Link to paper: [1506.02640] You Only Look Once: Unified, Real-Time Object Detection

This paper models object detection as a regression problem for bounding boxes and object class probabilities with a single pass through the CNN. The main contribution is the idea of dividing the image into a 7x7 grid, and having each cell predict a distribution over class labels as well as a bounding box for the object whose center falls into it. It's much faster than R-CNN and Fast R-CNN, as the additional step of extracting region proposals has been removed.

Strengths

• Works real-time. Base model runs at 45fps and a faster version goes up to 150fps, and they claim that it's more than twice as fast as other works on real-time detection.
• End-to-end model; Localization and classification errors can be jointly optimized.
• YOLO makes more localization errors and fewer background mistakes than Fast R-CNN, so using YOLO to eliminate false background detections from Fast R-CNN results in ~3% mAP gain (without much computational time as R-CNN is much slower).

Weaknesses / Notes

• Results fall short of state-of-the-art: 57.9% v/s 70.4% mAP (Faster R-CNN).
• Performs worse at detecting small objects, as at most one object per grid cell can be detected.