Explain semantic vs instance segmentation and how Mask R-CNN works.

Semantic labels every pixel by class; instance separates each object. The signal is knowing that one cannot count overlapping objects, why Mask R-CNN adds a mask branch on RoIAlign, and what RoIAlign fixed. Here is the answer.

Updated Aug 2026 · Grounded in real Applied AI Engineer interview loops and written to a senior-engineer editorial bar.