Region of Interest Pooling, or RoIPool, is an operation for extracting a small feature map (e.g., $7×7$) from each RoI in detection and segmentation based tasks. Features are extracted from each candidate box, and thereafter in models like Fast R-CNN, are then classified and bounding box regression performed.
The actual scaling to, e.g., $7×7$, occurs by dividing the region proposal into equally sized sections, finding the largest value in each section, and then copying these max values to the output buffer. In essence, RoIPool is max pooling on a discrete grid based on a box.
Image Source: Joyce XuSource: Rich feature hierarchies for accurate object detection and semantic segmentation
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