Yes it is binary, and there is only one detection (one 1) per image.
E.g. a ground truth matrix:
0 0 0 0 0 0 0
0 0 0 0 1 0 0
0 0 0 0 0 0 0
0 0 0 0 0 0 0
0 0 0 0 0 0 0
0 0 0 0 0 0 0
0 0 0 0 0 0 0
My Labeling Software works like this:
ROI --> green
detection grid --> red