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2.3 Video HMD
OHMD problems are partly solved by VHMD. VHMD devices merge
artificial graphics with the images coming from video cameras mounted on
them. A merged view is then projected on the display which is totally
Virtual and real images are perfectly synchronized by delaying real
images by the time taken by a scene generator. The main problem of such
devices is to correctly position the cameras so as to give a correct stereo-
scopic view to human eyes. Cameras are obviously mounted in places other
than eye's position. Therefore, a parallax error takes place and it can cause
users feel unwell when they stop using a VHMD and resume looking at the
real environment around them directly with their own eyes.
A z-buffer algorithm is used to manage the z -axis in a 3D system. This
turns out very useful when overlapping real and virtual images and lets
artificial images partly or totally overshadow real images behind them. This
way, a realistic view is then achieved where real and artificial images are
correctly rendered.
As in the case of OHMD, VHMD also cannot give the same resolution of
human view, and it becomes very difficult to make shadows exactly fall from
virtual objects on a real environment.
The AR goal is to perfectly mix two worlds, real and virtual. Image
rendering by means of HMD is, therefore, very important to let us feel both
worlds as one (Figure 3). Unfortunately, even a few pixels deviation between
the two representations are detected by human eye, and it can heavily affect
the vision of an augmented world. There are some techniques available
indeed to face the problem that, however, can affect image fluidity.
Figure 3: An example of augmented environment with perfect alignment
(on the right), where the digital augmentation of cups, floppy
disks, pencils and rubbers can be noted.
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