Figure 3.

Schematic of the principle of matching ground truth with a prior box. The green box in the esophagus image is the ground truth, the multiple red boxes are prior boxes, and the blue box is the boundary box that finally matches the ground truth. (a) A match between multiple prior frames and ground truth; (b) a negative sample that does not meet the first principle during the matching process; (d) a positive sample that conforms to the first principle, which later becomes a boundary box; (c) a positive sample that does not meet the first principle but meets the second principle and also becomes a bounding box; and (e) the final output bounding box, which conforms to the first and second principles, which is the final SSD prediction.