feat: run manual polygons through the shared margin pipeline

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/lib/margin.ts

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+import type { Mat } from '@techstark/opencv-js';
+import { traceMask } from './detect';
+import type { CV } from './opencv-loader';
+import type { Point, Shape } from './types';
+
+export function shapeFromPolygons(cv: CV, width: number, height: number, polygons: Shape, marginPx: number): Shape | null {
+  if (polygons.outer.length < 3) return null;
+  const mask = cv.Mat.zeros(height, width, cv.CV_8UC1) as Mat;
+  try {
+    fill(cv, mask, [polygons.outer], 255);
+    fill(cv, mask, polygons.holes.filter((h) => h.length >= 3), 0);
+    return traceMask(cv, mask, marginPx);
+  } finally {
+    mask.delete();
+  }
+}
+
+function fill(cv: CV, mask: Mat, rings: Point[][], value: number) {
+  if (rings.length === 0) return;
+  const mats = rings.map((ring) =>
+    cv.matFromArray(ring.length, 1, cv.CV_32SC2, ring.flatMap((p) => [Math.round(p.x), Math.round(p.y)])),
+  );
+  const vector = new cv.MatVector();
+  try {
+    mats.forEach((m) => vector.push_back(m));
+    cv.fillPoly(mask, vector, new cv.Scalar(value));
+  } finally {
+    vector.delete();
+    mats.forEach((m) => m.delete());
+  }
+}

tests/margin.test.ts

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+import { beforeAll, describe, expect, it } from 'vitest';
+import { bounds } from '../src/lib/geometry';
+import { shapeFromPolygons } from '../src/lib/margin';
+import { loadOpenCV, type CV } from '../src/lib/opencv-loader';
+
+let cv: CV;
+beforeAll(async () => {
+  ({ cv } = await loadOpenCV());
+});
+
+const outer = [{ x: 40, y: 30 }, { x: 160, y: 30 }, { x: 160, y: 130 }, { x: 40, y: 130 }];
+const hole = [{ x: 90, y: 70 }, { x: 110, y: 70 }, { x: 110, y: 90 }, { x: 90, y: 90 }];
+const near = (actual: number, expected: number) => expect(Math.abs(actual - expected)).toBeLessThanOrEqual(2);
+
+describe('shapeFromPolygons', () => {
+  it('traces the drawn outline and hole', () => {
+    const shape = shapeFromPolygons(cv, 200, 160, { outer, holes: [hole] }, 0)!;
+    const b = bounds(shape.outer);
+    near(b.minX, 40); near(b.maxX, 160); near(b.minY, 30); near(b.maxY, 130);
+    expect(shape.holes).toHaveLength(1);
+  });
+
+  it('applies the margin to outline and hole', () => {
+    const shape = shapeFromPolygons(cv, 200, 160, { outer, holes: [hole] }, -5)!;
+    near(bounds(shape.outer).minX, 45);
+    near(bounds(shape.holes[0]).minX, 84);
+  });
+
+  it('returns null for an outline with fewer than 3 points', () => {
+    expect(shapeFromPolygons(cv, 200, 160, { outer: outer.slice(0, 2), holes: [] }, 0)).toBeNull();
+  });
+});