
1. Contribution
The main contributions of this work are three-fold:
- We introduce a new perspective to design a single-shot instance segmentation framework, PolarMask, which predicts instance masks and rotated objects in the polar coordinate in an effective and efficient manner.
- With the polar representation, we propose the polar IoU loss and the soft polar centerness for instance center classification and dense coordinate regression.
- Rich experiments show that state-of-the-art performances of object instance segmentation and rotated object detection can be achieved with low computational overhead in multiple challenging benchmarks.
2. Method
The heads in PolarMask++ contain three branches, including a classification branch, a polar centerness branch, and a mask regression branch, which predict the class label, the polar centerness score and the length of each polar ray of each pixel respectively, where k and n indicate the number of categories and the number of rays.

2.1 Overview of Mask Segmentation in Polar Coordinate
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Polar Representation: n = 36,θ=10°\theta=10°θ=10°
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Mass Center
- We find that the mass center is more advantageous than the box center because the mass center has a larger probability of falling inside an instance compared to its box center. Although