3D Holoscopic Video Representation and Coding Technology - Novel 3D Media Technologies - page 76

x 10 4

a b

1.42

x 10 4

1.32

1.22

1.4

1.3 1.12

x

- MI j 0 + MI j

x 10 4 1.2

1.419

1.1

1.319

y

1.219

x

1.119

0

- MI i + MI i

y

Fig. 5.3 Example of inherent 3D holoscopic spatial correlation: (a) autocorrelation function for a
3D holoscopic image; and (b) projection onto x and y axis, showing the high correlation between
points spaced of about one micro-image size (MI)

Although there is a resemblance between 3D holoscopic video and 2D video
(as both are captured by an ordinary 2D sensor), a more careful analysis reveals
inherent correlations that are not exploited by state-of-the-art 2D video coding
solutions. Notably, in the spatial domain, a significant correlation between neigh-
boring micro-images can be identified through the autocorrelation function, as
illustrated in Fig. 5.3 . In particular, it can be seen that the pixel correlation in 3D
holoscopic content is not as smooth as in conventional 2D video content. A periodic
structure is evidenced by the autocorrelation function whose period is approxi-
mately one micro-image size (represented in each direction by MI j and MI i in
Fig. 5.3b ). Additionally, it should also be noted that each micro-image itself has
some degree of inter-pixel redundancy, as is also common in 2D images.
Some coding schemes in the literature have proposed to represent the 3D
holoscopic image by a stack of their composing micro-images, which can be
interpreted as a pseudo volumetric image (PVI) [ 8 , 9 ]—if stacking micro-images
are done along the third dimension—or a pseudo video sequence (PVS) [ 10 , 11 ]—if
stacking is done along the temporal dimension.
Briefly, consider a 3D holoscopic image, HI, as illustrated in Fig. 5.4a , captured
using a rectangular-packed square-based micro-lens array with resolution
MLA n MLA m and micro-image resolution of MI j

MI i . Each micro-image,
MI k , in the PVI or PVS representation can be obtained from HI at the position
k

( x , y ) represents the
pixel positions inside MI k . Alternatively, the holoscopic image can be expressed in
terms of its micro-images by the array in ( 5.2 ).

¼ ( k n , k m ) in the micro-lens array, as given by ( 5.1 ), where x ¼

MI k ¼ HI k n MI j þ x , k m MI i þ y ð 5 1 Þ

:

HI ¼ ½ MI k MLA n MLA m ð 5 2 Þ

: