The semantic line (SEL) dataset contains 1,750 outdoor images in total, which are split into 1,575 training and 175 testing images. Each semantic line is annotated by the coordinates of the two end-points on an image boundary. If an image has a single dominant line, it is set as the ground truth primary semantic line. If an image has multiple semantic lines, the line with the best rank by human annotators is set as the ground-truth primary line, and the others as additional ground-truth semantic lines. In SEL, 61% of the images contain multiple semantic lines.