To assess performance, we rely on the standard Jaccard Index, commonly known as the PASCAL VOC intersection-over-union metric IoU = TP ⁄ (TP+FP+FN) [1], where TP, FP, and FN are the numbers of true positive, false positive, and false negative pixels, respectively, determined over the whole test set. Owing to the two semantic granularities, i.e. classes and categories, we report two separate mean performance scores: IoUcategory and IoUclass. In either case, pixels labeled as void do not contribute to the score.
It is well-known that the global IoU measure is biased toward object instances that cover a large image area. In street scenes with their strong scale variation this can be problematic. Specifically for traffic participants, which are the key classes in our scenario, we aim to evaluate how well the individual instances in the scene are represented in the labeling. To address this, we additionally evaluate the semantic labeling using an instance-level intersection-over-union metric iIoU = iTP ⁄ (iTP+FP+iFN). Again iTP, FP, and iFN denote the numbers of true positive, false positive, and false negative pixels, respectively. However, in contrast to the standard IoU measure, iTP and iFN are computed by weighting the contribution of each pixel by the ratio of the class’ average instance size to the size of the respective ground truth instance. It is important to note here that unlike the instance-level task below, we assume that the methods only yield a standard per-pixel semantic class labeling as output. Therefore, the false positive pixels are not associated with any instance and thus do not require normalization. The final scores, iIoUcategory and iIoUclass, are obtained as the means for the two semantic granularities. The same policy is applied to other benchmarks.