Faster RCNN Object Detection
With Faster RCNN, the 3rd generation in the family of region-based detectors, we are replacing the selective search algorithm that is considered computationally expensive, with a neural network called the Region Proposal Network (RPN) that as the name implies produces the proposals. This allows us to call the detector differentiable and therefore train it end-to-end in a much more straightforward way.
Therefore, in this architecture there is one CNN network that not only produces a global feature map but also produces proposals from the feature map itself rather than the original image, using additional convolutional layers and a sliding window scheme detailed below. Since the RPN component is the key differentiator we limit the discussion to it.
Region Proposal Network (RPN)
The RPN produces proposals by sliding a window
The
The RPN network produces a classification score i.e. how confident we are that there is an object for each of the anchor boxes as well as the regression on the anchor box coordinates.