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and CAPELLINI, v.: ‘DCT-based
watermark recovering without resorting to the uncorrupted
original image’. Proc. IEEE Int. Conf. Image Processing, Vol I,
October 1997, pp. 520-523
HSU, c.-T., and wu, I.-L.: ‘Hidden signatures in images’. Proc. IEEE
Int. Conf. Image Processing, Vol 111, September 1996, pp. 223-226
PIVA, A., BARNI, M., BARTOLINI, F ,
The threshold can be increased, according to practical requirements.
Method for detecting all-zero DCT
coefficients ahead of discrete cosine
transformation and quantisation
Simulation results: In our simulations, we have applied the proposed method to the H.263 software written by Telenor R&D.
The frame frequency is ftxed to 10 frames per second. We tested
the ‘Miss America’, ‘Claire’ and ‘Susie’ sequences, and found the
proposed method to be very useful to these kinds of test
sequences, i.e. head-and-shoulder type video sequences mainly
used in video phone applications. When the quantisation level Q =
10, -40% of the blocks of the ‘Miss America’ and ‘Claire’
sequences and more than 20% of the blocks of the ‘Susie’
sequence can be determined to have all-zero DCT coefficients
without any incorrectly determined blocks.
Zhou Xuan, Yu Zhenghua and Yu Songyu
Table 1: Simulation results for SQ
The authors present a new algorithm for detecting all-zero DCT
coefficient blocks before discrete cosine transformation and
quantisation in very low bit rate video coding. The proposed
algorithm uses the sum of absolute difference (SAD) of each
motion compensation block as the criteria. Because the SAD can
be obtained after motion estimation, no additional computation is
required. Results show that the proposed algorithm can be used
to reduce the amount of computation significantly without
sacrificing video coding quality.
Introduction: Motion estimation, motion compensation, discrete
cosine transformation (DCT) and quantisation are major processing components of motion picture coding. In this entire process,
DCT is applied to compress motion compensation data in the spatial domain, and a special case for the encoder occurs when all the
coefficients from the DCT are quantised to zero. In this situation,
instead of sending multiple zeros to the decoder, the encoder sends
a special signal indicating the state. This makes its representation
very efficient. However, the detection of this state is quite computationally expensive using traditional methods, because it requires
the computation of one entire round of the DCT and quantisation, followed by a check to see if all the coefficients are zero. For
very low bit-rate coding, especially in videophone applications,
this all-zero state is quite common, so early detection of this state
could significantly reduce the amount of computation necessary.
In this Letter, a method for determining all-zero blocks before
DCT and quantisation is proposed. When this method is used in
H.263 [I], and the quantisation level is 10, -40% of the blocks of
the Miss America and Claire sequences can be determined to have
all-zero coefficients.
r
t
8Q
12Q
16Q
20Q
determination threshold while Q=10
24Q
15211
Fig. 1 Graph of all-zero block ratio against determination threshold
Proposed algorithm: The discrete cosine transform of a discrete
functionAx,y), x, y = 0, 1, ..., N-1 is defined as [2]
A Miss America
Claire
1 Susie
/
where
ku,k
-
1
-
u-fi
foru,v=O
=1
for u , v = 1 , 2,...,N
In H.263, N = 8, so eqn. 1 gives
c
l 7
IF(%
.)I < 4
a=O
abs(f(z, 9 ) )
-
I
(2)
y=o
The condition for all-zero DCT coefficients is
IF(u,v)l < 2Q
(3)
where U, v = 0, 1, ..., 7, and Q denotes the quantisation level. Thus
7
0Q
7
z=o y=o
16Q
20Q
24Q
45zizj
The above inequality gives the conditions under which the DCT
has all-zero coefficients. In eqn. 4, the left summation gives the
sum of absolute difference (SAD) of the motion compensation
block, which can be obtained during motion estimation. Therefore
no additional computation is required. Eqn. 4 is the full condition.
ELECTRONICS LETTERS
12Q
determination threshold while Q=10
(4)
77th September 7998
Pig. 2 Graph of incorrect determined block ratio against determination
threshold
A Miss America
Claire
W Susie
Vol. 34
No. 19
1839
Table 1 shows the simulation results for threshold 8Q. With
increasing quantisation level Q, the number of all-zero blocks
determined will increase very rapidly.
From eqn. 4, we know that the threshold 8Q represents the sufficient condition for all-zero DCT coefficients but not the necessary condition. In our research work, we also tried to increase the
determination threshold to reduce the amount of computation
necessary. Fig. 1 shows the ratio of determined all-zero blocks
against determination threshold, and Fig. 2 shows the ratio of
incorrectly determined blocks against determination thieshold. In
Figs. 1 and 2, the quantisation level Q = 10; from these Figures,
we found that 16Q gives a good tradeoff between image distortion
and computational burden.
of Fig. 1, all cameras, including the ‘virtual’ ones, have optical
centres on the X axis, and optical axes parallel to the 2 axis. In
general, under these assumptions, points corresponding to different IVs can be accurately reconstructed from the two original
images of the stereo pair, provided that the disparities have
already been computed. Consequently, the correspondence problem should be solved prior to the IV reconstruction.
We stress the importance of having reliable disparity maps to
reconstruct IVs, which is supported by prior work [l]. The accuracy of the disparity map strongly affects the quality of the IV
reconstruction. Hence, it is assumed here that the reason that
points are marked as not matched is on account of the non-existence of homologous points in the other image due to occlusion.
Conclusions: A new technique for the early determination of allzero DCT coefficient blocks has been presented. Using the technique, -40% of blocks in the ‘Miss America’ and ‘Claire’
sequences can be determined to be all-zero DCT coefficient
blocks. Because the technique uses the SAD as a criterion, no
additional computation is required. Moreover, the determination
threshold can be changed with the quantisation level self-adaptively. This new technique is very useful for very low bit rate video
coding, and can be applied to videophone and video conferencing
systems.
0 IEE 1998
Electronics Letters Online No: 19981308
Zhou Xuan, Yu Zhenghua and Yu Songyu (Institute
Communication and Information Processing, Shanghai
University, Shanghai, 200030, People’s Republic of China)
Image
Jiaotong
of
References
1 Draft ITU Rec. H.263: ‘Line transmission of non-telephone
signals: Video coding for low bit rate communication’. 5 December
1995
2 MAKHOUL, J.: ‘A fast cosine transform in one and two dimensions’,
ZEEE Trans., 1980, ASSP-28, (1), pp. 27-34
Stereo-based intermediate view synthesis
with realistic ’look around’ capability
M.M. Perez, C.L. Pagliari and T.J. Dennis
An algorithm is presented for synthesising intermediate views
from a stereo image pair that attempts to produce a realistic 3D
‘look around’ effect. The task is accomplished by suitably
incorporatinga term proportional to the corresponding disparities
to the co-ordinatesof the ‘virtual’ intermediate view points to be
reconstructed.
Introduction: In this Letter, we propose an image synthesis algorithm which generates views between those of the two camera
views that originally captured the scene. A special treatment for
occluded areas is also embedded in the algorithm. In previous
work [l, 21 the intermediate views (IVs) are synthesised by mapping the intensities of the points over the 2D image co-ordinates
of one of the two camera views that constitute the stereo pair. The
key-point of our approach is that the 2D co-ordinates of the new
intermediate view
points generated are established as functions of both the disparity data for the original stereo pair and the
desired position for the ‘virtual’ intermediate camera.
A basic motivation for the use of stereoscopic data is to give the
feeling of telepresence, which we try to enhance by allowing a realistic ‘look around’ capability. A further benefit occurs in scenes
shot by a stereo camera with a large baseline which may cause discomfort to viewers using stereoscopic displays [2]. This can be
avoided by generating more tolerable intermediate views.
Problem dejnition: As depicted in Fig. 1, the task of IV reconstruction is to generate images that would have been acquired
from ‘virtual’cameras, depicted in grey, located anywhere between
the original left and right extremes. According to the convention
1840
\U
7 July 1998
Fig. 1 Stereo cameras showing left view (L), intermediate views (pictured in grey) and right view (R)
Synthesis of intermediate views: A setup where the cameras have
parallel optical and vertical axes and coplanar image planes is
assumed, thus implying a linear relationship between the disparity
and the displacement, aB,of a virtual camera optical centre along
the real camera baseline, length B. Although the algorithm has
been developed assuming a parallel camera setup, IV synthesis
using the proposed algorithm is also feasible for scenes shot with a
camera convergence angle small enough for the induced vertical
disparity to be neglected.
The reconstructed image Z,, is a linear combination of the left
(IL)and right (IR)images and the disparity (d) map. The ‘virtual’
intermediate camera (corresponding to the IV)is related to the left
image plane by a translation aB,0 I a < 1. It can be shown,
through manipulations over the expressions for the projective
transform, that under such a translation the resulting disparity
map between the original left stereo view and the new IV is a
scaled version (by a factor a)of the original disparity map for the
left image of the stereo pair. As a consequence, the intensity of a
point in the new IV should not simply be mapped over the coordinates of one of its two homologous points, in one of the original stereo views. Instead, the new co-ordinates of each IV point
can be precisely computed, based on the co-ordinates of its homologous point in one of the original stereo views, and the scaled disparity map corresponding to this new IV.
Eqn. 1 represents the proposed interpolator applied to synthesise the IV for all matched points of an original M x N pixel size
stereo pair. The expression for the resulting co-ordinates of the IV
points is the left side of eqn. 1, whereas the right side corresponds
to the synthesis of the intensity of the new ‘virtual‘ point from a
weighted average of the intensities of its original homologous
points. Eqn. 1 takes the left image of the stereo pair as the reference view. This is consistent with the fact that the IV corresponds
to the left image for a = 0. In this case, d (which is computed
using [3]) is the disparity of the right image in relation to the left
image.
The remaining unmatched points receive a special treatment.
We assume that unmatched regions have near-constant depth
which is similar to that of adjacent background areas. This is not
an unreasonable assumption, considering that the depth variation
along these images patches is unpredictable, and also that they
cannot have the same depth as the foreground region, because
they could not then be occluded in one of the stereo views. For
unmatched points in the left image, each IV intensity point is
ELECTRONICS LETTERS
17th September 1998
Vol. 34
No. 19
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