Supersample the shard 3x3

Pass 2 magnifies the object sheet where the surface steepens at the cracks,
so a one-pixel silhouette showed up as hard blocks. Nine jittered rays per
pixel give the edge a soft ramp instead.

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);