Margin MCC: Chance-Robust Metric for Video Boundary Detection with Allowed Margin

Video boundary detection is a task to divide a video into several segments based on event changes such as scenes or actions. The most common evaluation is to judge whether the distance between predicted boundaries and the ground truth boundaries is lower than allowed margin and then compute F1 score. However, we found that the evaluation only by F1 measure can lead to wrong conclusions since even completely random model can achieve inflated F1 when the number of predictions is large. To design a robust metric against chance, we propose Margin Matthews Correlation Coefficient (MMCC) as an extension of Matthews Correlation Coefficient (MCC) to video boundary detection with allowed margin. Although MCC is a robust metric against chance, it is not obvious that the same is true in video boundary detection due to allowed margin. Specifically, some definitions of MCC do not keep a constant as for the number of predicted boundaries. Therefore, we design MMCC so that the expected MMCC for random guessing will be zero, based on mathematical analysis. We empirically examine if MMCC is robust against completely random guessing and oversegmentation/undersegmentation, while F1 is not.