Civil Engineering Reference
In-Depth Information
Converting local coordinates of an object to global coordinate frame
when a dependent virtual object is about to be placed independently in the
augmented scene.
*
In this work, the global coordinate system is defined by three major axes: X,
which is parallel to the equator with its positive direction pointing towards the
geographical east; Y, which is perpendicular to the earth's surface with its positive
location pointing upwards; and Z, which is parallel to the prime meridian with its
positive direction pointing towards the geographical south. X, Y, and Z readings in
the global coordinate system are used to determine the longitude, altitude, and
latitude of the user as well as CAD objects in the global space. At the same time, each
CAD object has its own unique local coordinate system. The definition of the three
major axes in a local coordinate system varies between CAD objects and is mainly
specified when the CADmodel is created in a graphical software, such as AutoCAD
or 3D Studio. However, to make sure that CAD objects are placed in the global
coordinate system with guaranteed consistency, their local coordinate frame
should be oriented in a way that it is exactly aligned with the global coordinate
frame (e.g. local X axis is parallel to the global X axis).
While objects can be placed in the scene by defining their global coordinates (i.e.
longitude, latitude, and altitude), all transformations (i.e. translations and rotations)
have to be done relative to the user. As a result, the application must be able to keep
track of the latest local position and orientation of each CAD object inside the user's
coordinate frame. As shown in Figure 5.9, although the user and all CAD objects have
their global position determined in the global coordinate system, CAD objects still
have to be placed relative to the user's eyes. Therefore, for each CAD object, an
additional step has to be executed in order to calculate the relative distance between
the user and that object and to use this distance to construct or update the
transformation matrix corresponding to that CAD object. Only after this matrix is
calculated, the CAD object can be correctly placed inside the user's viewing frustum.
Since the user is assumed to be moving, all transformation calculations must be
done in real time with the latest global position of CAD objects and the one for the
user being important input parameters to such calculations. Switching from local to
global coordinate frames and vice versa and adapting appropriate conversion
methods to achieve the expected result have been key challenges in designing
ARVISCOPE language statements. The language statements are designed in a way
that they can accept and handle positional values in both global and local
coordinate frames as arguments when necessary. Described in the following
sections are the details and methods followed in this research to design three
main ARVISCOPE language statements (i.e.
POSITION
,
ROUTE
, and
DISCON-
NECT
) that comply with such situations by automatically switching between global
and local coordinate frames.
5.7.1 On-site positional measurement problems
When the number of CAD objects increases the time required to measure the
longitude,
latitude, and altitude for each point grows dramatically. For an
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