feat(audio): sweep a pass-by through the stereo field on each exit

src/audio/drive-texture.ts

@@ -8,9 +8,13 @@ const CRUISE_ENGINE_GAIN = 0.055;
 const MAX_ENGINE_GAIN = 0.12;
 const CRUISE_ROAD_GAIN = 0.006;
 const MAX_ROAD_GAIN = 0.024;
+const PASS_BY_GAIN = 0.05;
+const PASS_BY_SECONDS = 1.6;
 
 export interface DriveTexture {
   setEnergy(sample: DriveEnergySample): void;
+  /** The exit gates going by: a filtered air sweep across the stereo field. */
+  passBy?(): void;
   dispose(): void;
 }
 
@@ -95,11 +99,29 @@ export const createDriveTexture: DriveTextureFactory = (context, destination) =>
   roadLowpass.connect(roadGain);
   roadGain.connect(destination);
 
+  // Pass-by: preallocated and silent until an exit, so a chapter change
+  // schedules automation only and never creates nodes.
+  const passSource = context.createBufferSource();
+  passSource.buffer = roadSource.buffer;
+  passSource.loop = true;
+  const passBand = context.createBiquadFilter();
+  passBand.type = 'bandpass';
+  passBand.frequency.value = 500;
+  passBand.Q.value = 0.9;
+  const passPan = context.createStereoPanner();
+  const passGain = context.createGain();
+  passGain.gain.value = 0;
+  passSource.connect(passBand);
+  passBand.connect(passPan);
+  passPan.connect(passGain);
+  passGain.connect(destination);
+
   const sources: readonly AudioScheduledSourceNode[] = [
     fundamental.source,
     harmonic.source,
     ignition.source,
     roadSource,
+    passSource,
   ];
   const ownedNodes: readonly AudioNode[] = [
     ...sources,
@@ -111,6 +133,9 @@ export const createDriveTexture: DriveTextureFactory = (context, destination) =>
     roadHighpass,
     roadLowpass,
     roadGain,
+    passBand,
+    passPan,
+    passGain,
   ];
   for (const source of sources) source.start();
 
@@ -135,6 +160,19 @@ export const createDriveTexture: DriveTextureFactory = (context, destination) =>
       automate(roadGain.gain, CRUISE_ROAD_GAIN + next * (MAX_ROAD_GAIN - CRUISE_ROAD_GAIN), duration);
       currentEnergy = next;
     },
+    passBy() {
+      if (disposed) return;
+      const now = context.currentTime;
+      for (const parameter of [passGain.gain, passBand.frequency, passPan.pan]) {
+        holdParameter(parameter, now);
+      }
+      passGain.gain.linearRampToValueAtTime(PASS_BY_GAIN, now + 0.5);
+      passGain.gain.linearRampToValueAtTime(0, now + PASS_BY_SECONDS);
+      passBand.frequency.linearRampToValueAtTime(2_600, now + 0.5);
+      passBand.frequency.linearRampToValueAtTime(380, now + PASS_BY_SECONDS);
+      passPan.pan.linearRampToValueAtTime(-0.75, now + 0.05);
+      passPan.pan.linearRampToValueAtTime(0.75, now + PASS_BY_SECONDS);
+    },
     dispose() {
       if (disposed) return;
       disposed = true;

src/audio/soundtrack-engine.ts

@@ -554,6 +554,9 @@ export function createSoundtrackEngine(
     },
     setChapterIntent(intent) {
       if (closed) return;
+      // The scene runs its exit gates on every chapter change; the drive
+      // texture answers with the air of the gates going by.
+      if (intent.sectionId !== latestChapter.sectionId) driveTexture?.passBy?.();
       latestChapter = intent;
       mixer.setChapterIntent(intent);
       if (!mainCommitted && introStarted) {