polygons are objects with an optional label field, and a vertices field which contains a list of objects with x and y attributes.
{
...
"polygons": [
{
"vertices": [
{
"y": 145,
"x": 356
},
{
"y": 103,
"x": 502
},
{
"y": 264,
"x": 482
}
],
"label": "building"
}
]
}