diff --git a/src/lib/detect.ts b/src/lib/detect.ts
new file mode 100644
index 0000000..295713e
--- /dev/null
+++ b/src/lib/detect.ts
@@ -0,0 +1,81 @@
+import type { Mat } from '@techstark/opencv-js';
+import type { CV } from './opencv-loader';
+import type { Point, RasterImage, Shape } from './types';
+
+/** Blurred grayscale; shared by thresholding and Otsu so both see the same pixels. */
+function toGray(cv: CV, image: RasterImage): Mat {
+  const src = cv.matFromImageData(image as unknown as ImageData);
+  const gray = new cv.Mat();
+  try {
+    cv.cvtColor(src, gray, cv.COLOR_RGBA2GRAY);
+    cv.GaussianBlur(gray, gray, new cv.Size(5, 5), 0);
+    return gray;
+  } finally {
+    src.delete();
+  }
+}
+
+export function otsuThreshold(cv: CV, image: RasterImage): number {
+  const gray = toGray(cv, image);
+  const out = new cv.Mat();
+  try {
+    return Math.round(cv.threshold(gray, out, 0, 255, cv.THRESH_BINARY | cv.THRESH_OTSU));
+  } finally {
+    gray.delete();
+    out.delete();
+  }
+}
+
+export function traceMask(cv: CV, mask: Mat, marginPx: number): Shape | null {
+  const radius = Math.round(Math.abs(marginPx));
+  // Padding keeps dilation from being clipped and contours off the image edge.
+  const pad = radius + 2;
+  const padded = new cv.Mat();
+  const contours = new cv.MatVector();
+  const hierarchy = new cv.Mat();
+  let kernel: Mat | null = null;
+  try {
+    cv.copyMakeBorder(mask, padded, pad, pad, pad, pad, cv.BORDER_CONSTANT, new cv.Scalar(0));
+    if (radius > 0) {
+      kernel = cv.getStructuringElement(cv.MORPH_ELLIPSE, new cv.Size(2 * radius + 1, 2 * radius + 1));
+      if (marginPx < 0) cv.erode(padded, padded, kernel);
+      else cv.dilate(padded, padded, kernel);
+    }
+    cv.findContours(padded, contours, hierarchy, cv.RETR_CCOMP, cv.CHAIN_APPROX_SIMPLE);
+
+    const count = contours.size();
+    const h = hierarchy.data32S; // per contour: [next, prev, firstChild, parent]
+    let best = -1;
+    let bestArea = 0;
+    for (let i = 0; i < count; i++) {
+      if (h[i * 4 + 3] !== -1) continue;
+      const c = contours.get(i);
+      const area = cv.contourArea(c);
+      c.delete();
+      if (area > bestArea) {
+        bestArea = area;
+        best = i;
+      }
+    }
+    if (best === -1) return null;
+
+    const ring = (i: number): Point[] => {
+      const c = contours.get(i);
+      const d = c.data32S;
+      const pts: Point[] = [];
+      for (let k = 0; k < d.length; k += 2) pts.push({ x: d[k] - pad, y: d[k + 1] - pad });
+      c.delete();
+      return pts;
+    };
+    const holes: Point[][] = [];
+    for (let i = 0; i < count; i++) {
+      if (h[i * 4 + 3] === best) holes.push(ring(i));
+    }
+    return { outer: ring(best), holes };
+  } finally {
+    padded.delete();
+    contours.delete();
+    hierarchy.delete();
+    kernel?.delete();
+  }
+}
diff --git a/src/lib/opencv-loader.ts b/src/lib/opencv-loader.ts
new file mode 100644
index 0000000..1059268
--- /dev/null
+++ b/src/lib/opencv-loader.ts
@@ -0,0 +1,18 @@
+export type CV = typeof import('@techstark/opencv-js');
+
+let loading: Promise<{ cv: CV }> | null = null;
+
+// Wrapped in an object: the cv module is a thenable, so resolving with it directly never settles.
+export function loadOpenCV(): Promise<{ cv: CV }> {
+  loading ??= (async () => {
+    const mod = (await import('@techstark/opencv-js')) as unknown as { default: CV };
+    const cv = mod.default;
+    if (!cv.Mat) {
+      await new Promise<void>((resolve) => {
+        (cv as unknown as { onRuntimeInitialized: () => void }).onRuntimeInitialized = () => resolve();
+      });
+    }
+    return { cv };
+  })();
+  return loading;
+}
diff --git a/tests/helpers/images.ts b/tests/helpers/images.ts
new file mode 100644
index 0000000..8e2d74d
--- /dev/null
+++ b/tests/helpers/images.ts
@@ -0,0 +1,35 @@
+import type { Mat } from '@techstark/opencv-js';
+import type { CV } from '../../src/lib/opencv-loader';
+import type { RasterImage } from '../../src/lib/types';
+
+/** Grayscale RGBA image where `paint(x, y)` returns the brightness 0–255. */
+export function grayImage(width: number, height: number, paint: (x: number, y: number) => number): RasterImage {
+  const data = new Uint8ClampedArray(width * height * 4);
+  for (let y = 0; y < height; y++) {
+    for (let x = 0; x < width; x++) {
+      const i = (y * width + x) * 4;
+      const v = paint(x, y);
+      data[i] = data[i + 1] = data[i + 2] = v;
+      data[i + 3] = 255;
+    }
+  }
+  return { width, height, data };
+}
+
+/** CV_8UC1 mask with 255 where `inside(x, y)`. Caller must delete it. */
+export function maskFrom(cv: CV, width: number, height: number, inside: (x: number, y: number) => boolean): Mat {
+  const mask = cv.Mat.zeros(height, width, cv.CV_8UC1) as Mat;
+  const data = mask.data;
+  for (let y = 0; y < height; y++) {
+    for (let x = 0; x < width; x++) {
+      if (inside(x, y)) data[y * width + x] = 255;
+    }
+  }
+  return mask;
+}
+
+export const inRect = (x: number, y: number, x0: number, y0: number, x1: number, y1: number) =>
+  x >= x0 && x < x1 && y >= y0 && y < y1;
+
+export const inCircle = (x: number, y: number, cx: number, cy: number, r: number) =>
+  (x - cx) ** 2 + (y - cy) ** 2 <= r * r;
diff --git a/tests/trace-mask.test.ts b/tests/trace-mask.test.ts
new file mode 100644
index 0000000..df41cc7
--- /dev/null
+++ b/tests/trace-mask.test.ts
@@ -0,0 +1,95 @@
+import { afterEach, beforeAll, describe, expect, it } from 'vitest';
+import type { Mat } from '@techstark/opencv-js';
+import { otsuThreshold, traceMask } from '../src/lib/detect';
+import { bounds } from '../src/lib/geometry';
+import { loadOpenCV, type CV } from '../src/lib/opencv-loader';
+import { grayImage, inCircle, inRect, maskFrom } from './helpers/images';
+
+let cv: CV;
+const mats: Mat[] = [];
+beforeAll(async () => {
+  ({ cv } = await loadOpenCV());
+});
+afterEach(() => {
+  mats.splice(0).forEach((m) => m.delete());
+});
+
+/** 200x160 plate: rect x 40..159, y 30..129 with a round hole r=15 at (100,80). */
+function plate(): Mat {
+  const m = maskFrom(cv, 200, 160, (x, y) => inRect(x, y, 40, 30, 160, 130) && !inCircle(x, y, 100, 80, 15));
+  mats.push(m);
+  return m;
+}
+
+describe('loadOpenCV', () => {
+  it('resolves to the same cv instance every time', async () => {
+    const again = await loadOpenCV();
+    expect(again.cv).toBe(cv);
+    expect(typeof cv.Mat).toBe('function');
+  });
+});
+
+describe('traceMask', () => {
+  it('returns the outer contour and the hole', () => {
+    const shape = traceMask(cv, plate(), 0)!;
+    const b = bounds(shape.outer);
+    expect(Math.abs(b.minX - 40)).toBeLessThanOrEqual(2);
+    expect(Math.abs(b.maxX - 159)).toBeLessThanOrEqual(2);
+    expect(Math.abs(b.minY - 30)).toBeLessThanOrEqual(2);
+    expect(Math.abs(b.maxY - 129)).toBeLessThanOrEqual(2);
+    expect(shape.holes).toHaveLength(1);
+    const h = bounds(shape.holes[0]);
+    expect(h.minX).toBeGreaterThan(82);
+    expect(h.maxX).toBeLessThan(118);
+  });
+
+  it('shrinks the outer edge and grows holes for a negative margin', () => {
+    const shape = traceMask(cv, plate(), -5)!;
+    const b = bounds(shape.outer);
+    expect(Math.abs(b.minX - 45)).toBeLessThanOrEqual(2);
+    expect(Math.abs(b.maxX - 154)).toBeLessThanOrEqual(2);
+    const h = bounds(shape.holes[0]);
+    expect(Math.abs(h.minX - 79)).toBeLessThanOrEqual(2);
+    expect(Math.abs(h.maxX - 121)).toBeLessThanOrEqual(2);
+  });
+
+  it('grows the outer edge for a positive margin, even past the image border', () => {
+    const shape = traceMask(cv, plate(), 5)!;
+    expect(Math.abs(bounds(shape.outer).minX - 35)).toBeLessThanOrEqual(2);
+
+    const edge = maskFrom(cv, 100, 100, (x, y) => inRect(x, y, 0, 20, 60, 80));
+    mats.push(edge);
+    expect(bounds(traceMask(cv, edge, 5)!.outer).minX).toBeLessThan(0);
+  });
+
+  it('keeps the larger part when erosion splits the piece in two', () => {
+    // 60x80 block and 40x40 block joined by a 7 px bridge; an 11 px erosion cuts the bridge.
+    const m = maskFrom(cv, 200, 120, (x, y) =>
+      inRect(x, y, 20, 20, 80, 100) || inRect(x, y, 120, 40, 160, 80) || inRect(x, y, 80, 56, 120, 63));
+    mats.push(m);
+    const b = bounds(traceMask(cv, m, -5)!.outer);
+    expect(b.maxX).toBeLessThan(100);
+  });
+
+  it('returns null when the margin removes the piece entirely', () => {
+    const m = maskFrom(cv, 50, 50, (x, y) => inRect(x, y, 20, 20, 30, 30));
+    mats.push(m);
+    expect(traceMask(cv, m, -8)).toBeNull();
+  });
+
+  it('does not modify the input mask', () => {
+    const m = plate();
+    const before = cv.countNonZero(m);
+    traceMask(cv, m, -5);
+    expect(cv.countNonZero(m)).toBe(before);
+  });
+});
+
+describe('otsuThreshold', () => {
+  it('lands between background and piece brightness', () => {
+    const image = grayImage(200, 160, (x, y) => (inRect(x, y, 40, 30, 160, 130) ? 220 : 30));
+    const t = otsuThreshold(cv, image);
+    expect(t).toBeGreaterThan(30);
+    expect(t).toBeLessThan(220);
+  });
+});
diff --git a/vite.config.ts b/vite.config.ts
index 02b567c..cc71282 100644
--- a/vite.config.ts
+++ b/vite.config.ts
@@ -11,5 +11,7 @@ export default defineConfig({
     include: ['tests/**/*.test.ts'],
     testTimeout: 30000,
     hookTimeout: 60000,
+    // OpenCV's CJS export is a thenable; copying it onto the namespace makes `import()` never settle.
+    deps: { interopDefault: false },
   },
 });