I have one system which predicts the location of boxes in the page. In the image below, the green rectangular is the correct location (label) and the red ones are the predicted. It is clear that the best performance of the system is when all red rectangular were located almost on the same location of green rectangular. My question is that how I can plot FROC (free-response ROC) curve for such a system to obtain the sensitivity of the predictor. How in this kind of problem True Positive, False Positive and etc are defined.

[![enter image description here](https://i.stack.imgur.com/Kx32j.png)](https://i.stack.imgur.com/Kx32j.png)