captured by using a charge-coupled device (CCD) camera
that moves in a plane parallel to the object plane. The paral-
lax images are then encoded into a holographic stereogram.
The encoding process can be considered as a 2-D Fourier
transformation of the parallax images. The reconstructed
images can be decomposed into a summation of recon-
structed rays [Fig. 2(b)]. At different viewing points, the
human eye would see different parallax images, correspond-
ing to reconstructed rays with different directions. Each of
these reconstructed rays is diffracted by a particula r spatial
frequency component in the holographic stereogram. For a
given point ðx
p
;y
p
;z
p
Þ, the spatial frequency is determined
by the angle of the reconstructed ray that joins this point to
the location of the viewer’s eye. In contrast with the method
based on interference, the depth value r
p
ðx; yÞ of the point
cannot be recorded in the stereogram generation process. As
a result, there is a stereo effect for binocular vision but not for
monocular vision.
In natural viewing, images arrive at the eyes with varying
degrees of binocular disparity. The brain perceives the spatial
relation of objects based on this disparity, resulting in 3-D
vision. When the viewer looks from one point to another,
the viewer must adjust his eye’s vergence (the angle between
the lines of sight). The distance between the point at which
two sight lines intersect and the viewer is defined as the ver-
gence distance (VD). The VD is one of the primary factors
that determine the 3-D perception of stereo displays.
Additionally, the viewer also adjusts the focal power of
each of his eyes to produce a clear image of the part of
the object that the eyes are fixated on. The distance at
which the eye must be focused to create a sharp retinal
image is defined as the accommodation distance (AD).
20
Accommodation is a weak depth cue, but is also very impor-
tant. Accommodation is represented as a blur gradient when
viewing real scenes. The retinal image blur varies with
changes in scene depth. The retinal image is sharpest for
objects at the distance at which the eye is focused, i.e.,
the accommodation distance, and blurred for nearer and far-
ther objects.
As shown in Fig. 3(a), the vergence and accommodation
responses are always coupled in natural viewing. When the
focus point of the eyes changes from A to B, the VD changes
according to the depth of the focus point (from VD1 to
VD2). The accommodation distance changes (from AD1
to AD2) with the vergence distance such that they remain
nearly identical. As discussed above, stereo vision is mainly
caused by the binocular disparity in conventional holo-
graphic stereograms. The vergence response rema ins the
same as it is in natural viewing. However, the focused
plane is fixed at the location of the holographic plane,
which means that the AD does not change with object points
in this system. Thus, the natural correlation between the ver-
gence and accommodation distances is disrupted [Fig. 3(b)].
This is the so-called vergence-accommodation conflict.
However, if the improved holographic stereograms we pro-
posed can achieve synchronous change of the vergence and
Object plane
Real Object
Camera plane
Camera
Parallax image
Holographic
stereogram
3D image
Viewing points
Eye
()
,,
ppp
xyz
(a) (b)
Fig. 2 Schematics of (a) the capturing of parallax images and (b) the reconstruction process in holo-
graphic stereograms.
Conventional holographic stereogram
Improved holographic stereogram
AD2
VD2
VD
1
A
D
A
B
A
B
A
B
AD1
VD1
VD2
VD
1
AD1
V
D2
AD2
(a) (b) (c)
Fig. 3 Vergence distance (VD) and accommodation distance (AD) in (a) natural viewing, (b) a conven-
tional holographic stereogram, and (c) an improved holographic stereogram.
Journal of Electronic Imaging 061109-3 Nov∕Dec 2014
•
Vol. 23(6)
Pei et al.: Method for generating full-parallax holographic stereograms without vergence-accommodation conflicts