The spec for the JSON output is consistent with the rest of the BMFont file spec.
Here is what a typical output looks like, omitting the full list of glyphs/kernings for brevity.
{
"pages": [
"sheet.png"
],
"chars": [
{
"id": 10,
"x": 281,
"y": 9,
"width": 0,
"height": 0,
"xoffset": 0,
"yoffset": 24,
"xadvance": 8,
"page": 0,
"chnl": 0
},
{
"id": 32,
"x": 0,
"y": 0,
"width": 0,
"height": 0,
"xoffset": 0,
"yoffset": 0,
"xadvance": 9,
"page": 0,
"chnl": 0
},
...
],
"kernings": [
{
"first": 34,
"second": 65,
"amount": -2
},
{
"first": 34,
"second": 67,
"amount": 1
},
...
],
"info": {
"face": "Nexa Light",
"size": 32,
"bold": 0,
"italic": 0,
"charset": "",
"unicode": 1,
"stretchH": 100,
"smooth": 1,
"aa": 2,
"padding": [
0,
0,
0,
0
],
"spacing": [
0,
0
]
},
"common": {
"lineHeight": 32,
"base": 24,
"scaleW": 1024,
"scaleH": 2048,
"pages": 1,
"packed": 0,
"alphaChnl": 0,
"redChnl": 0,
"greenChnl": 0,
"blueChnl": 0
}
}