Augmented Reality Markup Language

The definition of a Feature is reused from the Geography Markup Language (GML) and describes the physical object that should be augmented. The physical object is described by a set of metadata, including an ID, a name and a description. A Feature has one or more Anchors.

An Anchor describes the location of the physical object in the real world. Four different Anchor types are defined in ARML:

1. Geometries
2. Trackables
3. RelativeTo
4. ScreenAnchor

Geometries describe the location of an object through a set of fixed coordinates. WGS84 (latitude, longitude, altitude) is used as the default coordinate reference system, other arbitrary coordinate reference systems can be supplied if required. ARML allows 0- (Point), 1- (LineString) and 2-dimensional (Polygon) geometries. Geometry Anchors reuse the syntax as defined in GML3. As an example, the following snippet defines the location of the Wiener Riesenrad.

 <gml:Point gml:id="ferrisWheelViennaPoint">
  <gml:pos>
    48.216622 16.395901
  </gml:pos>
 </gml:Point>

Trackables are patterns that are searched, recognized and tracked in the video screen coming from the camera of the device. A wide variety of different tracking technologies exist, including QR codes, Natural features, 3D and Face Tracking. As all these tracking types use different algorithms and technologies, the definition of a Trackable is abstracted and split into two parts, a Tracker and its associated Trackables. A Tracker describes the technology (or algorithm) with which its associated Trackables should be tracked, using URIs identifying the algorithm. The Trackable itself describes the pattern the algorithm should look for in the video stream.

Example: A natural feature tracker and an associated Trackable

 <Tracker id="defaultImageTracker">
  <uri xlink:href="http://opengeospatial.org/arml/tracker/genericImageTracker" />
 </Tracker>
    
 <Trackable>
  <config>
   <tracker xlink:href="#defaultImageTracker" />
   <src>http://www.example.com/myMarker.jpg</src>
  </config>
  <size>0.20</size>
 </Trackable>

RelativeTo Anchors allow the definition of a location relative to other Anchors or the user's position. The former allows the setup of a scene and the location of all included virtual objects based on a single Anchor, like a Trackable placed on a table. The latter allows for scenarios where the actual location of the user is irrelevant. The virtual objects are simply placed around the user, regardless of his or her physical location.

Contrary to the previous three Anchor types, ScreenAnchors do not describe a location in the 3-dimensional virtual scene. Instead, they define an area on the device screen, allowing for status bars and the like.

VisualAssets describe the appearance of the virtual objects in the augmented scene. ARML allows various kinds of VisualAssets to be described, including plain text, images, HTML content and 3D models. VisualAssets can be oriented (either to always automatically face the user, or to maintain a specific static orientation) and scaled. Additionally, visibility conditions can be applied (i.e. the Asset is only visible on the screen if the distance to the user is within certain boundaries).