Visual Discomfort in 3DTV: Definitions, Causes, Measurement, and Modeling - Novel 3D Media Technologies - page 184

converge to the virtual object which is in front of or behind the screen plane.
However, the accommodation has to be performed at the screen itself, i.e. with
respect to the distance of the observer in front of the screen, which is unnatural and
will not happen in our daily life. The larger this discrepancy between the vergence
and accommodation gets, the higher the possibility that observers will perceive
visual discomfort.
To define the threshold of this discrepancy in which conditions viewers may not
experience visual discomfort, i.e., the comfortable viewing zone, numerous studies
have been conducted. Yano et al. [ 7 ] proposed that the depth of field (DOF), which
refers to the range of distances in image space within which an image appears in
sharp focus, can be used to define the comfortable viewing zone in terms of diopters
(D). A value of

0.2 D is suggested [ 8 , 9 ]. Another definition on comfortable
viewing zone is based on the results of empirical measurements, in which 1 arc

degree of visual angle is used [ 10 , 11 ]. If considering the screen disparity, the
comfortable viewing zone can be defined by a percentage of the horizontal screen
size. For 3D television, values of 3 % are suggested [ 12 ]. For cinema, values of

1 % for crossed and 2 % for uncrossed disparities are suggested [ 13 ].
Generally,

these definitions generate similar 3D spaces of comfortable

viewing [ 14 ].

10.2.2 Disparity Distribution

In addition to the Vergence-Accommodation conflict, some studies also showed
that the disparity distribution might introduce visual discomfort as well:

1. Excessive uncrossed disparity (behind screen) will induce less visual discomfort
compared to the crossed disparity (in front of the screen) when the angular
disparity magnitude is the same [ 15 ].
2. When most parts of an image are positioned behind the screen (or the averaged
disparity is uncrossed), there will be less visual discomfort compared with the
condition that they are distributed in front of the screen [ 16 ].
3. If the image is split into top and bottom parts, the stereoscopic image will be
more comfortable to watch when the top part of the image is distributed behind
the screen and the bottom of the image is in front of the screen [ 17 ].
4. In the condition of the same averaged value of disparity distribution, higher
dispersion of the disparity would lead to more visual discomfort due to the
Vergence-Accommodation conflict [ 16 ].

10.2.3 Binocular Distortions

Binocular distortions or binocular image asymmetries seriously reduce visual
comfort if present to a sufficient extent [ 18 ]. Asymmetries can be classified into