getArea for ImageStatic layer

Question

I am new to OpenLayers, I am using it for an office layout, so I use an ImageStatic layer as background, image size is 1024x968. As it is not an earth map, I want to draw a box on map and get the real size of the box. I don't know where I can set the ratio, I mean if 1024 equals to 1024 meter, how can I set this value? Or maybe 1024 pixels equal to 2048 meter for another image, how to set that too?

Below is the code I use to create map:

const extent = [0, 0, 1024, 968];
const projection = new Projection({
  code: 'mycompany',
  units: 'pixels',
  metersPerUnit: 10,
  extent,
});

const imageLayer = new ImageLayer({
  source: new ImageStatic({
    url: 'http://localhost:8081/static/a35.png',
    projection,
    imageExtent: extent,
  }),
});

const source = new Vector({ wrapX: false, features: this.features });

const vector = new VectorLayer({
  source,
});

const map = new Map({
  controls: Control.defaults().extend([new FullScreen()]),
  layers: [imageLayer, vector],
  overlays: [overlay],
  target: 'map',
  view: new View({
    projection,
    center: Extent.getCenter(extent),
    zoom: 2,
    minZoom: 2,
    maxZoom: 4,
  }),
});
map.addInteraction(draw);
this.createMeasureTooltip(map);
draw.on(
  'drawstart',
  evt => {
    this.sketch = evt.feature;
    this.listener = this.sketch.getGeometry().on('change', evtChange => {
      this.measureTooltipElement.innerHTML = this.formatArea(evtChange.target);
      this.measureTooltip.setPosition(evtChange.target.getInteriorPoint().getCoordinates());
    });
  },
  this,
);

I am using following function to get the area size:

formatArea(polygon) {
  const area = Sphere.getArea(polygon);
  let output;
  if (area > 10000) {
    output = `${Math.round(area / 1000000 * 100) / 100} km<sup>2</sup>`;
  } else {
    output = `${Math.round(area * 100) / 100} m<sup>2</sup>`;
  }
  return output;
}

But this just called getArea, and I don't understand how can it calculate the size in meter?

Whenever I change the metersPerUnit to 10 or 10000, the area are always same.

---

I changed my code as below and by changing radius I can get any number I want, but I don't know if it's a good way:

  const area = Sphere.getArea(polygon, {
    radius: 1000000,
  });