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Figure 9: Paper anaglyph glasses [48].
A more complex and performing passive stereo technology is based
on differently polarized light. Two projectors are used to display the
two images using orthogonally polarized light, and the images are view-
ed through glasses with orthogonally polarized lenses. Each lens lets
through light having the same direction of polarization, while filtering
all light whose polarization is orthogonal. Thus, each eye sees the
corresponding image, in full color but with half its brightness. Polarized
glasses are relatively cheap and the resulting image has a good quality.
For these reasons, polarized stereo is commonly used in cinemas for 3D
movies.
Active stereo is based on the use of more complex visualization
devices. The two most notable examples are shutter glasses and Head-
Mounted Displays (HMD). Shutter glasses are based on alternatively
displaying left and right images on the same display, at a very high
frequency, alternatively occluding the left and right eyes in sync with
the display. This way, each eye sees only the corresponding image. If
the alternating frequency is sufficiently high, the brain fuses the images
as two continuous streams.
Stereo-enabled HMDs use separate displays for each of the two eyes,
so that the eyes actually see two different video streams. Active stereo
device usually provide an image quality superior to passive stereo, al-
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