Definition
Faster R-CNN substituted the off-the-shelf region proposal of R-CNN with the region proposal network (RPN)
Architecture
Region Proposal Network
Region proposal network (RPN) is a CNN that operates on the feature maps produced by the backbone CNN. RPN slides over the CNN feature map. At each location, it makes multiple region proposals have various sizes and aspect ratios. For each proposal, the RPN predicts the objectness score and the bounding box refinement.