Revisiting Evaluation Metrics for Semantic Segmentation: Optimization
  and Evaluation of Fine-grained Intersection over Union
Zifu Wang Maxim Berman Amal Rannen-Triki Philip H. S. Torr Devis Tuia Tinne Tuytelaars Luc Van Gool Jiaqian Yu Matthew B. Blaschko
Semantic segmentation datasets often exhibit two types of imbalance:
\textit{class imbalance}, where some classes appear more frequently than others
and \textit{size imbalance}, where some objects occupy more pixels than others.
This causes traditional evaluation metrics to be biased towards
\textit{majority classes} (e.g. overall pixel-wise accuracy) and \textit{large
objects} (e.g. mean pixel-wise accuracy and per-dataset mean intersection over
union). To address these shortcomings, we propose the use of fine-grained mIoUs
along with corresponding worst-case metrics, thereby offering a more holistic
evaluation of segmentation techniques. These fine-grained metrics offer less
bias towards large objects, richer statistical information, and valuable
insights into model and dataset auditing. Furthermore, we undertake an
extensive benchmark study, where we train and evaluate 15 modern neural
networks with the proposed metrics on 12 diverse natural and aerial
segmentation datasets. Our benchmark study highlights the necessity of not
basing evaluations on a single metric and confirms that fine-grained mIoUs
reduce the bias towards large objects. Moreover, we identify the crucial role
played by architecture designs and loss functions, which lead to best practices
in optimizing fine-grained metrics. The code is available at
\href{https://github.com/zifuwanggg/JDTLosses}{https://github.com/zifuwanggg/JDTLosses}.