====== Mask RCNN ======

Extends Faster-RCNN

  * Additional branch for predicting segmentation masks on each Region of Interest (ROI)
  * Mask branch is a small FCN

Problem: Faster-RCNN is not designed for pixel-to-pixel alignment between input and output.
  * RoIPool operation for attending to instances performs coarse spatial quantization for feature extraction

Solution: Quantization-free layer that preserves spatial location (RoIAlign)

===== Architecture =====

  * Convolutional backbone used for feature extraction
    * Alternative 1: ResNet-50-C4 (features of final conv layer of 4-th stage (C4))
    * Alternative 2: Feature Pyramid Network (FPN): Extracts features from different scales
  * Network head for bounding-box recognition (classiciation and regression)
    * Extend Faster R-CNN box heads from ResNet/FPN paper with mask prediction branch.