Fluid backdrop behind the 3D model

New 'Sfondo Modello 3D' selector (Media tab): gradient or any of the
five fluid modes. The fluid sim paints the canvas first, then the model
renders on top clearing only the depth buffer (overlay mode skips its
own backdrop pass). Palette and speed are shared with the preset;
persisted and re-sent when the output restarts.

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

src/control.html

@@ -131,6 +131,15 @@
         <button id="btn-glb-load" class="wide">🧊 Carica un modello .glb…</button>
         <input type="file" id="glb-input" accept=".glb" hidden />
         <div class="hint-mini" id="glb-name">Il modello ruota a tempo di musica nella famiglia “Modello 3D”.</div>
+        <label>Sfondo Modello 3D</label>
+        <select id="glb-bg">
+          <option value="gradient">Gradiente</option>
+          <option value="ink">Fluido</option>
+          <option value="fire">Fluido Fuoco</option>
+          <option value="ring">Fluido Anello</option>
+          <option value="vortex">Fluido Vortici</option>
+          <option value="wave">Fluido Onda</option>
+        </select>
       </section>
 
       <section class="span2" id="effect-seq-section">

src/control.js

@@ -146,13 +146,23 @@ $('#svg-input').addEventListener('change', (e) => {
 // --- 3D model (GLB) for the 'Modello 3D' family -----------------------------
 // The path persists and is re-sent when the output window (re)starts.
 let glbPath = localStorage.getItem('glb3d') || '';
-function glbSend() { if (glbPath) send({ type: 'glb', path: glbPath }); }
+let glbBg = localStorage.getItem('glbbg') || 'gradient';
+function glbSend() {
+  if (glbPath) send({ type: 'glb', path: glbPath });
+  send({ type: 'modelBg', mode: glbBg });
+}
 function glbLabel(name) {
   $('#glb-name').textContent = name
     ? '🧊 ' + name + ' — attiva la famiglia “Modello 3D” per vederlo.'
     : 'Il modello ruota a tempo di musica nella famiglia “Modello 3D”.';
 }
 if (glbPath) glbLabel(glbPath.split('/').pop());
+$('#glb-bg').value = glbBg;
+$('#glb-bg').addEventListener('change', (e) => {
+  glbBg = e.target.value;
+  localStorage.setItem('glbbg', glbBg);
+  send({ type: 'modelBg', mode: glbBg });
+});
 $('#btn-glb-load').addEventListener('click', () => $('#glb-input').click());
 $('#glb-input').addEventListener('change', (e) => {
   const f = e.target.files[0];

src/model3d.js

@@ -929,7 +929,9 @@ class ModelSim {
   // pose (optional, from the camera-interactive mode): { yaw, leanX, hopY,
   // squash, rim } — extra rotation, sideways lean, jump height (in radii),
   // vertical squash & stretch, touch-glow 0..1.
-  render(timeSec, audio, e, canvas, pose) {
+  // overlay = true: another engine (e.g. the fluid sim) already painted the
+  // canvas this frame — draw only the model on top (clear depth, no backdrop).
+  render(timeSec, audio, e, canvas, pose, overlay) {
     const gl = this.gl;
     const speed = Math.min(2.5, e.speed || 1);
     const mix = e.audioMix !== undefined ? e.audioMix : 1;
@@ -938,10 +940,15 @@ class ModelSim {
 
     gl.viewport(0, 0, canvas.width, canvas.height);
     gl.disable(gl.BLEND);
-    gl.clearColor(0, 0, 0, 1);
+    if (overlay) {
-    gl.clear(gl.COLOR_BUFFER_BIT | gl.DEPTH_BUFFER_BIT);
+      gl.clear(gl.DEPTH_BUFFER_BIT);
+    } else {
+      gl.clearColor(0, 0, 0, 1);
+      gl.clear(gl.COLOR_BUFFER_BIT | gl.DEPTH_BUFFER_BIT);
+    }
 
     // backdrop
+    if (!overlay) {
     gl.useProgram(this.progBg);
     gl.bindBuffer(gl.ARRAY_BUFFER, this.quad);
     gl.enableVertexAttribArray(this.aBg);
@@ -953,6 +960,7 @@ class ModelSim {
     gl.uniform1f(this.ub.uBass, bass);
     gl.uniform1f(this.ub.uBeat, beat);
     gl.drawArrays(gl.TRIANGLES, 0, 3);
+    }
 
     // model
     const asp = canvas.width/Math.max(1, canvas.height);

src/output.js

@@ -320,6 +320,7 @@ djv.onControl(async (m) => {
       djv.report({ type: 'autoVj', on: autoVj });
       break;
     case 'svg': loadCustomTexture(m.dataUrl); break;
+    case 'modelBg': viz.modelBg = m.mode || 'gradient'; break;
     case 'glb':
       try {
         const bytes = await djv.readFile(m.path);

src/visualizer.js

@@ -114,7 +114,15 @@ class Visualizer {
     if (e.isModel3d && window.ModelSim) {
       if (!this.model3d) this.model3d = new window.ModelSim(gl);
       if (this._pendingGlb) { this.model3d.setModel(this._pendingGlb); this._pendingGlb = null; }
-      this.model3d.render(timeSec, audio, e, this.canvas);
+      // optional fluid backdrop behind the model (modelBg: ink/fire/ring/...)
+      const bg = this.modelBg || 'gradient';
+      if (bg !== 'gradient' && window.FluidSim) {
+        if (!this.fluid) this.fluid = new window.FluidSim(gl);
+        this.fluid.render(timeSec, audio, Object.assign({}, e, { fluidMode: bg }), this.canvas);
+        this.model3d.render(timeSec, audio, e, this.canvas, null, true);
+      } else {
+        this.model3d.render(timeSec, audio, e, this.canvas);
+      }
       return;
     }
     if (e.isInteractive && window.InteractiveSim) {