Pulse or digital communications – Bandwidth reduction or expansion – Television or motion video signal
Reexamination Certificate
1997-09-19
2003-02-25
Lee, Young (Department: 2613)
Pulse or digital communications
Bandwidth reduction or expansion
Television or motion video signal
Reexamination Certificate
active
06526096
ABSTRACT:
FIELD OF INVENTION
The present invention relates to a video coding system. In particular, it relates to a system for the compression of video sequences using motion compensated prediction.
BACKGROUND OF THE INVENTION
The schematic diagram of a system using motion compensated prediction is shown in FIG.
1
and
FIG. 2
of the accompanying drawings.
FIG. 1
illustrates an encoder having a motion estimation block and
FIG. 2
illustrates a corresponding decoder. Motion compensated prediction in such a system is outlined below.
In typical video sequences the change of the content of successive frames is to a great extent the result of the motion in the scene. This motion may be due to camera motion or due to motion of the objects depicted in the scene. Therefore typical video sequences are characterized by significant temporal correlation, which is highest along the trajectory of the motion. Efficient compression of video sequences requires exploitation of this property of video sequences.
Motion Compensated (MC) prediction is a widely recognized technique for compression of video. It utilizes the fact that in typical video sequence, image intensity value in a particular frame can be predicted using image intensities of some other already coded and transmitted frame, given motion trajectory between these two frames.
The operating principle of motion compensated video coders is to minimize the prediction error E
n
(x,y), i.e., the difference between the frame being coded I
n
(x,y) called the current frame and the prediction frame P
n
(x,y) (FIG.
1
):
E
n
(
x,y
)=
I
n
(
x,y
)−
P
n
(
x,y
) (1)
The prediction error E
n
(x,y) is compressed and the compression process typically introduces some loss of information. The compressed prediction error denoted {overscore (E)}
n
(x,y) is sent to the decoder. Prediction frame P
n
(x,y) is constructed by the motion compensated prediction block in FIG.
1
and FIG.
2
. The prediction frame is built using pixel values of the reference frame denoted R
n
(x,y) and the motion vectors of pixels between the current frame and the reference frame using formula
P
n
(
x,y
)=
R
n
[x+&Dgr;x
(
x,y
),
y+&Dgr;y
(
x,y
)]. (2)
Reference frame is one of previously coded and transmitted frames (e.g. frame preceding the one being coded) which at a given instant is available in the Frame Memory of the encoder and of the decoder. The pair of numbers [&Dgr;x(x,y), &Dgr;y(x,y)] is called the motion vector of the pixel in location (x,y) in the current frame. &Dgr;x(x,y) and &Dgr;y(x,y) are the values of horizontal and vertical displacements of this pixel, respectively. Motion vectors are calculated by the motion estimation block in the encoder shown in FIG.
1
. The set of motion vectors of all pixels of the current frame [&Dgr;x(·), &Dgr;y(·)] is called motion vector field and is transmitted to the decoder.
In the decoder, pixels of the coded current frame {overscore (I)}
n
(x,y) are reconstructed by finding the prediction pixels in the reference frame R
n
(·) using the received motion vectors and by adding the received prediction error {overscore (E)}
n
(x,y), i.e.,
{overscore (I)}
n
(
x,y
)=
R
n
[x+&Dgr;x
(
x,y
),
y+&Dgr;y
(
x,y
)]+{overscore (E)}
n
(
x,y
) (3)
For example, if the transmission channel available for the compressed video bit stream is very narrow, it is possible to reject the effect of prediction errors. Then it is not necessary to compress and transmit the prediction error, and the spare bits from the transmission channel and spare calculation power can be used for other purposes, e.g., to improve the frame rate of the video signal. The rejection of prediction errors leads to defective pixel elements in the visible video picture, but depending on the demands of the application in use it may be acceptable.
Due to the very large number of pixels in the frame it is not efficient to transmit a separate motion vector for each pixel. Instead, in most video coding schemes the current frame is divided into larger image segments so that all motion vectors of the segment can be described by few coefficients. Depending on the way the current frame is divided into the segments two types of motion compensated coders can be distinguished:
1 Block based coders where the current frame is divided into fixed and a priori known blocks, e.g., 16×16 pixels blocks in international standard ISO/IEC MPEG-1 or ITU-T H.261 codecs (
FIG. 3
a
).
2 Segmentation based (region based) coders where the current frame is divided into arbitrarily shaped segments, e.g., obtained by a segmentation algorithm (
FIG. 3
b
). (For examples refer to Centre de Morphologie Mathematique (CMM), “Segmentation algorithm by multicriteria region merging,” Document SIM(95)19, COST 211ter Project Meeting, May 1995 and P. Cicconi and H. Nicolas, “Efficient region-based motion estimation and symmetry oriented segmentation for image sequence coding,”
IEEE Transactions on Circuits and Systems for Video Technology
, Vol. 4, No. 3, June 1994, pp. 357-364)
In practice segments include at least few tens of pixels. In order to represent the motion vectors of these pixels compactly it is desirable that their values are described by a function of few coefficients. Such function is called motion vector field model.
Motion compensated video coding schemes may define the motion vectors of image segments by the following general formula:
Δ
x
(
x
,
y
)
=
∑
i
=
0
N
-
1
a
i
f
i
(
x
,
y
)
(
4
)
Δ
y
(
x
,
y
)
=
∑
i
=
0
M
-
1
b
i
g
i
(
x
,
y
)
(
5
)
where coefficients a
i
and b
i
are called motion coefficients and are transmitted to the decoder. Functions f
i
and g
i
are called motion field basis functions and have to be known both to the encoder and decoder.
Polynomial motion models are a widely used family of models. (See, for example H. Nguyen and E. Dubois, “Representation of motion information for image coding,” in
Proc. Picture Coding Symposium '
90, Cambridge, Mass., Mar. 26-18, 1990, pp. 841-845 and Centre de Morphologie Mathematique (CMM), “Segmentation algorithm by multicriteria region merging,” Document SIM(95)19, COST 211ter Project Meeting, May 1995). The values of motion vectors are described by functions which are linear combinations of 2D polynomial functions, the translational motion model is the simplest model and requires only two coefficients to describe motion vectors of each segment. The values of motion vectors are given by formula
&Dgr;
x
(
x,y
)=
a
0
&Dgr;
y
(
x,y
)=
b
0
(6)
This model is used in international standards (ISO MPEG-1, ITU-T Recommendation H.261) to describe motion of fixed 16×16 blocks. Two other widely used models are affine motion model given by the equation:
&Dgr;
x
(
x,y
)=
a
0
+a
1
x+a
2
y
&Dgr;
y
(
x,y
)=
b
0
+b
1
x+b
2
y
(7)
and quadratic motion model given by the equation:
&Dgr;
x
(
x,y
)=
a
0
+a
1
x+a
2
y+a
3
xy+a
4
x
2
+a
5
y
2
&Dgr;
y
(
x,y
)=b
0
+b
1
x+b
2
y+b
3
xy+b
4
x
2
+b
5
y
2
(8)
The Motion Estimation block calculates motion vectors [&Dgr;x(x,y),&Dgr;y(x,y)] of the pixels of a given segment S
k
which minimize some measure of prediction error in this segment. A meaningful additive measure of prediction error has the form
∑
(
x
i
y
i
)
∈
S
k
p
i
h
(
&LeftBracketingBar;
I
n
(
x
,
y
)
-
R
n
(
x
+
Δ
x
(
x
,
y
)
,
y
+
Δ
y
(
x
,
y
)
)
&RightBracketingBar;
)
(
9
)
where p
i
's are scalar constants, |.| denotes absolute value, and h is a non-decreasing function. A very popular measure is the square prediction error, in which case p
i
=1, and h(.)=(.)
2
:
∑
(
x
i
,
y
i
)
∈
S
k
(
I
n
(
x
,
y
)
-
R
n
(
x
Karczewicz Marta
Lainema Jani
Oktem Levent
Lee Young
Nokia Mobile Phones Limited
Perman & Green LLP
LandOfFree
Video coding system for estimating a motion vector field by... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Video coding system for estimating a motion vector field by..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Video coding system for estimating a motion vector field by... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3160254