Each of the boxes is specified with an object containing left, top, width, and height keys, and optional label.
{
"lines": [
...
],
"polygons": [
...
],
"boxes": [
{
"label": "car",
"height": 97,
"width": 147,
"top": 229,
"left": 300
}
],
"cuboids": [
...
]
}