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Object Detection — YOLO, Faster R-CNN, Anchor Boxes and mAP

Computer VisionDetection🟢 Free Lesson

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Computer Vision

Object Detection — Teaching Computers to Find and Identify Objects

Object detection localizes and classifies objects in images, predicting both bounding boxes and class labels. It is one of the most impactful applications of deep learning.

  • YOLO is Real-Time — Single-shot, grid-based prediction enables real-time detection at 30+ FPS
  • Faster R-CNN is Accurate — Two-stage detector with Region Proposal Network for high-precision detection
  • mAP is the Metric — Mean Average Precision over IoU thresholds is the standard evaluation measure

Object Detection — YOLO, Faster R-CNN, Anchor Boxes and mAP

Object detection localizes and classifies objects in images, predicting both bounding boxes and class labels. It is one of the most impactful applications of deep learning.


Detection vs. Classification

Object Detection Pipeline🚗🚶Input ImageBackbone(CNN)FeatureExtractionNeck(FPN)Multi-scaleFeaturesHead(Class + Box)PredictionOutput🚗 (0.95)🚶 (0.88)Bboxes +Classes

IoU (Intersection over Union)

IoU VisualizationGround TruthPredictedIntersectionUnion = Area(GT) + Area(Pred) - IntersectionIoU = 0.9ExcellentIoU = 0.5AcceptableIoU = 0.1Poor

Anchor Boxes


Non-Maximum Suppression (NMS)

Non-Maximum Suppression (NMS)Before NMSCar (0.9)Car (0.8)Car (0.7)Person (0.85)Person (0.6)After NMSCar (0.9) ✓Person (0.85) ✓

Two-Stage: Faster R-CNN

Faster R-CNN ArchitectureImageBackbone(CNN)FeatureMapRPNRegion ProposalsRoI PoolFixed sizeClass +Box RegDetsStage 1: RPN (proposals) → Stage 2: Classification + Box RegressionTwo-stage: accurate but slower

Single-Stage: YOLO

YOLO: Single-Shot Detection🚗🚶Input7×7 GridPredictions🚗 (0.95)🚶 (0.88)DetectionsSpeedYOLO: 30+ FPSFaster RCNN: 5 FPSReal-time!

Two-Stage vs Single-Stage

Two-Stage vs Single-Stage DetectorsTwo-Stage• R-CNN, Fast R-CNN, Faster R-CNN• Higher accuracy (mAP)• Slower (2-5 FPS)• Better for small objects• Complex pipeline• Mask R-CNN adds instance segmentationSingle-Stage• YOLO, SSD, RetinaNet• Faster (30+ FPS)• Lower latency• Real-time applications• Focal loss for class imbalance• YOLOv8: state-of-the-art speed/accuracy

Evaluation: mAP


Summary

  • Object detection predicts bounding boxes + class labels + confidence scores
  • IoU measures overlap between predicted and ground truth boxes
  • Anchor boxes provide reference shapes for regression
  • NMS removes duplicate detections
  • Two-stage (Faster R-CNN): accurate but slower
  • Single-stage (YOLO): faster, real-time capable
  • mAP is the standard evaluation metric

Next: Semantic Segmentation

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