feat: add OpenCV loader, Otsu threshold and margin-aware mask tracing

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

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();
+  }
+}