In this section, some general concepts are described, which apply to multiple
classes within the ARchitect library.
Scaled Distance Units (SDUs)
applications, the term "size of an object" cannot be defined just in pixels
or similar size measurements. Unlike in 2-dimensional applications, such as
maps, the size of an object inherently depends on the distance from the
viewer to the object.
Imagine a virtual 2-dimensional logo which should be placed right on top of a
certain store location. In a map-based application, the size can easily be
defined by specifying the width and height of the logo in pixels. In AR
applications, the logo should appear a big as the building itself, which is
dependent on the distance in a natural way.
The term "pixels" or similar measurements cannot be used in AR applications,
as they are defined in a static way and do not allow the application to scale
the objects in a natural way. Thus, the Wikitude ARchitect introduces Scaled
Distance Units. SDUs are defined not only in relation to the object's size
itself, but also in relation to the distance from the viewer to the object
For Drawables attached to GeoObjects, 1 SDU is defined as the screen-size of
an object which is 1 meter high and located 10 meters from the viewer. If a
drawable's width is defined as 5 SDU, it will exactly overlap an object which
is 5 meters wide and is located in 10 meters distance.
attached to Trackable2DObjects, SDUs are determined by the underlying
trackable's size. The Drawables anchor point will be set to the center of the
Trackable2DObject, and 1 SDU is defined to be the height of the image, as
described in the figure below:
A circle with
radius set to 0.25 will be rendered accordingly:
Distance Based Scaling (DBS)
important concept of an AR application is "Distance Based Scaling". Think
about the logo again, which represents the store location of a certain brand.
If we are in 10 meters viewing distance, the logo will perfectly overlap the
building, given that we have set the drawable's width and height to the right
SDUs. Now, we are moving away from the store location. Without the concept of
DSB, the logo will quickly vanish from the screen, as it will be too small to
see - especially when the SDUs are not set to a high value. A similar
situation appears when we are moving towards the location - the logo will
soon appear very big and obstruct the entire screen.
To avoid situations like this, where drawables are only clearly visible in a
certain viewing distance, and are otherwise soon too big or too small to see
respectively, the Wikitude ARchitect introduces Distance Based Scaling (DBS).
It ensures that drawables do not exceed certain limits of their original
We define two values, cutoff_min and cutoff_max.
cutoff_min is set to
10 meters and defines the viewing distance at which the drawables will not be
scaled anymore. Regardless if the viewing distance is 10 meters or even
closer to the original location, the drawable will still have the same size
on the screen. This avoids drawables obstructing the entire screen.
cutoff_max is set to 20 kilometers and defines the viewing distance beyond
which the drawables will not be scaled anymore. The drawable will still
appear in the same size on the screen, regardless if the distance to the
object is 20 kilometers, or even more. This avoids drawables getting too
small to see on the screen.
The cutoff_max size of the drawable is set
to 10% of its size at cutoff_min (the defined SDU-value), resulting in any
drawable not appearing smaller than 10% of its size if the object was only 10
Between cutoff_min and cutoff_max, the objects are scaled
based on a scaling function of x^(0.2). The effect of this particular scaling
function is that drawables representing near objects are downscaled fairly
quickly as the user is moving away from the object. The further the user
moves away from the object, the lesser the loss of size will be visible for
the user, to allow a smooth user experience.