diff --git a/8_ice_scene.frag b/8_ice_scene.frag
index c6a2a6c..3065966 100644
--- a/8_ice_scene.frag
+++ b/8_ice_scene.frag
@@ -100,21 +100,27 @@ void main() {
   float solid = 0.45; // sheet 1 keeps its plasma, but its alpha becomes coverage
   if (layer > 0.5 && layer < 1.5) {
     vec3 ro = vec3(0.0, 0.0, 6.0);
-    vec3 rd = normalize(vec3((uv - 0.5) * 2.0 * vec2(u_res.x / u_res.y, 1.0), -1.0));
-    float b = dot(ro, rd);
-    float disc = b * b - dot(ro, ro) + 2.4 * 2.4;
-    if (disc > 0.0) {
-      float near = max(-b - sqrt(disc), 0.0);
-      float t = near, d = 1.0;
-      for (int k = 0; k < 40; k++) {
-        d = sdShard(ro + rd * t, phase);
-        if (d < 0.003 || t > near + 5.0) break;
-        t += d;
-      }
-      if (d < 0.003) {
-        solid = 1.0;
-        vec3 pos = ro + rd * t;
+    vec3 lit = vec3(0.0);
+    float hits = 0.0;
+    // 3x3 supersample: pass 2 magnifies this sheet where the surface steepens at the
+    // cracks, and a one-pixel silhouette would show up there as big hard steps.
+    for (int sy = 0; sy < 3; sy++) {
+      for (int sx = 0; sx < 3; sx++) {
+        vec2 jitter = (vec2(float(sx), float(sy)) / 3.0 + 1.0 / 6.0 - 0.5) / u_res;
+        vec3 rd = normalize(vec3((uv + jitter - 0.5) * 2.0 * vec2(u_res.x / u_res.y, 1.0), -1.0));
+        float b = dot(ro, rd);
+        float disc = b * b - dot(ro, ro) + 2.4 * 2.4;
+        if (disc <= 0.0) continue;
+        float near = max(-b - sqrt(disc), 0.0);
+        float t = near, d = 1.0;
+        for (int k = 0; k < 40; k++) {
+          d = sdShard(ro + rd * t, phase);
+          if (d < 0.003 || t > near + 5.0) break;
+          t += d;
+        }
+        if (d >= 0.003) continue;
         float e2 = 0.004;
+        vec3 pos = ro + rd * t;
         vec3 nrm = normalize(vec3(
           sdShard(pos + vec3(e2, 0.0, 0.0), phase) - sdShard(pos - vec3(e2, 0.0, 0.0), phase),
           sdShard(pos + vec3(0.0, e2, 0.0), phase) - sdShard(pos - vec3(0.0, e2, 0.0), phase),
@@ -124,11 +130,16 @@ void main() {
         float key = max(dot(nrm, normalize(vec3(-0.45, 0.72, 0.52))), 0.0);
         float fill = max(dot(nrm, normalize(vec3(0.60, -0.20, -0.75))), 0.0);
         float rim = pow(1.0 - max(dot(nrm, -rd), 0.0), 4.0);
-        col = mix(vec3(0.004, 0.016, 0.026), vec3(0.060, 0.290, 0.340), key);
-        col += vec3(0.010, 0.060, 0.080) * fill;
-        col += vec3(0.20, 0.62, 0.72) * rim * 0.55;
+        vec3 c = mix(vec3(0.004, 0.016, 0.026), vec3(0.060, 0.290, 0.340), key);
+        c += vec3(0.010, 0.060, 0.080) * fill;
+        c += vec3(0.20, 0.62, 0.72) * rim * 0.55;
+        lit += c;
+        hits += 1.0;
       }
     }
+    float cov = hits / 9.0;
+    col = mix(col, lit / max(hits, 1.0), cov);
+    solid = mix(0.45, 1.0, cov);
   }
   float alpha = (layer > 0.5 && layer < 1.5) ? solid : (0.42 + layer * 0.26) / 2.6;
   gl_FragColor = vec4(col, alpha);
diff --git a/ice_object.html b/ice_object.html
index eafaaac..455bc40 100644
--- a/ice_object.html
+++ b/ice_object.html
@@ -16,7 +16,7 @@ canvas{display:block;width:100%;height:100%;touch-action:none}
 <div id="info" role="status"><strong>Сквозь лёд</strong><br><span id="help">Двигайте мышь · скорость − / =</span></div>
 <script>
 'use strict';
-const sceneSource = "// Pass 1: eight sheets, one atlas tile each; sheet 5 holds a shard frozen in the ice.\n// Alpha stores linear depth, so pass 2 absorbs it like any other sheet.\nprecision highp float;\nuniform vec2 u_res; // size of one tile\nuniform float u_time;\n\nfloat hash(vec2 p) { return fract(sin(dot(p, vec2(127.1, 311.7))) * 43758.5453); }\nfloat noise(vec2 p) {\n  vec2 i = floor(p), f = fract(p);\n  f = f*f*(3.0-2.0*f);\n  return mix(mix(hash(i), hash(i+vec2(1.0,0.0)), f.x),\n             mix(hash(i+vec2(0.0,1.0)), hash(i+vec2(1.0,1.0)), f.x), f.y);\n}\nfloat fbm(vec2 p) {\n  float sum = 0.0, amp = 0.5;\n  for (int i = 0; i < 4; i++) {\n    sum += amp * noise(p);\n    p = mat2(1.62, 1.17, -1.17, 1.62) * p;\n    amp *= 0.5;\n  }\n  return sum;\n}\nfloat fbm2(vec2 p) {\n  return 0.5 * noise(p) + 0.25 * noise(mat2(1.62, 1.17, -1.17, 1.62) * p);\n}\nconst float EXTENT = 3.8; // half-width of a sheet in world units; must match the ice pass\n\n// Signed distance to an elongated octahedron turning once per cycle. Sharp edges,\n// cheap to march: this is the one thing in the slab that is a real 3D object.\nvec3 shardLocal(vec3 p, float phase) {\n  float c1 = cos(phase), s1 = sin(phase);\n  p.xz = mat2(c1, -s1, s1, c1) * p.xz;\n  float c2 = cos(phase * 2.0 + 1.0), s2 = sin(phase * 2.0 + 1.0);\n  p.yz = mat2(c2, -s2, s2, c2) * p.yz;\n  p.yz /= vec2(1.0, 1.45);\n  return abs(p);\n}\nconst float SHARD = 0.95; // how big the shard is in sheet units\n// Cross two octahedra: the intersection is an eight-sided gem, so the shard still\n// reads as a cut stone from any angle instead of a flat kite.\nvec3 gemAxes(vec3 l, out vec2 r) {\n  r = mat2(0.70711, -0.70711, 0.70711, 0.70711) * l.xz;\n  return vec3(abs(r.x), l.y, abs(r.y));\n}\nfloat sdShard(vec3 p, float phase) {\n  vec3 l = shardLocal(p / SHARD, phase);\n  vec2 r;\n  vec3 m = gemAxes(l, r);\n  float a = (l.x + l.y * 0.86 + l.z - 1.0) * 0.45;\n  float b = (m.x * 1.34 + m.y * 0.86 + m.z * 1.34 - 1.0) * 0.40;\n  return max(a, b) * SHARD;\n}\n\nvoid main() {\n  vec2 tile = floor(gl_FragCoord.xy / u_res);\n  float layer = tile.x + tile.y * 4.0;\n  vec2 uv = fract(gl_FragCoord.xy / u_res);\n  vec2 p = (uv - 0.5) * 2.0 * vec2(EXTENT * u_res.x / u_res.y, EXTENT);\n  p *= 0.85 + 0.10 * layer; // far sheets carry finer detail, which reads as distance\n  float angle = hash(vec2(layer, 3.1)) * 6.2831853;\n  p = mat2(cos(angle), sin(angle), -sin(angle), cos(angle)) * p;\n\n  // Periodic time: every field returns exactly after 30 seconds.\n  float phase = u_time * 6.2831853 / 30.0;\n  vec2 o = vec2(cos(phase), sin(phase));\n  vec2 seed = vec2(layer * 17.3, layer * 9.1);\n\n  // Two nested warp stages, the marbled-plasma motion.\n  vec2 q = vec2(fbm(p * 0.50 + seed + o * 0.55),\n                fbm(p * 0.50 + seed + 23.1 - o * 0.55));\n  vec2 r = vec2(fbm(p * 0.85 + 2.0 * q + seed + vec2(o.y, -o.x) * 0.40),\n                fbm(p * 0.85 + 2.0 * q + seed + 31.7 - vec2(o.y, -o.x) * 0.40));\n  vec2 w = p * 1.10 + 2.6 * r + seed;   // warped coords of the density field\n\n  // The silhouette is shared by all sheets (slid a little per depth), so the gaps\n  // stay open right through the stack instead of filling in with overlap.\n  float mass = smoothstep(0.520, 0.548, fbm((p + o * 0.30 + vec2(0.10, -0.07) * layer) * 0.62 + r * 1.1 + 13.7));\n\n  // Light the density as a height surface: $cheap 2-octave gradient, strong terminator.\n  float h = 0.05;\n  vec2 slope = vec2(fbm2(w + vec2(h, 0.0)) - fbm2(w - vec2(h, 0.0)),\n                    fbm2(w + vec2(0.0, h)) - fbm2(w - vec2(0.0, h))) * (10.0 / (2.0 * h));\n  vec3 nrm = normalize(vec3(-slope, 1.0));\n  float lit = clamp(dot(nrm, normalize(vec3(-0.38, 0.52, 0.76))), 0.0, 1.0);\n\n  // Folded planes across the volume, feathered so they never read as a grid.\n  vec2 g = (p + r * 2.0) * 1.4;\n  vec2 cell = floor(g), f = fract(g);\n  float feather = smoothstep(0.0, 0.10, min(min(f.x, 1.0 - f.x), min(f.y, 1.0 - f.y)));\n  float facet = mix(1.0, mix(0.70, 1.25, hash(cell + seed + step(1.0, f.x + f.y) * 37.0)), feather);\n\n  float shade = 0.05 + 1.10 * lit * lit;                // lit tops, near-black undersides\n  float e = mass * facet * shade * 0.085\n          + mass * (1.0 - mass) * 4.0 * 0.040           // thin torn silhouette\n          + mass * pow(lit, 14.0) * 0.110;              // glint where a fold faces the light\n  vec3 col = e * mix(vec3(0.02, 0.42, 0.60), vec3(0.08, 0.64, 0.88), facet * 0.5);\n  // Faint dye in the open gaps so darkness reads as depth, not as a hole.\n  col += smoothstep(0.40, 0.85, fbm(p * 0.30 + seed * 0.4 + o * 0.3)) * vec3(0.0003, 0.0011, 0.0015);\n\n  float solid = 0.45; // sheet 1 keeps its plasma, but its alpha becomes coverage\n  if (layer > 0.5 && layer < 1.5) {\n    vec3 ro = vec3(0.0, 0.0, 6.0);\n    vec3 rd = normalize(vec3((uv - 0.5) * 2.0 * vec2(u_res.x / u_res.y, 1.0), -1.0));\n    float b = dot(ro, rd);\n    float disc = b * b - dot(ro, ro) + 2.4 * 2.4;\n    if (disc > 0.0) {\n      float near = max(-b - sqrt(disc), 0.0);\n      float t = near, d = 1.0;\n      for (int k = 0; k < 40; k++) {\n        d = sdShard(ro + rd * t, phase);\n        if (d < 0.003 || t > near + 5.0) break;\n        t += d;\n      }\n      if (d < 0.003) {\n        solid = 1.0;\n        vec3 pos = ro + rd * t;\n        float e2 = 0.004;\n        vec3 nrm = normalize(vec3(\n          sdShard(pos + vec3(e2, 0.0, 0.0), phase) - sdShard(pos - vec3(e2, 0.0, 0.0), phase),\n          sdShard(pos + vec3(0.0, e2, 0.0), phase) - sdShard(pos - vec3(0.0, e2, 0.0), phase),\n          sdShard(pos + vec3(0.0, 0.0, e2), phase) - sdShard(pos - vec3(0.0, 0.0, e2), phase)));\n        // A key light and a cool fill from the plasma side: flat faces take one tone\n        // each, which is what makes the shard read as a cut stone rather than a blob.\n        float key = max(dot(nrm, normalize(vec3(-0.45, 0.72, 0.52))), 0.0);\n        float fill = max(dot(nrm, normalize(vec3(0.60, -0.20, -0.75))), 0.0);\n        float rim = pow(1.0 - max(dot(nrm, -rd), 0.0), 4.0);\n        col = mix(vec3(0.004, 0.016, 0.026), vec3(0.060, 0.290, 0.340), key);\n        col += vec3(0.010, 0.060, 0.080) * fill;\n        col += vec3(0.20, 0.62, 0.72) * rim * 0.55;\n      }\n    }\n  }\n  float alpha = (layer > 0.5 && layer < 1.5) ? solid : (0.42 + layer * 0.26) / 2.6;\n  gl_FragColor = vec4(col, alpha);\n}\n";
+const sceneSource = "// Pass 1: eight sheets, one atlas tile each; sheet 5 holds a shard frozen in the ice.\n// Alpha stores linear depth, so pass 2 absorbs it like any other sheet.\nprecision highp float;\nuniform vec2 u_res; // size of one tile\nuniform float u_time;\n\nfloat hash(vec2 p) { return fract(sin(dot(p, vec2(127.1, 311.7))) * 43758.5453); }\nfloat noise(vec2 p) {\n  vec2 i = floor(p), f = fract(p);\n  f = f*f*(3.0-2.0*f);\n  return mix(mix(hash(i), hash(i+vec2(1.0,0.0)), f.x),\n             mix(hash(i+vec2(0.0,1.0)), hash(i+vec2(1.0,1.0)), f.x), f.y);\n}\nfloat fbm(vec2 p) {\n  float sum = 0.0, amp = 0.5;\n  for (int i = 0; i < 4; i++) {\n    sum += amp * noise(p);\n    p = mat2(1.62, 1.17, -1.17, 1.62) * p;\n    amp *= 0.5;\n  }\n  return sum;\n}\nfloat fbm2(vec2 p) {\n  return 0.5 * noise(p) + 0.25 * noise(mat2(1.62, 1.17, -1.17, 1.62) * p);\n}\nconst float EXTENT = 3.8; // half-width of a sheet in world units; must match the ice pass\n\n// Signed distance to an elongated octahedron turning once per cycle. Sharp edges,\n// cheap to march: this is the one thing in the slab that is a real 3D object.\nvec3 shardLocal(vec3 p, float phase) {\n  float c1 = cos(phase), s1 = sin(phase);\n  p.xz = mat2(c1, -s1, s1, c1) * p.xz;\n  float c2 = cos(phase * 2.0 + 1.0), s2 = sin(phase * 2.0 + 1.0);\n  p.yz = mat2(c2, -s2, s2, c2) * p.yz;\n  p.yz /= vec2(1.0, 1.45);\n  return abs(p);\n}\nconst float SHARD = 0.95; // how big the shard is in sheet units\n// Cross two octahedra: the intersection is an eight-sided gem, so the shard still\n// reads as a cut stone from any angle instead of a flat kite.\nvec3 gemAxes(vec3 l, out vec2 r) {\n  r = mat2(0.70711, -0.70711, 0.70711, 0.70711) * l.xz;\n  return vec3(abs(r.x), l.y, abs(r.y));\n}\nfloat sdShard(vec3 p, float phase) {\n  vec3 l = shardLocal(p / SHARD, phase);\n  vec2 r;\n  vec3 m = gemAxes(l, r);\n  float a = (l.x + l.y * 0.86 + l.z - 1.0) * 0.45;\n  float b = (m.x * 1.34 + m.y * 0.86 + m.z * 1.34 - 1.0) * 0.40;\n  return max(a, b) * SHARD;\n}\n\nvoid main() {\n  vec2 tile = floor(gl_FragCoord.xy / u_res);\n  float layer = tile.x + tile.y * 4.0;\n  vec2 uv = fract(gl_FragCoord.xy / u_res);\n  vec2 p = (uv - 0.5) * 2.0 * vec2(EXTENT * u_res.x / u_res.y, EXTENT);\n  p *= 0.85 + 0.10 * layer; // far sheets carry finer detail, which reads as distance\n  float angle = hash(vec2(layer, 3.1)) * 6.2831853;\n  p = mat2(cos(angle), sin(angle), -sin(angle), cos(angle)) * p;\n\n  // Periodic time: every field returns exactly after 30 seconds.\n  float phase = u_time * 6.2831853 / 30.0;\n  vec2 o = vec2(cos(phase), sin(phase));\n  vec2 seed = vec2(layer * 17.3, layer * 9.1);\n\n  // Two nested warp stages, the marbled-plasma motion.\n  vec2 q = vec2(fbm(p * 0.50 + seed + o * 0.55),\n                fbm(p * 0.50 + seed + 23.1 - o * 0.55));\n  vec2 r = vec2(fbm(p * 0.85 + 2.0 * q + seed + vec2(o.y, -o.x) * 0.40),\n                fbm(p * 0.85 + 2.0 * q + seed + 31.7 - vec2(o.y, -o.x) * 0.40));\n  vec2 w = p * 1.10 + 2.6 * r + seed;   // warped coords of the density field\n\n  // The silhouette is shared by all sheets (slid a little per depth), so the gaps\n  // stay open right through the stack instead of filling in with overlap.\n  float mass = smoothstep(0.520, 0.548, fbm((p + o * 0.30 + vec2(0.10, -0.07) * layer) * 0.62 + r * 1.1 + 13.7));\n\n  // Light the density as a height surface: $cheap 2-octave gradient, strong terminator.\n  float h = 0.05;\n  vec2 slope = vec2(fbm2(w + vec2(h, 0.0)) - fbm2(w - vec2(h, 0.0)),\n                    fbm2(w + vec2(0.0, h)) - fbm2(w - vec2(0.0, h))) * (10.0 / (2.0 * h));\n  vec3 nrm = normalize(vec3(-slope, 1.0));\n  float lit = clamp(dot(nrm, normalize(vec3(-0.38, 0.52, 0.76))), 0.0, 1.0);\n\n  // Folded planes across the volume, feathered so they never read as a grid.\n  vec2 g = (p + r * 2.0) * 1.4;\n  vec2 cell = floor(g), f = fract(g);\n  float feather = smoothstep(0.0, 0.10, min(min(f.x, 1.0 - f.x), min(f.y, 1.0 - f.y)));\n  float facet = mix(1.0, mix(0.70, 1.25, hash(cell + seed + step(1.0, f.x + f.y) * 37.0)), feather);\n\n  float shade = 0.05 + 1.10 * lit * lit;                // lit tops, near-black undersides\n  float e = mass * facet * shade * 0.085\n          + mass * (1.0 - mass) * 4.0 * 0.040           // thin torn silhouette\n          + mass * pow(lit, 14.0) * 0.110;              // glint where a fold faces the light\n  vec3 col = e * mix(vec3(0.02, 0.42, 0.60), vec3(0.08, 0.64, 0.88), facet * 0.5);\n  // Faint dye in the open gaps so darkness reads as depth, not as a hole.\n  col += smoothstep(0.40, 0.85, fbm(p * 0.30 + seed * 0.4 + o * 0.3)) * vec3(0.0003, 0.0011, 0.0015);\n\n  float solid = 0.45; // sheet 1 keeps its plasma, but its alpha becomes coverage\n  if (layer > 0.5 && layer < 1.5) {\n    vec3 ro = vec3(0.0, 0.0, 6.0);\n    vec3 lit = vec3(0.0);\n    float hits = 0.0;\n    // 3x3 supersample: pass 2 magnifies this sheet where the surface steepens at the\n    // cracks, and a one-pixel silhouette would show up there as big hard steps.\n    for (int sy = 0; sy < 3; sy++) {\n      for (int sx = 0; sx < 3; sx++) {\n        vec2 jitter = (vec2(float(sx), float(sy)) / 3.0 + 1.0 / 6.0 - 0.5) / u_res;\n        vec3 rd = normalize(vec3((uv + jitter - 0.5) * 2.0 * vec2(u_res.x / u_res.y, 1.0), -1.0));\n        float b = dot(ro, rd);\n        float disc = b * b - dot(ro, ro) + 2.4 * 2.4;\n        if (disc <= 0.0) continue;\n        float near = max(-b - sqrt(disc), 0.0);\n        float t = near, d = 1.0;\n        for (int k = 0; k < 40; k++) {\n          d = sdShard(ro + rd * t, phase);\n          if (d < 0.003 || t > near + 5.0) break;\n          t += d;\n        }\n        if (d >= 0.003) continue;\n        float e2 = 0.004;\n        vec3 pos = ro + rd * t;\n        vec3 nrm = normalize(vec3(\n          sdShard(pos + vec3(e2, 0.0, 0.0), phase) - sdShard(pos - vec3(e2, 0.0, 0.0), phase),\n          sdShard(pos + vec3(0.0, e2, 0.0), phase) - sdShard(pos - vec3(0.0, e2, 0.0), phase),\n          sdShard(pos + vec3(0.0, 0.0, e2), phase) - sdShard(pos - vec3(0.0, 0.0, e2), phase)));\n        // A key light and a cool fill from the plasma side: flat faces take one tone\n        // each, which is what makes the shard read as a cut stone rather than a blob.\n        float key = max(dot(nrm, normalize(vec3(-0.45, 0.72, 0.52))), 0.0);\n        float fill = max(dot(nrm, normalize(vec3(0.60, -0.20, -0.75))), 0.0);\n        float rim = pow(1.0 - max(dot(nrm, -rd), 0.0), 4.0);\n        vec3 c = mix(vec3(0.004, 0.016, 0.026), vec3(0.060, 0.290, 0.340), key);\n        c += vec3(0.010, 0.060, 0.080) * fill;\n        c += vec3(0.20, 0.62, 0.72) * rim * 0.55;\n        lit += c;\n        hits += 1.0;\n      }\n    }\n    float cov = hits / 9.0;\n    col = mix(col, lit / max(hits, 1.0), cov);\n    solid = mix(0.45, 1.0, cov);\n  }\n  float alpha = (layer > 0.5 && layer < 1.5) ? solid : (0.42 + layer * 0.26) / 2.6;\n  gl_FragColor = vec4(col, alpha);\n}\n";
 const iceSource = "// Pass 2: clean ice. Refraction, chromatic shear, absorption by true path length.\n// Nothing is added to empty space, so the gaps between sheets stay black.\nprecision highp float;\nuniform vec2 u_res;\nuniform float u_time;\nuniform vec2 u_mouse; // normalized, bottom-left origin\nuniform sampler2D u_scene;\nuniform vec2 u_tile_res;\n\nconst float EXTENT = 3.8;                 // must match the plasma pass\nconst float FRONT  = -2.0;                // ice surface, camera at the origin looking down -z\nconst vec3  SIGMA  = vec3(0.66, 0.26, 0.11); // absorption per unit: red dies first, so depth turns blue\nconst float IOR    = 1.31;\nconst float OBJECT_DEPTH = 0.68; // sheet 1 holds the shard, at a fixed depth\n\nfloat hash(vec2 p) { return fract(sin(dot(p, vec2(127.1, 311.7))) * 43758.5453); }\nfloat noise(vec2 p) {\n  vec2 i = floor(p), f = fract(p);\n  f = f*f*(3.0-2.0*f);\n  return mix(mix(hash(i), hash(i+vec2(1.0,0.0)), f.x),\n             mix(hash(i+vec2(0.0,1.0)), hash(i+vec2(1.0,1.0)), f.x), f.y);\n}\nfloat fbm(vec2 p) {\n  float sum = 0.0, amp = 0.5;\n  for (int i = 0; i < 3; i++) { sum += amp * noise(p); p = mat2(1.62, 1.17, -1.17, 1.62) * p; amp *= 0.5; }\n  return sum;\n}\nfloat fbm2(vec2 p) {\n  return 0.5 * noise(p) + 0.25 * noise(mat2(1.62, 1.17, -1.17, 1.62) * p);\n}\nvec2 atlasUV(vec2 world, float layer) {\n  vec2 uv = world / (2.0 * EXTENT) + 0.5;\n  uv = clamp(uv, 0.5 / u_tile_res, 1.0 - 0.5 / u_tile_res); // keep bilinear taps inside the tile\n  return (vec2(mod(layer, 4.0), floor(layer / 4.0)) + uv) / vec2(4.0, 2.0);\n}\n// The slab's fracture lines: two families of long, slightly wandering cracks. Shared\n// by the height field and the sheen, so each crack is both a bright seam and a step.\nvec2 fracture(vec2 x) {\n  float d = fbm(x * 0.62 + 3.0) - 0.5;                 // zero set: long winding cracks\n  float e = fbm(x * 1.35 - 7.0) - 0.5;                 // a sparser second family\n  return vec2(d, e);\n}\n// Frozen surface: fine ridged grain over the plates the cracks have lifted apart.\n// Static in world space; only the plasma behind it moves.\nfloat height(vec2 x) {\n  vec2 fr = fracture(x);\n  return 0.090 * (1.0 - abs(2.0 * fbm2(x * 2.2 + 2.0) - 1.0))\n       + 0.055 * (1.0 - abs(2.0 * fbm2(x * 6.4 - 5.0) - 1.0))\n       + 0.014 * clamp(fr.x * 110.0, -1.0, 1.0)  // the slab sits at a different level\n       + 0.009 * clamp(fr.y * 110.0, -1.0, 1.0); // on each side of a crack\n}\nvoid main() {\n  vec2 screen = gl_FragCoord.xy / u_res;\n  vec2 p = (gl_FragCoord.xy - 0.5 * u_res) / u_res.y;\n  vec2 look = clamp(u_mouse - 0.5, -0.35, 0.35); // orbiting the slab centre\n  vec3 ray = normalize(vec3(p - look * 2.2, -1.6));\n\n  // Front surface along the view ray; two fixed-point steps settle on the swell.\n  float t = FRONT / ray.z;\n  for (int i = 0; i < 2; i++) t = (FRONT + height(ray.xy * t)) / ray.z;\n  vec3 hit = ray * t;\n\n  float e = 0.008;\n  vec2 slope = vec2(height(hit.xy + vec2(e, 0.0)) - height(hit.xy - vec2(e, 0.0)),\n                    height(hit.xy + vec2(0.0, e)) - height(hit.xy - vec2(0.0, e))) / (2.0 * e);\n  float bevel = smoothstep(0.14, 0.0, min(min(screen.x, 1.0 - screen.x), min(screen.y, 1.0 - screen.y)));\n  slope += bevel * (screen - 0.5) * 0.55; // the slab turns away at the frame edge\n  vec3 normal = normalize(vec3(-slope, 1.0));\n  float facing = max(dot(-ray, normal), 0.0);\n\n  // Refracted ray; red and blue bend a little differently, which is what makes ice ring.\n  vec3 inside = refract(ray, normal, 1.0 / IOR);\n  vec2 shearR = (refract(ray, normal, 1.0 / (IOR - 0.012)) - inside).xy;\n  vec2 shearB = (refract(ray, normal, 1.0 / (IOR + 0.019)) - inside).xy;\n  inside = normalize(inside);\n  inside.z = min(inside.z, -0.40); // bound grazing rays at the crack steps\n\n  // Recentre the parallax on the middle of the sheet stack: near and far sheets\n  // then slide past each other at different rates as the pointer moves.\n  vec2 par = look * 4.54;\n\n  vec3 acc = vec3(0.0);\n  float trans = 1.0;\n  for (int i = 0; i < 8; i++) {\n    float layer = float(i);\n    bool solid = i == 1; // sheet 1 is the shard: its alpha is coverage, not depth\n    float depth = solid ? OBJECT_DEPTH\n                        : texture2D(u_scene, atlasUV(hit.xy + par, layer)).a * 2.6;\n    float path = (hit.z - (FRONT - depth)) / -inside.z;                     // ice crossed to reach this sheet\n    vec2 at = (hit + inside * path).xy + par;\n    vec3 plasma;\n    plasma.r = texture2D(u_scene, atlasUV(at + shearR * path, layer)).r;\n    plasma.g = texture2D(u_scene, atlasUV(at, layer)).g;\n    plasma.b = texture2D(u_scene, atlasUV(at + shearB * path, layer)).b;\n    vec3 c = plasma * exp(-SIGMA * path);\n    // Near plumes are nearly opaque, so the sheets behind them show through the gaps\n    // instead of averaging into a wash. That is what makes the stack read as depth.\n    // The shard is a solid, so its coverage comes straight from pass 1.\n    float alpha = solid ? texture2D(u_scene, atlasUV(at, layer)).a * 0.92\n                        : clamp(max(max(c.r, c.g), c.b) * 1.8, 0.0, 0.80);\n    acc += trans * c;\n    trans *= 1.0 - alpha;\n  }\n\n  // Gentle shoulder: overlapping glow stays coloured instead of clipping to white.\n  vec3 col = vec3(1.0) - exp(-acc * 1.5);\n\n  // Glass: reflection and a rim only where the surface turns away. Empty space stays dark.\n  float fresnel = pow(1.0 - facing, 5.0);\n  vec3 sky = mix(vec3(0.004, 0.016, 0.030), vec3(0.075, 0.220, 0.320), normal.y * 0.5 + 0.5);\n  col += sky * (fresnel * 1.2 + bevel * 0.03);\n  // Steep faces of the crests catch a thin sheen: the edge of the frozen surface,\n  // painted only where the ice actually tilts, so the open water stays black.\n  // Only the crack walls are this steep, so this sheen outlines the seams alone\n  // instead of washing the whole grain.\n  float tilt = smoothstep(1.00, 2.60, length(slope));\n  col += vec3(0.022, 0.062, 0.078) * tilt;\n  vec2 fr = fracture(hit.xy);\n  float crack = (1.0 - smoothstep(0.0, 0.004, abs(fr.x)))\n              + (1.0 - smoothstep(0.0, 0.004, abs(fr.y)));\n  col += vec3(0.020, 0.055, 0.068) * crack * smoothstep(0.58, 0.80, fbm2(hit.xy * 0.30 + 21.0));\n\n  gl_FragColor = vec4(pow(clamp(col, 0.0, 1.0), vec3(0.4545)), 1.0);\n}\n";
 const vertexSource = 'attribute vec2 p; void main(){gl_Position=vec4(p,0.0,1.0);}';
 const canvas = document.getElementById('view'), help = document.getElementById('help');