Instance Segmentation — Mask R-CNN

Add a tiny mask branch to a Faster R-CNN detector and you have instance segmentation. The hard part is RoIAlign, and it is harder than it looks.

Build + Learn 1.3 hr Python

Prerequisites: Phase 4 Lesson 06 (YOLO), Phase 4 Lesson 07 (U-Net)

#Learning Objectives

Trace the Mask R-CNN architecture end-to-end: backbone, FPN, RPN, RoIAlign, box head, mask head
Implement RoIAlign from scratch and explain why RoIPool is no longer used
Use the torchvision maskrcnn_resnet50_fpn_v2 pretrained model for production-quality instance masks and read its output format correctly
Fine-tune Mask R-CNN on a small custom dataset by replacing the box and mask heads and keeping the backbone frozen

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The text and code of this lesson are based on AI Engineering from Scratch by Rohit Ghumare, MIT licensed.