The method described for tracking user ID cards is
extended for tracking the shared whiteboard card. There are
six markers in the Virtual Shared White Board, aligned
around the outside of the board as shown in figure 7. The
orientation of the White Board is found by fitting lines
around the fiducial markers and using an extension of the
technique described for tracking user ID cards.
Using all six markers to find the board orientation and
align virtual images in the interior produces very good
registration results. However, when a user draws a virtual
annotation, some markers may be occluded by user's hands,
or they may move their head so only a subset of the markers
are in view. The transformation matrix for Virtual Shared
White Board has to be estimated from visible markers so
errors are introduced when fewer markers are available. To
reduce errors the line fitting equations are found by both
considering individual markers and sets of aligned markers.
Each marker has a unique letter in its interior that enables
the system to identify markers which should be horizontally
or vertically aligned and so estimate the board rotation.
Though line equations in the camera screen coordinates
frame are independently generated for each of markers, the
alignment of the six markers in Virtual Shared White Board
means that some line equations are identical. Therefore by
extracting all aligned sides from visible markers for the linefitting