phase 3

ui/detection-app/src/cv/tracks.ts

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+/**
+ * Motion track matching and rendering.
+ *
+ * Matches bounding boxes across frames by IoU (Intersection over Union).
+ * Uses Hungarian-style optimal assignment so every track gets its best match.
+ * Renders VFX-style tracks: × at start/end, dotted trace in between.
+ * Returns ImageData (RGBA, transparent background) for use as a FrameOverlay.
+ */
+
+export type TrackPoint = { seq: number; cx: number; cy: number; w: number; h: number }
+export type Track = { id: number; points: TrackPoint[] }
+
+type Box = { x: number; y: number; w: number; h: number }
+
+function iou(a: Box, b: Box): number {
+  const x1 = Math.max(a.x, b.x)
+  const y1 = Math.max(a.y, b.y)
+  const x2 = Math.min(a.x + a.w, b.x + b.w)
+  const y2 = Math.min(a.y + a.h, b.y + b.h)
+  const inter = Math.max(0, x2 - x1) * Math.max(0, y2 - y1)
+  if (inter === 0) return 0
+  const union = a.w * a.h + b.w * b.h - inter
+  return union > 0 ? inter / union : 0
+}
+
+/**
+ * Match bounding boxes across frames by IoU.
+ *
+ * For each pair of consecutive frames, computes an IoU cost matrix and
+ * assigns tracks to boxes using greedy-best-first on IoU score (descending).
+ * minIoU: minimum overlap to consider a match (default 0.15).
+ */
+export function matchTracks(
+  regionsByFrame: Record<number, Box[]>,
+  minIoU = 0.15,
+): Track[] {
+  const seqs = Object.keys(regionsByFrame).map(Number).sort((a, b) => a - b)
+  if (seqs.length === 0) return []
+
+  let nextId = 0
+  const active: Track[] = []
+  const finished: Track[] = []
+
+  for (const seq of seqs) {
+    const boxes = regionsByFrame[seq]
+
+    if (active.length === 0) {
+      for (const b of boxes) {
+        active.push({
+          id: nextId++,
+          points: [{ seq, cx: b.x + b.w / 2, cy: b.y + b.h / 2, w: b.w, h: b.h }],
+        })
+      }
+      continue
+    }
+
+    // Build cost matrix: IoU between each active track's last box and each new box
+    const costs: Array<{ trackIdx: number; boxIdx: number; score: number }> = []
+    for (let t = 0; t < active.length; t++) {
+      const last = active[t].points[active[t].points.length - 1]
+      const trackBox: Box = { x: last.cx - last.w / 2, y: last.cy - last.h / 2, w: last.w, h: last.h }
+      for (let b = 0; b < boxes.length; b++) {
+        const score = iou(trackBox, boxes[b])
+        if (score >= minIoU) {
+          costs.push({ trackIdx: t, boxIdx: b, score })
+        }
+      }
+    }
+
+    // Greedy-best-first assignment (sorted by descending IoU)
+    costs.sort((a, b) => b.score - a.score)
+    const matchedTracks = new Set<number>()
+    const matchedBoxes = new Set<number>()
+    const next: Track[] = []
+
+    for (const { trackIdx, boxIdx } of costs) {
+      if (matchedTracks.has(trackIdx) || matchedBoxes.has(boxIdx)) continue
+      matchedTracks.add(trackIdx)
+      matchedBoxes.add(boxIdx)
+      const b = boxes[boxIdx]
+      active[trackIdx].points.push({
+        seq, cx: b.x + b.w / 2, cy: b.y + b.h / 2, w: b.w, h: b.h,
+      })
+      next.push(active[trackIdx])
+    }
+
+    // Unmatched tracks → finished
+    for (let t = 0; t < active.length; t++) {
+      if (!matchedTracks.has(t)) finished.push(active[t])
+    }
+
+    // Unmatched boxes → new tracks
+    for (let b = 0; b < boxes.length; b++) {
+      if (!matchedBoxes.has(b)) {
+        const box = boxes[b]
+        next.push({
+          id: nextId++,
+          points: [{ seq, cx: box.x + box.w / 2, cy: box.y + box.h / 2, w: box.w, h: box.h }],
+        })
+      }
+    }
+
+    active.length = 0
+    active.push(...next)
+  }
+
+  return [...finished, ...active]
+}
+
+/**
+ * Render tracks to ImageData (RGBA, transparent background).
+ *
+ * Only draws tracks that span ≥2 frames (single-frame detections are skipped).
+ * × at start, × at end, dotted trace in between.
+ * Filled dot at currentSeq.
+ */
+export function renderTracksToImageData(
+  tracks: Track[],
+  width: number,
+  height: number,
+  currentSeq: number,
+  color = '#00e5ff',
+): ImageData {
+  const canvas = new OffscreenCanvas(width, height)
+  const ctx = canvas.getContext('2d')!
+
+  ctx.clearRect(0, 0, width, height)
+  ctx.strokeStyle = color
+  ctx.fillStyle = color
+
+  for (const track of tracks) {
+    const pts = track.points
+    // Skip single-frame detections — no temporal signal
+    if (pts.length < 2) continue
+
+    // Dotted trace connecting all centers
+    ctx.globalAlpha = 0.75
+    ctx.lineWidth = 1.5
+    ctx.setLineDash([4, 4])
+    ctx.beginPath()
+    ctx.moveTo(pts[0].cx, pts[0].cy)
+    for (let i = 1; i < pts.length; i++) {
+      ctx.lineTo(pts[i].cx, pts[i].cy)
+    }
+    ctx.stroke()
+    ctx.setLineDash([])
+
+    // × at first point
+    ctx.globalAlpha = 0.9
+    ctx.lineWidth = 1.5
+    drawX(ctx, pts[0].cx, pts[0].cy, 7)
+
+    // × at last point
+    drawX(ctx, pts[pts.length - 1].cx, pts[pts.length - 1].cy, 7)
+
+    // Filled dot at current frame
+    const curr = pts.find(p => p.seq === currentSeq)
+    if (curr) {
+      ctx.globalAlpha = 1
+      ctx.beginPath()
+      ctx.arc(curr.cx, curr.cy, 4, 0, Math.PI * 2)
+      ctx.fill()
+    }
+  }
+
+  return ctx.getImageData(0, 0, width, height)
+}
+
+function drawX(
+  ctx: OffscreenCanvasRenderingContext2D,
+  cx: number,
+  cy: number,
+  size: number,
+): void {
+  ctx.beginPath()
+  ctx.moveTo(cx - size, cy - size)
+  ctx.lineTo(cx + size, cy + size)
+  ctx.moveTo(cx + size, cy - size)
+  ctx.lineTo(cx - size, cy + size)
+  ctx.stroke()
+}