fix: harden ray tracer against unphysical rays

src/common/model/optics/RayTracer.ts

@@ -197,7 +197,21 @@ export class RayTracer {
     }
 
     const totalBrightness = ray.brightnessS + ray.brightnessP;
-    if (totalBrightness < this.config.minBrightness) {
+    // Cull dim rays. The negated `>=` form also catches a non-finite brightness
+    // (a NaN produced by a degenerate element): `NaN < minBrightness` is false in
+    // IEEE-754, so a bare `<` check would let an unphysical ray propagate uncapped
+    // to maxRayDepth. Non-finite brightness contributes 0 to the truncation total
+    // so it cannot poison the accumulator.
+    if (!(totalBrightness >= this.config.minBrightness)) {
+      return Number.isFinite(totalBrightness) ? totalBrightness : 0;
+    }
+
+    // Cull rays with a degenerate direction. `normalize()` returns the zero vector
+    // for a zero input, so a degenerate element (e.g. a SegmentMirror with p1 === p2)
+    // can feed a zero-length or non-finite direction into the tracer. Such a ray has
+    // no meaningful propagation and would otherwise emit a zero-length or NaN segment.
+    const dir = ray.direction;
+    if (!(Number.isFinite(dir.x) && Number.isFinite(dir.y)) || (dir.x === 0 && dir.y === 0)) {
       return totalBrightness;
     }
 

tests/fuzz/fuzz.spec.ts

@@ -10,9 +10,9 @@
 import { expect, test } from "@playwright/test";
 
 const FUZZ_DURATION = parseInt(process.env["FUZZ_DURATION"] || "15", 10) * 1000;
-const FUZZ_SEED = process.env["FUZZ_SEED"] || Math.floor(Math.random() * 1_000_000).toString();
-const FUZZ_RATE = process.env["FUZZ_RATE"] || "100";
-const FUZZ_POINTERS = process.env["FUZZ_POINTERS"] || "1";
+const FUZZ_SEED: string = process.env["FUZZ_SEED"] || Math.floor(Math.random() * 1_000_000).toString();
+const FUZZ_RATE: string = process.env["FUZZ_RATE"] || "100";
+const FUZZ_POINTERS: string = process.env["FUZZ_POINTERS"] || "1";
 
 interface ConsoleMessage {
   type: string;

tests/review-regressions.test.ts

@@ -37,7 +37,7 @@ import { ArcMirror } from "../src/common/model/mirrors/ArcMirror.js";
 import { deserializeElement } from "../src/common/model/optics/elementSerialization.js";
 import { arcBounds, point } from "../src/common/model/optics/Geometry.js";
 import { OpticsScene } from "../src/common/model/optics/OpticsScene.js";
-import type { SimulationRay } from "../src/common/model/optics/OpticsTypes.js";
+import type { OpticalElement, SimulationRay } from "../src/common/model/optics/OpticsTypes.js";
 import { RayTracer } from "../src/common/model/optics/RayTracer.js";
 
 // ── Helpers ──────────────────────────────────────────────────────────────────
@@ -276,3 +276,75 @@ describe("beam-type sources tag emitted rays for per-source grouping", () => {
     expect(rays[0]?.sourceId).toBe(src.id);
   });
 });
+
+// ── 5. Tracer culls unphysical rays at the boundary ─────────────────────────
+//
+// The brightness cutoff at the top of processRayEntry uses `!(b >= min)` rather
+// than `b < min` so that a non-finite (NaN) brightness — which a degenerate
+// element could produce — is culled too; `NaN < min` is false in IEEE-754, so a
+// bare `<` would let a NaN ray propagate uncapped to maxRayDepth. A ray with a
+// zero-length or non-finite direction (normalize() returns the zero vector for a
+// zero input) is likewise dropped before it can emit a zero-length or NaN segment.
+
+/**
+ * Minimal light source that emits exactly the rays it is handed. Used to inject a
+ * single crafted ray into the tracer without going through a real source's fan.
+ */
+class StubSource implements OpticalElement {
+  public readonly id = "stub-source";
+  public readonly type = "stubSource";
+  public readonly category = "lightSource" as const;
+  private readonly rays: SimulationRay[];
+  public constructor(rays: SimulationRay[]) {
+    this.rays = rays;
+  }
+  public emitRays(): SimulationRay[] {
+    return this.rays.map((r) => ({ ...r }));
+  }
+  public checkRayIntersection(): null {
+    return null;
+  }
+  public onRayIncident(): { isAbsorbed: boolean } {
+    return { isAbsorbed: true };
+  }
+  public serialize(): Record<string, unknown> {
+    return { type: this.type, id: this.id };
+  }
+  public getBounds(): { minX: number; minY: number; maxX: number; maxY: number } {
+    return { minX: 0, minY: 0, maxX: 0, maxY: 0 };
+  }
+  public dispose(): void {
+    // no-op
+  }
+}
+
+describe("tracer culls unphysical rays at the boundary", () => {
+  it("drops a ray with non-finite (NaN) brightness instead of propagating it", () => {
+    const nanRay = makeRay({ x: 0, y: 0 }, { x: 1, y: 0 });
+    nanRay.brightnessS = Number.NaN;
+    const result = new RayTracer([new StubSource([nanRay])]).trace();
+    // Culled before any segment is emitted; the truncation total stays finite.
+    expect(result.segments).toHaveLength(0);
+    expect(Number.isFinite(result.truncationError)).toBe(true);
+  });
+
+  it("drops a ray whose direction is the zero vector", () => {
+    const degenerateRay = makeRay({ x: 0, y: 0 }, { x: 0, y: 0 });
+    const result = new RayTracer([new StubSource([degenerateRay])]).trace();
+    expect(result.segments).toHaveLength(0);
+  });
+
+  it("still traces a normal ray from the same stub source", () => {
+    const goodRay = makeRay({ x: 0, y: 0 }, { x: 1, y: 0 });
+    const result = new RayTracer([new StubSource([goodRay])]).trace();
+    // A normal ray with nothing to hit escapes and records exactly one segment,
+    // with finite endpoints.
+    expect(result.segments).toHaveLength(1);
+    const seg = result.segments[0];
+    expect(seg).toBeDefined();
+    if (!seg) {
+      return;
+    }
+    expect(Number.isFinite(seg.p2.x) && Number.isFinite(seg.p2.y)).toBe(true);
+  });
+});