Agent Skills: When to use

When to use

• Detect moving objects in scenes with camera motion; produce sparse masks aligned to sampled frames.

Workflow

1. Global alignment: warp previous gray frame to current using estimated affine/homography.
2. Valid region: also warp an all-ones mask to get valid pixels, avoiding border fill.
3. Difference + adaptive threshold: diff = abs(curr - warp_prev); on diff[valid] compute median + 3×MAD; use a reasonable minimum threshold to avoid triggering on noise.
4. Morphology + area filter: open then close; keep connected components above a minimum area (tune as fraction of image area or a fixed pixel threshold).
5. CSR encoding: for final bool mask
   • rows, cols = nonzero(mask)
   • indices = cols.astype(int32); data = ones(nnz, uint8)
   • counts = bincount(rows, minlength=H); indptr = cumsum(counts, prepend=0)
   • store as f_{i}_data/indices/indptr

Code sketch

warped_prev = cv2.warpAffine(prev_gray, M, (W,H), flags=cv2.INTER_LINEAR, borderValue=0)
valid = cv2.warpAffine(np.ones((H,W),uint8), M, (W,H), flags=cv2.INTER_NEAREST)>0
diff = cv2.absdiff(curr_gray, warped_prev)
vals = diff[valid]
thr = max(20, np.median(vals) + 3*1.4826*np.median(np.abs(vals - np.median(vals))))
raw = (diff>thr) & valid
m = cv2.morphologyEx(raw.astype(uint8)*255, cv2.MORPH_OPEN, k3)
m = cv2.morphologyEx(m, cv2.MORPH_CLOSE, k7)
n, cc, stats, _ = cv2.connectedComponentsWithStats(m>0, connectivity=8)
mask = np.zeros_like(raw, dtype=bool)
for cid in range(1,n):
    if stats[cid, cv2.CC_STAT_AREA] >= min_area:
        mask |= (cc==cid)

Self-check

• [ ] Masks only for sampled frames; keys match sampled indices.
• [ ] shape stored as [H, W] int32; len(indptr)==H+1; indptr[-1]==indices.size.
• [ ] Border fill not treated as foreground; threshold stats computed on valid region only.
• [ ] Threshold + morphology + area filter applied.