Intersection over union (IoU)
DefinitionIntersection over union measures how much two shapes overlap: the area they share divided by the area they cover together. It runs from 0 for no overlap to 1 for identical shapes.
Example
Two boxes each cover 100 square units and share 50. Together they cover 150, so IoU = 50 / 150 = 0.33.
Uses
- Judging whether two annotators drew the same box or polygon.
- Scoring an object detector's boxes against human boxes.
- Matching time segments in audio, where the overlap is measured in seconds.
Thresholds
A threshold of 0.5 is the common cutoff for calling two boxes the same object. Stricter tasks use 0.75 or higher.
How Azmaval uses it
Azmaval treats two boxes, polygons or time segments with the same label as the same mark when their IoU is at least 0.5, and keeps a mark when a majority of reviewers made it.
Frequently asked questions
Does IoU work for polygons?
Yes. The overlap is the shared filled area. Azmaval compares polygons by filling them on a grid.
Why 0.5?
It is the long-standing convention for object detection benchmarks. Pick a stricter value when tight boundaries matter.