fix(lenia): read params at the start of each frame

A preset switch set params after the frame, so the first frame used
the old mu/sigma. This broke Scutium and made Gyrorbium unstable.
Now LeniaSimulation reads params at each frame start (readParams).
Add a ROADMAP item for a display zoom (Bicaudatus tails).

ROADMAP.md

@@ -19,6 +19,7 @@ Open ideas for the playground frontend. Tick a box when done.
 
 ## Features
 
+- [ ] Lenia: display zoom so small creatures (e.g. Orbium bicaudatus tails) are visible
 - [ ] **Shareable demo state**: keep demo settings in query params (fractal zoom/center, sorting algorithm, seeds). Opening the link restores the view.
 - [ ] **New algorithm demos** (pattern: route + `RouterConstants` + `AlgorithmsService` entry + i18n + information panel):
   - [x] Boids / flocking (GPU path like cloth)

src/app/pages/algorithms/lenia/lenia-simulation.spec.ts

@@ -1,4 +1,4 @@
-import {LeniaSimulation} from './lenia-simulation';
+import {LeniaParams, LeniaSimulation} from './lenia-simulation';
 import {DEFAULT_SETTINGS} from './lenia.model';
 
 const renderCalls: {name: string; target: unknown}[] = [];
@@ -59,6 +59,10 @@ function fakeScene() {
   };
 }
 
+function paramsWithSteps(stepsPerFrame: number): LeniaParams {
+  return {settings: {...DEFAULT_SETTINGS, stepsPerFrame}, kernelSum: 1, brush: {u: 0, v: 0, active: false}};
+}
+
 describe('LeniaSimulation', () => {
   it('keepsSeedWhileStepShaderCompiles', () => {
     renderCalls.length = 0;
@@ -66,8 +70,7 @@ describe('LeniaSimulation', () => {
     ready['leniaStep'] = false;
     const scene = fakeScene();
     const simulation = new LeniaSimulation();
-    simulation.init(scene as never, {} as never, 'float');
-    simulation.updateParams({settings: {...DEFAULT_SETTINGS, stepsPerFrame: 1}, kernelSum: 1, brush: {u: 0, v: 0, active: false}});
+    simulation.init(scene as never, {} as never, 'float', () => paramsWithSteps(1));
     simulation.seed(new Uint8Array(4));
 
     scene.frame();
@@ -81,4 +84,22 @@ describe('LeniaSimulation', () => {
     expect(firstStep.name).toBe('leniaStep');
     expect(firstStep.target).not.toBe(seededTarget);
   });
+
+  it('readsParamsAtFrameStart', () => {
+    renderCalls.length = 0;
+    ready['leniaCopy'] = true;
+    ready['leniaStep'] = true;
+    let stepsPerFrame = 1;
+    const scene = fakeScene();
+    const simulation = new LeniaSimulation();
+    simulation.init(scene as never, {} as never, 'float', () => paramsWithSteps(stepsPerFrame));
+    simulation.seed(new Uint8Array(4));
+    scene.frame();
+
+    stepsPerFrame = 3;
+    renderCalls.length = 0;
+    scene.frame();
+
+    expect(renderCalls.filter((call) => call.name === 'leniaStep')).toHaveLength(3);
+  });
 });

src/app/pages/algorithms/lenia/lenia-simulation.ts

@@ -25,12 +25,14 @@ export class LeniaSimulation {
   private next: RenderTargetTexture | null = null;
   private material: ShaderMaterial | null = null;
   private plane: Mesh | null = null;
-  private params: LeniaParams | null = null;
+  private readParams: (() => LeniaParams) | null = null;
+  private frameParams: LeniaParams | null = null;
   private seeded = false;
   private readonly massBuffer = new Float32Array(GRID_SIZE * GRID_SIZE * FLOATS_PER_PIXEL);
 
-  init(scene: Scene, engine: WebGPUEngine | Engine, textureType: StateTextureType): void {
+  init(scene: Scene, engine: WebGPUEngine | Engine, textureType: StateTextureType, readParams: () => LeniaParams): void {
     this.scene = scene;
+    this.readParams = readParams;
     this.renderer = new EffectRenderer(engine);
     this.current = createStateTarget('leniaA', scene, GRID_SIZE, textureType);
     this.next = createStateTarget('leniaB', scene, GRID_SIZE, textureType);
@@ -61,10 +63,6 @@ export class LeniaSimulation {
     scene.onBeforeRenderObservable.add(() => this.frame());
   }
 
-  updateParams(params: LeniaParams): void {
-    this.params = params;
-  }
-
   /** Uploads a start field (one byte per cell). The next frame copies it into the state texture. */
   seed(fieldData: Uint8Array): void {
     this.field?.update(fieldData);
@@ -109,7 +107,8 @@ export class LeniaSimulation {
     this.next = null;
     this.material = null;
     this.plane = null;
-    this.params = null;
+    this.readParams = null;
+    this.frameParams = null;
     this.scene = null;
   }
 
@@ -138,10 +137,10 @@ export class LeniaSimulation {
 
   private applyStepUniforms(): void {
     const effect = this.stepWrapper?.effect;
-    if (!effect || !this.params || !this.current) {
+    if (!effect || !this.frameParams || !this.current) {
       return;
     }
-    const {settings, kernelSum, brush} = this.params;
+    const {settings, kernelSum, brush} = this.frameParams;
     effect.setTexture('state', this.current);
     effect.setFloat('texel', 1 / GRID_SIZE);
     effect.setInt('radius', Math.min(settings.radius, MAX_RADIUS));
@@ -154,9 +153,11 @@ export class LeniaSimulation {
   }
 
   private frame(): void {
-    if (!this.renderer || !this.copyWrapper || !this.stepWrapper || !this.current || !this.next || !this.params) {
+    if (!this.renderer || !this.copyWrapper || !this.stepWrapper || !this.current || !this.next || !this.readParams) {
       return;
     }
+    // Read before the seed copy, so a new seed never steps with old params.
+    this.frameParams = this.readParams();
     if (!this.seeded) {
       if (!this.copyWrapper.effect.isReady()) {
         return;
@@ -168,7 +169,7 @@ export class LeniaSimulation {
     if (!this.stepWrapper.effect.isReady()) {
       return;
     }
-    for (let i = 0; i < this.params.settings.stepsPerFrame; i++) {
+    for (let i = 0; i < this.frameParams.settings.stepsPerFrame; i++) {
       this.renderer.render(this.stepWrapper, this.next);
       [this.current, this.next] = [this.next, this.current];
     }

src/app/pages/algorithms/lenia/lenia.component.ts

@@ -26,7 +26,7 @@ import {
 } from './lenia.model';
 import {kernelSum} from './lenia-kernel';
 import {buildSeedField, decodeRle, SeedKind} from './lenia-seed';
-import {LeniaSimulation} from './lenia-simulation';
+import {LeniaParams, LeniaSimulation} from './lenia-simulation';
 
 interface SettingSlider {
   key: keyof LeniaSettings;
@@ -153,9 +153,9 @@ export class LeniaComponent implements OnDestroy {
     this.backend.set(`${backend} (${type === 'float' ? 'float32' : 'float16'})`);
     this.simulation?.dispose();
     this.simulation = new LeniaSimulation();
-    this.simulation.init(event.scene, event.engine, type);
+    this.simulation.init(event.scene, event.engine, type, () => this.currentParams());
     this.seed('creature');
-    this.startParamUpdateLoop(event.scene);
+    this.startFrameLoop(event.scene);
     this.attachPointer(event.scene);
   }
 
@@ -189,11 +189,8 @@ export class LeniaComponent implements OnDestroy {
     this.simulation?.seed(buildSeedField(kind, pattern, GRID_SIZE));
   }
 
-  private startParamUpdateLoop(scene: Scene): void {
-    scene.onAfterRenderObservable.clear();
-    scene.onAfterRenderObservable.add(() => {
-      this.fpsCounter.tick();
-      this.simulation?.updateParams({
+  private currentParams(): LeniaParams {
+    return {
       settings: {
         radius: this.radius(),
         mu: this.mu(),
@@ -203,8 +200,12 @@ export class LeniaComponent implements OnDestroy {
       },
       kernelSum: this.kernelSum(),
       brush: this.brush
-      });
-    });
+    };
+  }
+
+  private startFrameLoop(scene: Scene): void {
+    scene.onAfterRenderObservable.clear();
+    scene.onAfterRenderObservable.add(() => this.fpsCounter.tick());
     // First in line, so the read is issued before the steps of the frame and the GPU is idle.
     scene.onBeforeRenderObservable.add(() => this.updateMass(), undefined, true);
   }