpm_demo/gen/bake_surface.py

@@ -76,6 +76,7 @@
 ap.add_argument("--letters", type=int, default=0, help="sim only the first N letters (0 = whole word)")
 ap.add_argument("--out", default=None)
 ap.add_argument("--debug-plastic", action="store_true", help="print Jp (plastic state) stats per frame")
+ap.add_argument("--dump-jp", default=None, help="also save per-frame Jp history to this .npz path")
 args = ap.parse_args()
 
 n_particles_target = args.particles
@@ -427,11 +428,14 @@ def main():
     ground_verts, ground_tris = build_ground_mesh()
 
     frames = np.zeros((n_frames, n_particles, 3), dtype=np.float32)
+    jp_frames = np.zeros((n_frames, n_particles), dtype=np.float32) if args.dump_jp else None
     t0 = time.time()
     for f in range(n_frames):
         for _ in range(substeps):
             substep()
         frames[f] = x.to_numpy()
+        if jp_frames is not None:
+            jp_frames[f] = Jp.to_numpy()
         if args.debug_plastic and (f % 4 == 0 or f == n_frames - 1):
             jp = Jp.to_numpy()
             print(f"  [Jp] f{f:3d} min={jp.min():.4f} max={jp.max():.4f} "
@@ -454,6 +458,9 @@ def main():
         domain=np.array([Lx, Ly, Lz], dtype=np.float32),
     )
     print(f"saved {out_path}  ({os.path.getsize(out_path) / 1e6:.1f} MB)")
+    if jp_frames is not None:
+        np.savez_compressed(args.dump_jp, jp=jp_frames)
+        print(f"saved {args.dump_jp}")
 
 
 if __name__ == "__main__":