How to measure with a projection using zoomify?

Question

i'm trying to measure distance using the measure example.

but the difference is that i'm displaying an image using iiifimage like this example

I also have a scale line on my map which working well (after setting the projection below on my map).

const projection = new Projection({
  code: 'ZOOMIFY',
  units: 'PIXELS',
  metersPerUnit,
  extent: [0, 0, options.size[0], options.size[1]],
  getPointResolution: a => a
});

With my measure tool , in order to get the length of my feature i'm doing this :

const length = getLength(geom, { projection: map.getView().getProjection() });

Unfortunately, my measured distance is totally wrong (compared to my scaleLine and my real image).

What am i doing wrong ?

Thanks a lot.