Unmei/resource-pack/assets/unmei/font/custom_ui.json

{
    "providers": [
      {
        "type": "bitmap",
        "chars" : ["\uE108"],
        "file": "unmei:gui/test.png",
        "ascent": 13,
        "height": 256
      },
      {
        "type": "space",
        "advances" : { "\uE000": -8, "\uE001": -169 }
      }
    ]
}