intersection over union (IoU)

The overlap between a predicted region and the ground truth, divided by their combined area.

Intersection over union (IoU) scores the overlap between two regions $A$ and $B$ (boxes or masks) as $\text{IoU}=\lvert A\cap B\rvert/\lvert A\cup B\rvert$ , ranging from $0$ (no overlap) to $1$ (identical regions). It is the standard test for whether a predicted box or mask counts as a match to a ground-truth one, usually applied against a threshold such as $0.5$ .

Examples

A predicted box overlapping ground truth with intersection area $40$ and union area $100$ has $\text{IoU}=0.4$ , below the usual $0.5$ match threshold.