How to detect when a marker is found in AR.js

问题:

I'm trying to detect when a marker if found/lost in ar.js, while using a-frame.

From what I see in the source code, when the marker is found, a 'getMarker' event should be fired, moreover artoolkit seems to dispatch a markerFound event.

I tried to listen to those events on the <a-scene>, or on the <a-marker>, but it seems I'm either mistaken, or i need to get deeper to the arController, or arToolkit objects.

When i log the scene, or the marker, i only get references to the attributes, which don't seem to have the above objects attached.(like marker.arController, or marker.getAttribute('artoolkitmarker').arController)

Did anybody tried this and have any tips how to do this ?

回答1:

PR303 introduces events when a marker is found and lost

• markerFound
• markerLost

You can use them by simply adding an event listener:

anchorRef.addEventListener("markerFound", (e)=>{ // your code here}

with a simple setup like this:

<a-marker id="anchor">
  <a-entity>
</a-marker>

example here. Please note, that as of sep 18', you need to use the dev branch to use the above.

---

ORIGINAL ANWSER

---

Apparently it's not yet possible to listen to any events, unless You implement them.
But detecting if the marker is found is possible by checking if the marker is visible when needed (other event, or on tick): if(document.querySelector("a-marker").object3D.visible == true)

For example:

init: function() {
   this.marker = document.querySelector("a-marker")
   this.markerVisible = false
},
tick: function() {
   if (!this.marker) return
   if (this.marker.object3D.visible) {
      if (!this.markerVisible) {
         // marker detected
         this.markerVisible = true
      }
   } else {
      if (this.markerVisbile) {
         // lost sight of the marker
         this.markerVisible = false
      }
   }
}

---

As adrian li noted, it doesn't work with a-marker-camera, only with a-markers

回答2:

I went with a dirty hack diving into the internals, bear in mind that what I've provided might not suffice because the event get's called every time the marker is found, sadly I couldn't find an event to hook into for markers being lost.

```

const arController = document.querySelector("a-scene").systems.arjs._arSession.arContext.arController;

arController.addEventListener("getMarker", (evt) => {
    const markerType = evt.data.type;
    const patternType = 0;

    //console.log("onMarkerFound!!");

    if (markerType == patternType) {
        //console.log("onMarkerFound out pattern!!");

        //Do stuff...
    }
});

```