Mobile and Pervasive Construction Visualization Using Outdoor Augmented Reality - Mobile and Pervasive Computing in Construction - page 74

Figure 5.16 Relationship between coordinates frames of different CAD objects
InFigure5.16, aCADmeta-objectofa towercrane is showninwhichthehighest level
parent node is the Base node. The Tower , Boom ,and Cable nodes are connected in the
scene graph to form the complete tower crane object. This is achieved by first placing
the Base node globally in the scene. The positionof the Tower is defined inside the local
coordinate frame of the Base .The Boom is placed locally inside the Tower coordinate
frame. Also, the Cable node is a child of the Boom node. As shown in this figure, a Steel
Section has been defined as a child object to the Cable , and hence is transformed (i.e.
picked up, rotated, and installed) in the augmented scene by manipulating the
transformational values of its immediate parent node (i.e. Cable node).
As soon as the steel section is in its final position and has to be separated from
the cable and installed on the foundation, its global coordinate values have to be
calculated and used to find its relative transformation to the user for subsequent
animation frames. The problem of conversion from local to global coordinate
values has been addressed in this research using the concept of “transformation
chain”, which is shown in Figure 5.17.
In Figure 5.17, to find the transformation values of the lowest level node (i.e. end
gripper of the robotic arm) inside the coordinate frame of the highest level node
(which is the global coordinate frame), and considering the fact that the definition
of different axes in different levels of the hierarchy is consistent, all the transfor-
mation matrices connected to nodes at different layers (starting from the lowest
level node to the highest level node) are multiplied together. The result of this
matrix operation is a new transformation matrix describing the transformation of
the child object inside the global coordinate frame. Since the position of the user in
terms of longitude, latitude, and altitude values are also known in the same
coordinate frame, the method previously described in Section 5.7.1 can be applied
to find the global position of the child object as well. Following this method, the
Reference Point represents the user while the Target Point describes the position of
the newly disconnected child node. This is shown in Figure 5.18.