feat: engine off at the destination

On arrival the drive winds down to a stop, the tail and running lamps
drop to parking level, the sky darkens about 10% with more stars and
the grid fades. The engine and road noise take a 1.2 s low-pass sweep
to idle. The letterbox stays within 8vh and the closing credit types
into the lower bar; reduced motion keeps only the bars and dimming.

src/scene/world/car.ts

@@ -15,6 +15,8 @@ export interface CarWorld {
     acceleration: number,
     elapsedSeconds: number,
     distance?: number,
+    /** 0 driving, 1 parked at the destination: lamps drop to parking level. */
+    engineOff?: number,
   ): void;
 }
 
@@ -417,7 +419,8 @@ export function createCar(
   return {
     group,
     setDetail,
-    update(speed, acceleration, elapsedSeconds) {
+    update(speed, acceleration, elapsedSeconds, _distance, engineOff = 0) {
+      const lamps = 1 - clamp01(engineOff) * 0.62;
       const safeSpeed = clamp01(speed);
       const response = clamp01(acceleration);
       const elapsed = Number.isFinite(elapsedSeconds)
@@ -441,9 +444,9 @@ export function createCar(
       group.rotation.z =
         Math.sin(elapsed * 0.78) * safeSpeed * 0.0024 +
         Math.sin(elapsed * 1.31) * response * 0.0014;
-      tailMaterial.emissiveIntensity = 1.25 + response * 1.35;
+      tailMaterial.emissiveIntensity = (1.25 + response * 1.35) * lamps;
       underglowUniforms.uOpacity.value =
-        0.1 + safeSpeed * 0.055 + response * 0.07;
+        (0.1 + safeSpeed * 0.055 + response * 0.07) * lamps;
       contact.uniforms.uOpacity.value = 0.2 + response * 0.06;
     },
   };
@@ -600,7 +603,10 @@ function createAuthoredCar(
       detail = nextDetail;
       setScale(detail);
     },
-    update(speed, acceleration, elapsedSeconds, distance) {
+    update(speed, acceleration, elapsedSeconds, distance, engineOff = 0) {
+      // Parked at the destination: the tail bar and the cyan running lamps
+      // drop to parking level, the underglow almost out.
+      const lamps = 1 - clamp01(engineOff) * 0.62;
       const safeSpeed = clamp01(speed);
       const response = clamp01(acceleration);
       const elapsed = Number.isFinite(elapsedSeconds)
@@ -634,12 +640,12 @@ function createAuthoredCar(
       group.rotation.x = 0;
       group.rotation.z = Math.sin(elapsed * 0.8) * safeSpeed * 0.0018;
       for (const material of tailMaterials)
-        material.emissiveIntensity = 2.2 + response * 1.2;
-      haloUniforms.uOpacity.value = 0.2 + response * 0.16;
+        material.emissiveIntensity = (2.2 + response * 1.2) * lamps;
+      haloUniforms.uOpacity.value = (0.2 + response * 0.16) * lamps;
       for (const material of cyanMaterials)
-        material.emissiveIntensity = 2.2 + response * 1.6;
+        material.emissiveIntensity = (2.2 + response * 1.6) * lamps;
       underglowUniforms.uOpacity.value =
-        0.09 + safeSpeed * 0.04 + response * 0.075;
+        (0.09 + safeSpeed * 0.04 + response * 0.075) * (1 - clamp01(engineOff) * 0.8);
       contact.uniforms.uOpacity.value = 0.21 + response * 0.055;
     },
   };

src/scene/world/environment.ts

@@ -307,9 +307,17 @@ export async function createProceduralSceneGeneration(
       terrainSpectrumPhase =
         (terrainSpectrumPhase + deltaSeconds * overdrive * 0.38) % 1;
       writeChapterPalette(targetPalette, snapshot.chapter, snapshot.chapter, 0);
+      // Engine off: parked at the destination the sky drops about 10%, more
+      // stars come out, and the drive winds down to a stop.
+      const engineOff = THREE.MathUtils.smoothstep(outroProgress, 0.12, 0.7);
+      if (engineOff > 0) {
+        targetPalette.sky.lerp(ENGINE_OFF_NIGHT, 0.1 * engineOff);
+        targetPalette.fog.lerp(ENGINE_OFF_NIGHT, 0.1 * engineOff);
+        targetPalette.starVisibility *= 1 + 0.8 * engineOff;
+      }
       dampPalette(currentPalette, targetPalette, deltaSeconds);
       const driveDistance = travelDistance.advance(
-        deltaSeconds,
+        deltaSeconds * (1 - engineOff),
         snapshot.driveSpeed,
         response.roadEnergy,
       );
@@ -317,7 +325,7 @@ export async function createProceduralSceneGeneration(
         0.18 + response.roadEnergy * 0.82,
         0.18,
         1,
-      );
+      ) * (1 - engineOff);
       road.update(
         driveDistance,
         visualSpeed,
@@ -347,6 +355,7 @@ export async function createProceduralSceneGeneration(
         overdrive,
         terrainSurge,
         terrainSpectrumPhase,
+        engineOff,
       );
       atmosphericTime += deltaSeconds;
       const elapsedSeconds = atmosphericTime;
@@ -354,7 +363,7 @@ export async function createProceduralSceneGeneration(
       skyline.update(currentPalette, elapsedSeconds);
       traffic.update(
         deltaSeconds,
-        18 + snapshot.driveSpeed * 54 + response.roadEnergy * 48,
+        (18 + snapshot.driveSpeed * 54 + response.roadEnergy * 48) * (1 - engineOff),
         elapsedSeconds,
       );
       sun.update(
@@ -369,6 +378,7 @@ export async function createProceduralSceneGeneration(
         response.suspensionEnergy,
         elapsedSeconds,
         driveDistance,
+        engineOff,
       );
       speedWake.update(
         overdrive,
@@ -634,6 +644,7 @@ const INTRO_SECONDS = 4.2;
 const OUTRO_SECONDS = 5.5;
 const OUTRO_RETURN_SECONDS = 1.6;
 const OUTRO_REACH = 0.62;
+const ENGINE_OFF_NIGHT = new THREE.Color(0x000000);
 /** Plates rise back over 720 ms with up to three 90 ms staggers. */
 const INTERLUDE_RISE_MS = 1100;
 

src/scene/world/terrain.ts

@@ -24,6 +24,8 @@ export interface TerrainWorld {
     overdrive?: number,
     surge?: number,
     spectrumPhase?: number,
+    /** 0 driving, 1 parked: the grid fades out under the end shot. */
+    engineOff?: number,
   ): void;
 }
 
@@ -59,6 +61,7 @@ export function createTerrain(
     uSurge: { value: 0 },
     uSpectrumPhase: { value: 0 },
     uRouteProgress: { value: 0 },
+    uEngineOff: { value: 0 },
     uTerrainEnergy: { value: initialPalette.terrainEnergy },
     uOctaves: { value: quality.terrainOctaves },
     uRelief: { value: 49 },
@@ -180,6 +183,7 @@ export function createTerrain(
       uniform vec3 uFogColor;
       uniform vec3 uHazeColor;
       uniform float uRouteProgress;
+      uniform float uEngineOff;
       uniform float uOverdrive;
       uniform float uSurge;
       varying float vHeight;
@@ -200,7 +204,8 @@ export function createTerrain(
         float road = 1.0 - smoothstep(7.6, 8.4, abs(vTerrainPosition.x));
         float apron = 1.0 - smoothstep(12.0, 32.0, abs(vTerrainPosition.x));
         float surfaceStrength = mix(0.72, 0.12 + uOverdrive * 0.3, road);
-        float alpha = vDepthFade * pulse * routeCalm * mix(surfaceStrength, 0.91, ridgeMix);
+        float alpha = vDepthFade * pulse * routeCalm * mix(surfaceStrength, 0.91, ridgeMix)
+          * (1.0 - uEngineOff * 0.72);
         float charge = uOverdrive * (0.35 + vFlow * 0.8) * (0.5 + apron * 0.5);
         charge += uSurge * (0.28 + vCrest * 0.5) * (1.0 - road);
         gl_FragColor = vec4(color * (1.46 + ridgeMix * 0.54 + charge), alpha);
@@ -282,7 +287,9 @@ export function createTerrain(
       overdrive = 0,
       surge = 0,
       spectrumPhase = 0,
+      engineOff = 0,
     ) {
+      uniforms.uEngineOff.value = clamp01(engineOff);
       const chapterPalette = CHAPTER_PALETTES[chapter];
       uniforms.uDistance.value =
         ((finiteOrZero(distance) % NOISE_PERIOD) + NOISE_PERIOD) % NOISE_PERIOD;