Object Detection

Object Detection is the computer vision task of localizing and classifying all objects in an image — outputting bounding boxes and class labels, and serving as the foundation for autonomous driving, surveillance, robotics, and medical imaging.

Detection Paradigms

Two-Stage (R-CNN Family):
- Stage 1: Region Proposal Network (RPN) → generate candidate regions.
- Stage 2: Classify and refine each region independently.
- Examples: R-CNN, Fast R-CNN, Faster R-CNN, Mask R-CNN.
- Pros: Higher accuracy. Cons: Slower (~5 FPS).

One-Stage (YOLO Family):
- Single forward pass predicts all boxes simultaneously.
- Divide image into SxS grid; each cell predicts B bounding boxes.
- YOLOv1 (2016) → YOLOv8 (2023): Accuracy improved to match two-stage.
- YOLOv8: 50+ FPS on GPU, 55 mAP on COCO — standard for real-time detection.

Anchor-Based vs. Anchor-Free

• Anchor boxes: Predefined aspect ratios/sizes. Network predicts offsets from anchors.
• Problem: Anchor hyperparameters, many candidates, slow.
• Anchor-free (FCOS, CenterNet): Predict from center or feature point directly.
• Simpler, faster, better on objects with unusual aspect ratios.

Feature Pyramid Network (FPN)

• Multi-scale feature extraction: Top-down pathway with lateral connections.
• Small objects detected at high-resolution features (early layers).
• Large objects detected at low-resolution features (later layers).
• Standard in all modern detectors.

DETR (Detection Transformer, 2020)

• Transformer encoder-decoder with learned object queries.
• No anchors, no NMS — set prediction with Hungarian matching loss.
• Global attention captures long-range relationships.
• Deformable DETR: 10x faster convergence with deformable attention.

Key Metrics

• mAP (mean Average Precision): Standard benchmark metric at IoU thresholds.
• COCO dataset: mAP@[.5:.95] — standard benchmark.
• State-of-art (2024): 60+ mAP with ensemble/large models.

Object detection is the gateway task for visual understanding of scenes — its algorithms power every camera-based safety system, content moderation tool, and autonomous navigation system deployed at scale today.