diff --git a/README.md b/README.md
index ab195ab..d272877 100644
--- a/README.md
+++ b/README.md
@@ -7,8 +7,8 @@ RDWorks (or LightBurn/Inkscape) whether your design fits on it. Built for Makers
 
 1. Photograph the piece from straight above, on a contrasting surface, with something of known length visible.
 2. Load the photo, click **Measure**, click both ends of the known length, and enter it in mm.
-3. Check the green outline. If needed, adjust the threshold, tick "darker than background",
-   click the piece, or draw the outline by hand.
+3. Check the green outline. If needed, click the piece (this also tells the app whether
+   it is darker or lighter than the background), adjust the threshold, or draw the outline by hand.
 4. Set the safety margin (default −2 mm shrinks the usable area a little).
 5. Download DXF and import it in your laser software at 1:1, in mm.
 
diff --git a/src/components/Sidebar.svelte b/src/components/Sidebar.svelte
index d9bdcf4..14117be 100644
--- a/src/components/Sidebar.svelte
+++ b/src/components/Sidebar.svelte
@@ -60,7 +60,6 @@
   {#if app.mode === 'auto'}
     <label>Threshold <input type="range" min="0" max="255" bind:value={app.threshold} /> {app.threshold}</label>
     <button onclick={() => app.autoThreshold()} disabled={!app.cv || !app.image}>Auto</button>
-    <label><input type="checkbox" bind:checked={app.invert} /> Piece is darker than background</label>
     <button class:active={app.tool === 'seed'} onclick={() => toggle('seed')} disabled={!app.image}>Click to pick piece</button>
   {/if}
   <button class:active={app.tool === 'draw'} onclick={() => toggle('draw')} disabled={!app.image}>
@@ -73,7 +72,7 @@
     <button onclick={() => app.clearManual()}>Back to automatic</button>
   {/if}
   {#if nothingDetected}
-    <p class="warning">Nothing detected, try adjusting the threshold or invert, or draw manually.</p>
+    <p class="warning">Nothing detected, try adjusting the threshold, clicking the piece, or draw manually.</p>
   {/if}
 </section>
 
diff --git a/src/lib/app-state.svelte.ts b/src/lib/app-state.svelte.ts
index 4ce5c7d..10b394b 100644
--- a/src/lib/app-state.svelte.ts
+++ b/src/lib/app-state.svelte.ts
@@ -22,7 +22,6 @@ export class AppState {
   scalePoints = $state<Point[]>([]);
   lengthMm = $state<number | null>(null);
   threshold = $state(DEFAULT_THRESHOLD);
-  invert = $state(false);
   seed = $state<Point | null>(null);
   marginMm = $state(-2);
   mode = $state<'auto' | 'manual'>('auto');
@@ -45,7 +44,7 @@ export class AppState {
       return shapeFromPolygons(cv, image.width, image.height, { outer: this.manualOuter, holes: this.manualHoles }, this.marginPx);
     }
     return detect(cv, image, {
-      threshold: this.threshold, invert: this.invert, seed: this.seed, marginPx: this.marginPx,
+      threshold: this.threshold, seed: this.seed, marginPx: this.marginPx,
       minHolePx: this.pxPerMm ? MIN_HOLE_MM * this.pxPerMm : 0,
     });
   });
diff --git a/src/lib/detect.ts b/src/lib/detect.ts
index 207ae6f..4a498f5 100644
--- a/src/lib/detect.ts
+++ b/src/lib/detect.ts
@@ -2,21 +2,91 @@ import type { Mat } from '@techstark/opencv-js';
 import type { CV } from './opencv-loader';
 import type { DetectParams, 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 {
+/** Blurred, lighting-corrected grayscale; shared by thresholding and Otsu so both see the same pixels. */
+function toGray(cv: CV, image: RasterImage): { gray: Mat; background: number } {
   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;
+    return { gray, background: flattenLighting(gray) };
   } finally {
     src.delete();
   }
 }
 
+/**
+ * Removes smooth lighting gradients (side light, vignetting) by fitting a quadratic surface to the
+ * photo's border band, which is almost always background. Returns the background brightness.
+ */
+function flattenLighting(gray: Mat): number {
+  const w = gray.cols;
+  const h = gray.rows;
+  const g = gray.data;
+  const band = Math.max(1, Math.round(Math.min(w, h) * 0.08));
+  const step = Math.max(1, Math.round(Math.min(w, h) / 400));
+  const terms = (x: number, y: number) => {
+    const u = x / w - 0.5;
+    const v = y / h - 0.5;
+    return [1, u, v, u * u, u * v, v * v];
+  };
+  const samples: { t: number[]; v: number }[] = [];
+  for (let y = 0; y < h; y += step) {
+    for (let x = 0; x < w; x += step) {
+      if (x >= band && x < w - band && y >= band && y < h - band) continue;
+      samples.push({ t: terms(x, y), v: g[y * w + x] });
+    }
+  }
+  // Keep samples near the median first, so a piece touching the border barely affects the fit.
+  const sorted = samples.map((s) => s.v).sort((a, b) => a - b);
+  const median = sorted[sorted.length >> 1];
+  const mad = samples.map((s) => Math.abs(s.v - median)).sort((a, b) => a - b)[samples.length >> 1];
+  const nearMedian = samples.filter((s) => Math.abs(s.v - median) <= 3.7 * mad + 2);
+  let coef = fitSurface(nearMedian);
+  // Second pass drops outliers (a piece touching the border) so they don't skew the fit.
+  const residuals = nearMedian.map((s) => s.v - dot(coef, s.t));
+  const sd = Math.sqrt(residuals.reduce((a, r) => a + r * r, 0) / residuals.length);
+  const inliers = nearMedian.filter((_, i) => Math.abs(residuals[i]) <= 2 * sd + 1);
+  if (inliers.length >= 6) coef = fitSurface(inliers);
+  const level = inliers.reduce((a, s) => a + s.v, 0) / inliers.length;
+
+  for (let y = 0; y < h; y++) {
+    for (let x = 0; x < w; x++) {
+      const i = y * w + x;
+      g[i] = Math.max(0, Math.min(255, Math.round(g[i] - dot(coef, terms(x, y)) + level)));
+    }
+  }
+  return level;
+}
+
+const dot = (a: number[], b: number[]) => a.reduce((sum, ai, i) => sum + ai * b[i], 0);
+
+/** Least-squares fit via the normal equations, solved with Gaussian elimination. */
+function fitSurface(samples: { t: number[]; v: number }[]): number[] {
+  const n = 6;
+  const m = Array.from({ length: n }, () => new Array<number>(n + 1).fill(0));
+  for (const { t, v } of samples) {
+    for (let i = 0; i < n; i++) {
+      for (let j = 0; j < n; j++) m[i][j] += t[i] * t[j];
+      m[i][n] += t[i] * v;
+    }
+  }
+  for (let c = 0; c < n; c++) {
+    let pivot = c;
+    for (let r = c + 1; r < n; r++) if (Math.abs(m[r][c]) > Math.abs(m[pivot][c])) pivot = r;
+    [m[c], m[pivot]] = [m[pivot], m[c]];
+    if (Math.abs(m[c][c]) < 1e-12) continue;
+    for (let r = 0; r < n; r++) {
+      if (r === c) continue;
+      const f = m[r][c] / m[c][c];
+      for (let k = c; k <= n; k++) m[r][k] -= f * m[c][k];
+    }
+  }
+  return m.map((row, i) => (Math.abs(row[i]) < 1e-12 ? 0 : row[n] / row[i]));
+}
+
 export function otsuThreshold(cv: CV, image: RasterImage): number {
-  const gray = toGray(cv, image);
+  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));
@@ -81,20 +151,24 @@ export function traceMask(cv: CV, mask: Mat, marginPx: number): Shape | null {
 }
 
 export function detect(cv: CV, image: RasterImage, params: DetectParams): Shape | null {
-  const gray = toGray(cv, image);
+  const { gray, background } = toGray(cv, image);
   const mask = new cv.Mat();
   const kernel = cv.getStructuringElement(cv.MORPH_ELLIPSE, new cv.Size(5, 5));
   const labels = new cv.Mat();
   const stats = new cv.Mat();
   const centroids = new cv.Mat();
   try {
-    const type = params.invert ? cv.THRESH_BINARY_INV : cv.THRESH_BINARY;
-    cv.threshold(gray, mask, params.threshold, 255, type);
-    cv.morphologyEx(mask, mask, cv.MORPH_OPEN, kernel);
-    cv.morphologyEx(mask, mask, cv.MORPH_CLOSE, kernel);
-    const count = cv.connectedComponentsWithStats(mask, labels, stats, centroids, 8, cv.CV_32S);
-
-    const label = pickComponent(cv, labels, stats, count, image.width, image.height, params.seed);
+    const segment = (dark: boolean, seed: Point | null) => {
+      cv.threshold(gray, mask, params.threshold, 255, dark ? cv.THRESH_BINARY_INV : cv.THRESH_BINARY);
+      cv.morphologyEx(mask, mask, cv.MORPH_OPEN, kernel);
+      cv.morphologyEx(mask, mask, cv.MORPH_CLOSE, kernel);
+      const count = cv.connectedComponentsWithStats(mask, labels, stats, centroids, 8, cv.CV_32S);
+      return pickComponent(cv, labels, stats, count, image.width, image.height, seed);
+    };
+    const seedDark = seedIsDark(gray, params.threshold, params.seed);
+    let label = seedDark === null ? null : segment(seedDark, params.seed);
+    // A click on the background makes it "the piece", which is rejected; fall back to the automatic choice.
+    if (label === null) label = segment(background > params.threshold, null);
     if (label === null) return null;
 
     const lab = labels.data32S;
@@ -173,3 +247,12 @@ function fillSmallHoles(cv: CV, mask: Mat, minHolePx: number) {
     hierarchy.delete();
   }
 }
+
+/** The clicked pixel's side of the threshold is the piece's; null without a usable click. */
+function seedIsDark(gray: Mat, threshold: number, seed: Point | null): boolean | null {
+  if (!seed) return null;
+  const x = Math.round(seed.x);
+  const y = Math.round(seed.y);
+  if (x < 0 || y < 0 || x >= gray.cols || y >= gray.rows) return null;
+  return gray.ucharAt(y, x) <= threshold;
+}
diff --git a/src/lib/types.ts b/src/lib/types.ts
index 532b689..3deadbf 100644
--- a/src/lib/types.ts
+++ b/src/lib/types.ts
@@ -8,7 +8,6 @@ export type RasterImage = { width: number; height: number; data: Uint8ClampedArr
 
 export type DetectParams = {
   threshold: number;
-  invert: boolean;
   seed: Point | null;
   /** Negative shrinks the piece (and grows holes). */
   marginPx: number;
diff --git a/tests/detect.test.ts b/tests/detect.test.ts
index b6dcc10..a3a18ba 100644
--- a/tests/detect.test.ts
+++ b/tests/detect.test.ts
@@ -1,5 +1,5 @@
 import { beforeAll, describe, expect, it } from 'vitest';
-import { detect } from '../src/lib/detect';
+import { detect, otsuThreshold } from '../src/lib/detect';
 import { bounds } from '../src/lib/geometry';
 import { loadOpenCV, type CV } from '../src/lib/opencv-loader';
 import type { DetectParams } from '../src/lib/types';
@@ -11,7 +11,7 @@ beforeAll(async () => {
 });
 
 const params = (overrides: Partial<DetectParams> = {}): DetectParams => ({
-  threshold: 128, invert: false, seed: null, marginPx: 0, minHolePx: 0, ...overrides,
+  threshold: 128, seed: null, marginPx: 0, minHolePx: 0, ...overrides,
 });
 
 /** Light plate x 40..159, y 30..129 with hole r=15 at (100,80), plus a small light blob, on dark ground. */
@@ -31,24 +31,35 @@ describe('detect', () => {
     expect(shape.holes).toHaveLength(1);
   });
 
-  it('handles a dark piece on a light background with invert', () => {
+  it('finds a dark piece on a light background without any setting', () => {
     const inverted = grayImage(200, 160, (x, y) => 250 - (inRect(x, y, 40, 30, 160, 130) ? 220 : 30));
-    expect(detect(cv, inverted, params())).toBeNull();
-    const b = bounds(detect(cv, inverted, params({ invert: true }))!.outer);
+    const b = bounds(detect(cv, inverted, params())!.outer);
     near(b.minX, 40); near(b.maxX, 159);
   });
 
+  it('takes the polarity from the clicked pixel', () => {
+    const inverted = grayImage(200, 160, (x, y) => 250 - (inRect(x, y, 40, 30, 160, 130) ? 220 : 30));
+    const b = bounds(detect(cv, inverted, params({ seed: { x: 100, y: 80 } }))!.outer);
+    near(b.minX, 40); near(b.maxX, 159);
+  });
+
+  it('corrects uneven lighting across the photo', () => {
+    // Light from the right: the floor ramps 120 → 220 and the piece is 50 darker than the floor around it.
+    const sideLit = grayImage(400, 300, (x, y) => 120 + (100 * x) / 400 - (inRect(x, y, 100, 75, 300, 225) ? 50 : 0));
+    const shape = detect(cv, sideLit, params({ threshold: otsuThreshold(cv, sideLit) }))!;
+    const b = bounds(shape.outer);
+    near(b.minX, 100); near(b.maxX, 299); near(b.minY, 75); near(b.maxY, 224);
+  });
+
   it('selects the clicked piece even when another piece is larger', () => {
     const shape = detect(cv, plateScene, params({ seed: { x: 177, y: 146 } }))!;
     const b = bounds(shape.outer);
     near(b.minX, 170); near(b.maxX, 184);
   });
 
-  it('falls back to the automatic choice when the seed is on background or in a hole', () => {
-    for (const seed of [{ x: 5, y: 5 }, { x: 100, y: 80 }]) {
-      const b = bounds(detect(cv, plateScene, params({ seed }))!.outer);
-      near(b.minX, 40);
-    }
+  it('falls back to the automatic choice when the click is on the background', () => {
+    const b = bounds(detect(cv, plateScene, params({ seed: { x: 5, y: 5 } }))!.outer);
+    near(b.minX, 40);
   });
 
   it('ignores a seed outside the image', () => {
diff --git a/tests/fixtures.test.ts b/tests/fixtures.test.ts
index 8c0250d..159ee71 100644
--- a/tests/fixtures.test.ts
+++ b/tests/fixtures.test.ts
@@ -10,10 +10,9 @@ import type { Point, RasterImage } from '../src/lib/types';
 type FixtureCase = {
   file: string;
   threshold: number | null;
-  invert: boolean;
   seed: Point | null;
   pxPerMm: number;
-  expected: { widthMm: number; heightMm: number; holes: number };
+  expected: { widthMm: number; heightMm: number; holes?: number };
   toleranceMm: number;
 };
 
@@ -34,12 +33,12 @@ describe.skipIf(cases.length === 0)('detect on real photos', () => {
   it.each(cases)('$file', (c) => {
     const image = readJpeg(dir + c.file);
     const threshold = c.threshold ?? otsuThreshold(cv, image);
-    const shape = detect(cv, image, { threshold, invert: c.invert, seed: c.seed, marginPx: 0, minHolePx: 3 * c.pxPerMm });
+    const shape = detect(cv, image, { threshold, seed: c.seed, marginPx: 0, minHolePx: 3 * c.pxPerMm });
     expect(shape).not.toBeNull();
     const mm = prepareForExport(shape!, c.pxPerMm)!;
     const size = shapeSize(mm);
     expect(Math.abs(size.width - c.expected.widthMm)).toBeLessThanOrEqual(c.toleranceMm);
     expect(Math.abs(size.height - c.expected.heightMm)).toBeLessThanOrEqual(c.toleranceMm);
-    expect(mm.holes).toHaveLength(c.expected.holes);
+    if (c.expected.holes !== undefined) expect(mm.holes).toHaveLength(c.expected.holes);
   });
 });
diff --git a/tests/fixtures/README.md b/tests/fixtures/README.md
index 3cef82f..937c6a9 100644
--- a/tests/fixtures/README.md
+++ b/tests/fixtures/README.md
@@ -16,7 +16,6 @@ To add one:
 {
   "file": "plywood-1.jpg",
   "threshold": null,
-  "invert": false,
   "seed": null,
   "pxPerMm": 3.21,
   "expected": { "widthMm": 412, "heightMm": 287, "holes": 2 },
@@ -24,4 +23,5 @@ To add one:
 }
 ```
 
-`threshold: null` means Otsu's automatic value. `seed` is `{ "x": ..., "y": ... }` or `null`.
+`threshold: null` means Otsu's automatic value. `holes` is optional; leave it out when
+background texture creates noise holes. Without a known scale, use `pxPerMm: 1` and sizes in px. `seed` is `{ "x": ..., "y": ... }` or `null`.
diff --git a/tests/fixtures/manifest.json b/tests/fixtures/manifest.json
index fe51488..ecbebb9 100644
--- a/tests/fixtures/manifest.json
+++ b/tests/fixtures/manifest.json
@@ -1 +1,10 @@
-[]
+[
+  {
+    "file": "mdf_floor.jpeg",
+    "threshold": null,
+    "seed": null,
+    "pxPerMm": 1,
+    "expected": { "widthMm": 1714, "heightMm": 675 },
+    "toleranceMm": 60
+  }
+]
diff --git a/tests/fixtures/mdf_floor.jpeg b/tests/fixtures/mdf_floor.jpeg
new file mode 100644
index 0000000..51b70e5
Binary files /dev/null and b/tests/fixtures/mdf_floor.jpeg differ