sections/intro/faster_rcnn_approach.md

For example, a FasterRCNN approach uses the following steps:
- the image features are given out by a FPN (Feature Pyramid Net) over a ResNet backbone, and
- then a RPN (Regision Proposal Network) layer detects proposals in those features, and
- then the ROI (Region of Interest) heads get the box proposals in the original image, and
- then the boxes are selected using a NMS (Non-max suppression),
- and then the features for selected boxes are used as visual features.