tldr: working glitch here.
Learn the area (the image is in the assets), click the accept button, and toggle the marker helpers.
Now, a bit of details:
1) Loading saved area data
Upon initialisation, when ar.js detects, that you want to use the area marker preset, it tries to grab a localStorage reference:
localStorage.get("ARjsMultiMarkerFile")
The most important data there is an array of pairs {markerPreset, url.patt} which will be used to create the area.
Note: By default it's just the hiro marker.
2) Creating an area data file
When you have debugUIEnabled set to true:
<a-scene embedded arjs='sourceType: webcam; debugUIEnabled: true'>
There shows up a button "Learn-new-marker-area".
At first glance it redirects you to a screen where you can save the area file.
There is one but: by default the loaded learner site is on another domain.
Strictly speaking: ARjs.Context.baseURL = 'https://jeromeetienne.github.io/AR.js/three.js/
Any data saved there won't be loaded on our website, for local storage is isolated per origin.
To save and use the marker area, you have to create your own learner.html. It can be identical to the original, just keep in mind you have to keep it on the same domain.
To make the debugUI button redirect the user to your learner html file, you need to set
ARjs.AnchorDebugUI.MarkersAreaLearnerURL = "myLearnerUrl.html"
before the <a-marker>s are initialized. Just do it in the <head>.
Once on the learner site, make sure the camera sees all the markers, and approve the learning.
It should look like this:
Once approved, you will be redirected back to your website, the area file will be loaded, and the data will be used.
