Lane C S13 gate 2.2: the front wall grounds on the horizon, ends feathered

Two QA sightings from in-yard eye height, both geometry, both reproduced on
dev_skyfx before touching anything. (1) The band stopped AT the camera's
horizontal plane, but the ground's true horizon sits ~0.6 deg below it (the
dip), so a bright sky sliver showed under the base — the floating slab. The
band now overshoots to ~12 deg below the equator; the ground occludes the
overshoot by depth, and it survives scale.y's 0.3 rise floor. (2) The arc-end
fade (sin^0.75) left the outer tenth at ~0.36 alpha — the hard vertical seam.
Now sin^1.6 with the arc widened 2.2->2.6 rad so the solid core keeps its
span while the ends drop under 0.12.

Verified by eye on dev_skyfx (southerly t=24, front 0.51): base meets the
ground line, no sliver, no seam. Selftest 338/0/0 (+1 test pinning both in
numbers). Mutation-checked: the old thetaLength and the old exponent each red.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

web/world/js/skyfx.js

@@ -656,15 +656,25 @@ export function createSkyFx(o = {}) {
   // fixed post-change instant — def + seed in, same wall out, every run.
   const FRONT_LEAD = 12;      // s before the change the wall starts rising
   const FRONT_FADE = 8;       // s after the swing completes for it to clear
-  const FRONT_ARC = 2.2;      // radians of horizon the wall spans (~126°)
+  const FRONT_ARC = 2.6;      // radians of horizon the wall spans (~149°)
   const FRONT_MAX_OP = 0.85;
   const frontEvents = (def.events || [])
     .filter((e) => e.type === 'windchange' && Number.isFinite(e.t))
     .map((e) => ({ t0: e.t, over: e.over ?? 6, az: null }));
-  // Band from ~34° above the horizon down to it, centred (in local space) on
-  // azimuth π: SphereGeometry's (x,z) = (-cosφ, sinφ)·sinθ, so φ=0 sits at
-  // atan2(z,x) = π. rotation.y = π − A then carries the centre to azimuth A.
-  const frontGeo = new THREE.SphereGeometry(170, 24, 8, -FRONT_ARC / 2, FRONT_ARC, Math.PI * 0.30, Math.PI * 0.20);
+  // Band from ~34° above the horizon down to ~12° BELOW it, centred (in local
+  // space) on azimuth π: SphereGeometry's (x,z) = (-cosφ, sinφ)·sinθ, so φ=0
+  // sits at atan2(z,x) = π. rotation.y = π − A then carries the centre to A.
+  //
+  // SPRINT13 gate 2.2 — why the band overshoots the equator: the old band
+  // stopped AT the horizontal plane through the camera, but from in-yard eye
+  // height the ground's true horizon sits ~0.6° BELOW that plane (the dip),
+  // so a sliver of bright sky showed under the wall's base and the bank read
+  // as a floating slab — the QA sighting, reproduced on dev_skyfx before this
+  // fix. Overshooting to −12° buries the base behind the ground from any eye
+  // height that matters (it survives scale.y's 0.3 floor: −37 m compressed to
+  // −11 m is still −3.7° elevation, below the dip). The ground plane occludes
+  // the overshoot by depth, so nothing is drawn twice.
+  const frontGeo = new THREE.SphereGeometry(170, 24, 10, -FRONT_ARC / 2, FRONT_ARC, Math.PI * 0.30, Math.PI * 0.27);
   {
     // Soft edges via vertex alpha, or the band reads as a floating rectangle
     // (checked by eye on dev_skyfx.html — the hard phi/theta cut was exactly
@@ -672,11 +682,15 @@ export function createSkyFx(o = {}) {
     // the arc ends and across the top, so it sits ON the horizon like a bank
     // of weather instead of hanging in the sky like a screen.
     // Sphere uv: x runs 0..1 across the arc, y is 1 at the band top, 0 at the
-    // horizon edge.
+    // (now sub-horizon) bottom edge.
+    // The 1.6 exponent is gate 2.2's other half: 0.75 left the outer tenth of
+    // the arc at ~0.36 alpha, which from in-yard eye height is the "hard
+    // vertical seam" the QA saw at the bank's ends. 1.6 holds the same solid
+    // core (the arc is wider to compensate) but takes the ends to <0.12.
     const uv = frontGeo.attributes.uv;
     const rgba = new Float32Array(uv.count * 4);
     for (let i = 0; i < uv.count; i++) {
-      const across = Math.pow(Math.sin(Math.PI * uv.getX(i)), 0.75);
+      const across = Math.pow(Math.sin(Math.PI * uv.getX(i)), 1.6);
       const up = 1 - smoothstep(0.45, 1, uv.getY(i));
       rgba[i * 4] = 1; rgba[i * 4 + 1] = 1; rgba[i * 4 + 2] = 1;
       rgba[i * 4 + 3] = across * up;

web/world/js/tests/c.test.js

@@ -227,6 +227,31 @@ export default async function run(t) {
       'dispose did not hand the sun back exactly (intensity / colour / shadow radius)');
   });
 
+  // SPRINT13 gate 2.2 — the wall's edges, judged from in-yard eye height.
+  // Two QA sightings, both geometry: a sliver of bright sky under the bank's
+  // base (it stopped AT the camera's horizontal plane, above the ground's true
+  // horizon), and a hard vertical seam at the arc ends (0.36 alpha at the
+  // outer tenth). The band now overshoots the equator and the end fade is
+  // steeper; these pins are what "grounded" and "feathered" mean in numbers.
+  t.test('the change-front wall grounds below the horizon and feathers its ends', () => {
+    const wind = createWind(storms.storm_03_southerly);
+    const sky = createSkyFx({ wind });
+    const geo = sky.front.geometry;
+    geo.computeBoundingBox();
+    assert(geo.boundingBox.min.y < -20,
+      `front base at y=${geo.boundingBox.min.y.toFixed(1)} — a band stopping at eye level floats a sky sliver under the wall`);
+    const uv = geo.attributes.uv, col = geo.attributes.color;
+    assert(col && col.itemSize === 4, 'front lost its vertex alpha — the edges are hard cuts again');
+    let edgeMax = 0;
+    for (let i = 0; i < uv.count; i++) {
+      const u = uv.getX(i);
+      if (u <= 0.09 || u >= 0.91) edgeMax = Math.max(edgeMax, col.getW(i));
+    }
+    assert(edgeMax < 0.2,
+      `arc-end alpha ${edgeMax.toFixed(2)} — the vertical seam QA saw at the bank's ends`);
+    sky.dispose();
+  });
+
   // --- SPRINT2 §Lane C.3: rain has to stop at the cloth ---
   // Driven with a synthetic 4×4 m panel rather than a whole cloth sim: the thing
   // under test is the projection, and a flat panel makes the right answer