radiance: corona sparks now have a lifecycle — born on the rim, drift outward, shrink and fizzle out; cursor pushes carry them, nothing pulls them back

src/scripts/orb.js

@@ -133,32 +133,48 @@ vec3 homeOf(uint idx, float epoch) {
 vec3 bodyToView(vec3 p) { return rotX(rotY(p, uTime * 0.045 + 2.4 + uTilt.x), uTilt.y); }
 vec3 viewToBody(vec3 v) { return rotY(rotX(v, -uTilt.y), -(uTime * 0.045 + 2.4 + uTilt.x)); }
 
-// ---- corona: sparks thrown off the limb, laid out in screen space; strongest over the top
+// ---- corona: sparks born on the rim, laid out in screen space, that drift outward and fizzle out.
+// Each is reborn at a fresh spot on the rim (favouring tufts over the top) when its life ends.
 struct P {
-  vec2 xy;
-  float n;
-  float reach;
-  float fade;
+  vec2 xy;      // where the spark is, before any cursor push
+  float n;      // strength of the tuft it was born in
+  float life;   // 0 at birth .. 1 at death
+  float fade;   // fades in at birth, out toward death
   vec4 r;
 };
 
+float coronaLen(uint idx) { return mix(3.5, 8.5, h01(idx * 8u + 1u)); }
+float coronaLT(uint idx, float t) { return t / coronaLen(idx) + h01(idx * 8u + 2u); }
+
 P corona(uint idx) {
   P o;
   o.r = vec4(h01(idx * 8u), h01(idx * 8u + 1u), h01(idx * 8u + 2u), h01(idx * 8u + 3u));
-  vec4 q = vec4(h01(idx * 8u + 4u), h01(idx * 8u + 5u), h01(idx * 8u + 6u), h01(idx * 8u + 7u));
-  float t = uTime;
-  float a = q.x * 6.2831853;
-  vec2 cd = vec2(cos(a), sin(a));
-  float n = fbm(vec3(cd * 4.2, t * 0.10));
-  float up = smoothstep(-0.75, 0.9, cd.y);
+  float lt = coronaLT(idx, uTime);
+  float epoch = floor(lt);
+  o.life = fract(lt);
+  float tSpawn = (epoch - o.r.z) * coronaLen(idx);
+
+  // birthplace: best of three rim angles, scored by tuft strength and how far over the top it is
+  uint base = pcg(idx * 2654435761u + uint(epoch) * 40503u + 91u);
+  vec2 cd = vec2(0.0, 1.0);
+  float n = 0.0, up = 0.0, best = -1.0;
+  for (int i = 0; i < 3; i++) {
+    float a = h01(base + uint(i) * 374761393u) * 6.2831853;
+    vec2 c = vec2(cos(a), sin(a));
+    float nn = fbm(vec3(c * 4.2, tSpawn * 0.10));
+    float u = smoothstep(-0.75, 0.9, c.y);
+    float score = (0.25 + 0.75 * u) * (0.3 + nn);
+    if (score > best) { best = score; cd = c; n = nn; up = u; }
+  }
   float spike = pow(n, 1.6) * 1.6;
-  float reach = pow(o.r.x, 1.3) * (0.07 + 0.5 * spike) * (0.3 + 0.7 * up);
-  reach *= 0.88 + 0.12 * sin(t * 0.7 + o.r.w * 20.0);
-  reach += 0.012 * sin(t * 0.9 + o.r.z * 30.0);
+  float travel = pow(o.r.x, 1.3) * (0.07 + 0.5 * spike) * (0.3 + 0.7 * up) + 0.02;
+  float ease = 1.0 - pow(1.0 - o.life, 1.35);         // a steady drift that slows toward the end
+  float ang = (o.r.w - 0.5) * 0.08 * o.life;           // a little sideways drift
+  float ca = cos(ang), sa = sin(ang);
+  vec2 dir = vec2(ca * cd.x - sa * cd.y, sa * cd.x + ca * cd.y);
   o.n = n;
-  o.reach = reach;
-  o.fade = 1.0 - smoothstep(0.0, 0.42, reach);
-  o.xy = cd * (1.0 + 0.004 + reach);
+  o.xy = dir * (1.004 + travel * ease);
+  o.fade = smoothstep(0.0, 0.04, o.life) * (1.0 - smoothstep(0.12, 1.0, o.life));   // brightest at the rim, dissolving with distance
   return o;
 }
 `;
@@ -337,23 +353,22 @@ void main() {
   float t = uTime;
 
   if (idx >= uInner) {
-    // corona: a spring pulls each spark home, the flow and a light wind shake it
+    // corona: the cursor flow and a light wind carry a spark along; nothing pulls it back,
+    // it just lives out its life and dies (a rebirth clears the push)
     P p = corona(idx);
     vec2 disp = s0.xy;
     vec2 vel = s1.xy;
+    if (uInit == 1 || floor(coronaLT(idx, t)) != floor(coronaLT(idx, t - uDt))) { disp = vec2(0.0); vel = vec2(0.0); }
     vec2 flow = flowAt(p.xy + disp);
     vec3 np = vec3(p.xy * 2.4 + vec2(0.0, t * 0.05), t * 0.16);
     float e = 0.03;
     float nx = vnoise(np + vec3(e, 0.0, 0.0)) - vnoise(np - vec3(e, 0.0, 0.0));
     float ny = vnoise(np + vec3(0.0, e, 0.0)) - vnoise(np - vec3(0.0, e, 0.0));
     vec2 wind = vec2(ny, -nx) / (2.0 * e);
-    float k = 10.5 * (0.65 + 0.7 * p.r.w);
-    float c = 2.0 * sqrt(k) * 0.45;
-    vec2 acc = -k * disp - c * vel + 15.0 * (flow * uGain * 1.8 - vel) + wind * 0.84 * uAmb;
-    vel += acc * uDt;
+    vel += (flow * uGain * 1.8 + wind * 0.05 * uAmb - vel) * (1.0 - exp(-uDt * 3.0));
     disp += vel * uDt;
     float m = length(disp);
-    if (m > 0.9) disp *= 0.9 / m;
+    if (m > 1.2) disp *= 1.2 / m;
     oPos = vec4(disp, 0.0, 0.0);
     oVel = vec4(vel, 0.0, 0.0);
     return;
@@ -474,21 +489,18 @@ void main() {
   } else {
     P p = corona(idx);
     vec2 disp = s0.xy;
-    float stir = clamp(length(disp) * 1.6 + length(s1.xy) * 0.35, 0.0, 1.5);
+    float stir = clamp(length(s1.xy) * 0.35, 0.0, 1.0);
     vp = p.xy + disp;
     float tw = 0.5 + 0.5 * sin(t * (1.0 + p.r.y * 3.0) + p.r.z * 60.0);
-    col = vec4(1.0, 0.98, 0.88, (0.45 + 0.55 * p.fade) * (0.5 + 0.5 * smoothstep(0.2, 0.75, p.n)));
+    col = vec4(1.0, 0.98, 0.88, 0.95 * p.fade * (0.55 + 0.45 * smoothstep(0.2, 0.75, p.n)));
     col.a *= mix(0.55, 1.0, tw);
     // a minority of sparks are bright, haloed points; the rest stay faint dust
     float lead = step(0.7, p.r.x);
     col.a *= mix(0.7, 1.0, lead);
-    size = (1.0 + p.r.y * p.r.y * 2.8 * (0.3 + p.fade)) * mix(0.9, 1.5, lead);
+    size = (1.0 + p.r.y * p.r.y * 2.8) * mix(0.9, 1.5, lead) * (1.0 - 0.5 * p.life);   // shrinking as it dies
     vGlow = mix(0.4, 1.0, lead);
-    // sparks that get knocked off the limb flare, then fade out while they are away from home
-    float away = smoothstep(0.03, 0.5, length(disp));
-    col.a *= 1.0 - 0.8 * away;
     col.a = min(1.0, col.a * (1.0 + stir * 0.8));
-    size *= 1.0 + stir * 0.9;
+    size *= 1.0 + stir * 0.5;
   }
 
   vec2 px = uCenter + vec2(vp.x, -vp.y) * uR;