improve(panel-detection): enhance OpenCV detection with connected components

- Replace Canny edge detection with connected components analysis
- Add Otsu thresholding and morphological operations for better border detection
- Add 2-page spread detection and split processing
- Improve panel size and aspect ratio filtering
- Better preprocessing for comic/manga images with imperfect borders

panel-detection/opencv.js

@@ -1,62 +1,211 @@
 // panel-detection/opencv.js
-// OpenCV-based edge detection for panel boundaries
-export async function detectPanelsOpenCV(imageData, cv) {
+// OpenCV-based panel detection optimized for comic/manga panels
+// Extract a portion of image data for 2-page spread splitting
+function extractImageDataRegion(imageData, startX, startY, width, height) {
+  const result = new ImageData(
+    new Uint8ClampedArray(width * height * 4),
+    width,
+    height,
+  );
+
+  for (let y = 0; y < height; y++) {
+    for (let x = 0; x < width; x++) {
+      const srcIdx = ((startY + y) * imageData.width + (startX + x)) * 4;
+      const destIdx = (y * width + x) * 4;
+      result.data[destIdx] = imageData.data[srcIdx];
+      result.data[destIdx + 1] = imageData.data[srcIdx + 1];
+      result.data[destIdx + 2] = imageData.data[srcIdx + 2];
+      result.data[destIdx + 3] = imageData.data[srcIdx + 3];
+    }
+  }
+
+  return result;
+}
+// Preprocess image to isolate comic panel borders
+function preprocessComicImage(imageData, cv) {
   const src = cv.matFromImageData(imageData);
   const gray = new cv.Mat();
-  const blurred = new cv.Mat();
-  const edges = new cv.Mat();
-  const contours = new cv.MatVector();
-  const hierarchy = new cv.Mat();
+  const binary = new cv.Mat();
+  const processed = new cv.Mat();
 
   try {
+    // Convert to grayscale
     cv.cvtColor(src, gray, cv.COLOR_RGBA2GRAY, 0);
-    cv.GaussianBlur(gray, blurred, new cv.Size(5, 5), 0, 0, cv.BORDER_DEFAULT);
-    cv.Canny(blurred, edges, 50, 150, 3, false);
-    cv.findContours(
-      edges,
-      contours,
-      hierarchy,
-      cv.RETR_EXTERNAL,
-      cv.CHAIN_APPROX_SIMPLE,
-    );
 
-    const panels = [];
+    // Threshold using Otsu's method (auto-detects optimal threshold)
+    // THRESH_BINARY_INV makes dark borders white, light content black
+    cv.threshold(gray, binary, 0, 255, cv.THRESH_BINARY_INV + cv.THRESH_OTSU);
+
+    // Morphological operations to connect gaps in imperfect borders
+    // Larger kernel (7x7) helps connect gaps in newspaper-style borders
+    const kernel = cv.getStructuringElement(cv.MORPH_RECT, new cv.Size(7, 7));
+
+    // Dilate to connect gaps
+    cv.dilate(binary, processed, kernel, new cv.Point(-1, -1), 2);
+
+    // Erode to refine borders back to reasonable thickness
+    cv.erode(processed, processed, kernel, new cv.Point(-1, -1), 1);
+
+    return { src, gray, binary, processed };
+  } catch (e) {
+    // Clean up on error
+    src.delete();
+    gray.delete();
+    binary.delete();
+    processed.delete();
+    throw e;
+  }
+}
+// Detect panels in a single image (not split)
+function detectPanelsInImage(imageData, cv) {
+  const { src, gray, binary, processed } = preprocessComicImage(imageData, cv);
+
+  try {
     const imgWidth = imageData.width;
     const imgHeight = imageData.height;
 
-    for (let i = 0; i < contours.size(); i++) {
-      const rect = cv.boundingRect(contours.get(i));
-      const minSize = Math.min(imgWidth, imgHeight) * 0.08;
-      const aspectRatio = rect.width / rect.height;
+    // Connected components analysis
+    const labels = new cv.Mat();
+    const stats = new cv.Mat();
+    const centroids = new cv.Mat();
 
-      if (rect.width < minSize || rect.height < minSize) continue;
-      if (aspectRatio < 0.2 || aspectRatio > 8) continue;
+    const numComponents = cv.connectedComponentsWithStats(
+      processed,
+      labels,
+      stats,
+      centroids,
+      8, // connectivity type (8 = all neighbors)
+      cv.CV_32S,
+    );
 
+    const panels = [];
+    const minArea = imgWidth * imgHeight * 0.05; // Minimum 5% of image area
+    const minDimension = Math.min(imgWidth, imgHeight) * 0.15; // Minimum 15% of smallest dimension
+
+    // Start from 1 to skip background component (label 0)
+    for (let i = 1; i < numComponents; i++) {
+      // Extract component statistics
+      const x = stats.data32S[i * 5]; // CC_STAT_LEFT
+      const y = stats.data32S[i * 5 + 1]; // CC_STAT_TOP
+      const width = stats.data32S[i * 5 + 2]; // CC_STAT_WIDTH
+      const height = stats.data32S[i * 5 + 3]; // CC_STAT_HEIGHT
+      const area = stats.data32S[i * 5 + 4]; // CC_STAT_AREA
+
+      // Skip if too small (likely noise, text, or speech bubbles)
+      if (area < minArea) continue;
+
+      // Skip if dimensions too small
+      if (width < minDimension || height < minDimension) continue;
+
+      // Check aspect ratio (comic panels are typically rectangular)
+      const aspectRatio = width / height;
+      if (aspectRatio < 0.3 || aspectRatio > 5) continue;
+
+      // Convert to percentages
       panels.push({
         id: `opencv-${i}`,
-        x: (rect.x / imgWidth) * 100,
-        y: (rect.y / imgHeight) * 100,
-        width: (rect.width / imgWidth) * 100,
-        height: (rect.height / imgHeight) * 100,
-        reading_order: i,
+        x: (x / imgWidth) * 100,
+        y: (y / imgHeight) * 100,
+        width: (width / imgWidth) * 100,
+        height: (height / imgHeight) * 100,
+        reading_order: panels.length,
       });
     }
-    console.log("[OpenCV] Detected", panels.length, "potential panels");
 
-    panels.sort((a, b) => {
-      const rowA = Math.floor(a.y / 20);
-      const rowB = Math.floor(b.y / 20);
-      if (rowA !== rowB) return rowA - rowB;
-      return a.x - b.x;
-    });
+    // Clean up
+    labels.delete();
+    stats.delete();
+    centroids.delete();
 
-    return panels.map((p, i) => ({ ...p, reading_order: i }));
+    return panels;
   } finally {
     src.delete();
     gray.delete();
-    blurred.delete();
-    edges.delete();
-    contours.delete();
-    hierarchy.delete();
+    binary.delete();
+    processed.delete();
   }
 }
+// Main detection function with 2-page spread handling
+export async function detectPanelsOpenCV(imageData, cv) {
+  const imgWidth = imageData.width;
+  const imgHeight = imageData.height;
+  const aspectRatio = imgWidth / imgHeight;
+
+  console.log(
+    `[OpenCV] Processing ${imgWidth}x${imgHeight}px image (aspect ratio: ${aspectRatio.toFixed(2)})`,
+  );
+
+  // Detect if this is a 2-page spread (aspect ratio > 1.3 suggests two pages)
+  if (aspectRatio > 1.3) {
+    console.log("[OpenCV] Detected 2-page spread, splitting for processing");
+
+    const halfWidth = Math.floor(imgWidth / 2);
+
+    // Split into left and right pages
+    const leftData = extractImageDataRegion(
+      imageData,
+      0,
+      0,
+      halfWidth,
+      imgHeight,
+    );
+    const rightData = extractImageDataRegion(
+      imageData,
+      halfWidth,
+      0,
+      imgWidth - halfWidth,
+      imgHeight,
+    );
+
+    try {
+      const leftPanels = detectPanelsInImage(leftData, cv);
+      const rightPanels = detectPanelsInImage(rightData, cv);
+
+      console.log(
+        `[OpenCV] Left page: ${leftPanels.length} panels, Right page: ${rightPanels.length} panels`,
+      );
+
+      // Adjust right panel coordinates (they're relative to right half)
+      const adjustedRightPanels = rightPanels.map((panel) => ({
+        ...panel,
+        x: 50 + panel.x / 2, // Map 0-100% of right half to 50-100% of full width
+        width: panel.width / 2,
+        id: `opencv-right-${panel.id}`,
+      }));
+
+      // Combine panels
+      const allPanels = [...leftPanels, ...adjustedRightPanels];
+
+      console.log(`[OpenCV] Total detected panels: ${allPanels.length}`);
+
+      // Sort by reading order (left-to-right, top-to-bottom)
+      allPanels.sort((a, b) => {
+        const rowA = Math.floor(a.y / 20);
+        const rowB = Math.floor(b.y / 20);
+        if (rowA !== rowB) return rowA - rowB;
+        return a.x - b.x;
+      });
+
+      // Reassign reading_order
+      return allPanels.map((p, i) => ({ ...p, reading_order: i }));
+    } finally {
+      // Clean up image data regions
+      // (ImageData doesn't need explicit cleanup like OpenCV Mats)
+    }
+  }
+
+  // Single page processing
+  const panels = detectPanelsInImage(imageData, cv);
+
+  console.log(`[OpenCV] Detected ${panels.length} panels in single page`);
+
+  // Sort by reading order
+  panels.sort((a, b) => {
+    const rowA = Math.floor(a.y / 20);
+    const rowB = Math.floor(b.y / 20);
+    if (rowA !== rowB) return rowA - rowB;
+    return a.x - b.x;
+  });
+
+  return panels.map((p, i) => ({ ...p, reading_order: i }));
+}