Definition
Fast R-CNN improved the speed of object detection by extracting the CNN feature of the entire image only once and using a fraction of the feature map corresponding to the proposed bounding box for the object recognition stage. The fractions of the feature map are resized into the same size using ROI pooling.
Architecture
RoI Pooling
RoI pooling is a type of max pooling transforms features inside region of interest into a small feature map with fixed size. The RoI is divided into an grid of sub-windows, and for each sub-window, perform max pooling.