Pulse or digital communications – Bandwidth reduction or expansion – Television or motion video signal
Reexamination Certificate
2000-07-26
2001-09-04
Lee, Young (Department: 2613)
Pulse or digital communications
Bandwidth reduction or expansion
Television or motion video signal
Reexamination Certificate
active
06285713
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a video coding/decoding system and a video coder and a video decoder used with the same system for implementing a motion compensation method in which all the pixels associated with the same patch are not restricted to have a common motion vector and in which the horizontal and vertical components of a motion vector of a pixel can assume an arbitrary value other than an integral multiple of the distance between adjacent pixels.
2. Description of the Related Art
In the high-efficiency coding and decoding of image sequences, a motion compensation method utilizing the analogy between temporally-proximate frames is well known to have a great advantage in compressing the amount of information.
FIGS. 1A and 1B
are diagrams showing a general circuit configuration of a video coder
1
and a video decoder
2
to which the motion compensation method described above is applied.
In
FIG. 1A
, a frame memory
2
-
1
has stored therein a reference image R providing a decoded image of the previous frame already coded. A motion estimation section
3
-
1
estimates a motion and outputs motion information using the original image I of the current frame to be coded and the reference image R read out of the frame memory
2
-
1
. A predicted image synthesis circuit
4
-
1
synthesizes a predicted image P for the original image I using the motion information and the reference image R. A subtractor
5
-
1
calculates the difference between the original image I and the predicted image P and outputs a prediction error. The prediction error is subjected to the DCT conversion or the like at a prediction error coder
6
-
1
, and transmits the prediction error information together with the motion information to the receiving end. At the same time, the prediction error information is decoded by the inverted DCT conversion or the like at a prediction error decoder
7
-
1
. An adder
8
-
1
adds the coded prediction error to the predicted image P and outputs a decoded image of the current frame. The decoded image of the current frame is newly stored in the memory
2
-
1
as a reference image R.
In
FIG. 1B
, a frame memory
2
-
2
has stored therein a reference image R providing a decoded image of the previous frame. A synthesis circuit
4
-
2
synthesizes a predicted image P using the reference image R read out of the frame memory
2
-
2
and the motion information received. The received prediction error information is decoded by being subjected to the inverse DCT conversion or the like by a prediction error decoder
7
-
2
. An adder
8
-
2
adds the decoded prediction error to the predicted image P and outputs a decoded image of the current frame. The decoded image of the current frame is newly stored in the frame memory
2
-
2
as a reference image P.
A motion compensation method constituting the main stream of the current video coding and decoding techniques depends on the “block matching of half-pixel accuracy” employed by MPEG
1
and MPEG
2
providing the international standard of video coding/decoding method.
In the “block matching of half-pixel accuracy”, the original image of the current frame to be coded is segmented into a number n of blocks at the motion estimation section
3
-
1
in
FIG. 1A
, and a motion vector is determined for each block as a motion information. The horizontal and vertical components of this motion vector have a minimum unit length equal to one-half of the distance between horizontally and vertically adjacent pixels, respectively. In the description that follows, let the horizontal component of the motion vector of the ith block (1≦i≦n) be ui and the vertical component thereof be vi. In a method most widely used for estimating the motion vector (ui,vi), a search range such as −15≦ui≦15, −15≦vi≦15 is predetermined, and a motion vector (ui,vi) which minimizes the prediction error Ei(ui,vi) in the block is searched for. The prediction error Ei(ui,vi) is expressed by Equation 1 using a mean absolute error (MAE) as an evaluation standard.
Ei
(
ui
,
vi
)
=
1
Ni
∑
(
x
,
y
)
∈
BI
&LeftBracketingBar;
I
(
x
,
y
)
-
R
(
x
-
ui
,
y
-
vi
)
&RightBracketingBar;
(
1
)
In Equation 1, I(x,y) denotes the original image of the current frame to be coded, and R(x,y) a reference image stored in memory. In this equation, it is assumed that pixels exist at points of which the x and y coordinates are an integer on the original image I and the reference image R. Bi designates the pixels contained in the ith block of the original image I, and Ni the number of pixels contained in the ith block of the original image I. The process of evaluating the prediction error for motion vectors varying from one block to another and searching for a motion vector associated with the smallest prediction error is called the matching. Also, the process of calculating Ei(ui,vi) for all vectors (ui,vi) conceivable within a predetermined search range and searching for the minimum value of the vector is called the full search.
In the motion estimation for the “block matching of half-pixel accuracy”, ui and vi are determined with one half of the distance between adjacent pixels, i.e., ½ as a minimum unit. As a result, (x−ui,y−vi) is not necessarily an integer, and a luminance value of a point lacking a pixel must actually be determined on the reference image R when calculating the prediction error using Equation 1. The process for determining the luminance value of a point lacking a pixel is called the interpolation, and the point where interpolation is effected is referred to as an interpolated point or an intermediate point. A bilinear interpolation is often used as an interpolation process using four pixels around the interpolated point.
When the process of bilinear interpolation is described in a formula, the luminance value R(x+p,y+q) at the interpolated point (x+p,y+q) of the reference image R can be expressed by Equation 2 with the fractional components of the coordinate value of the interpolated point given as p and q (0≦p<1, 0≦q<1).
R
(
x
+
p
,
y
+
q
)
=
(
1
-
q
)
{
(
1
-
p
)
R
(
x
,
y
)
+
pR
(
x
+
1
,
y
)
}
+
q
{
(
1
-
p
)
R
(
x
,
y
+
1
)
+
pR
(
x
+
1
,
y
+
1
)
}
(
2
)
In the motion estimation by “block matching of half-pixel accuracy”, a two-step search is widely used in which, first, the full-search of single-pixel accuracy is effected for a wide search range to estimate a motion vector approximately, followed by the full search of half-pixel accuracy for a very small range defined by, say, plus/minus a half pixel in horizontal and vertical directions around the motion vector. In the second-step search, a method is frequently used in which the luminance value of an interpolated point on the reference image R is determined in advance. An example of the process according to this method is shown in
FIGS. 2A
, B, C and D. In this example, a block containing four pixels each in longitudinal and lateral directions is used. In
FIGS. 2A
, B, C and D, the points assuming an integral coordinate value and originally having a pixel in a reference image are expressed by a white circle ◯, and the interpolated points for which a luminance value is newly determined are represented by X. Also, the pixels in a block of the original image of the current frame are expressed by a white square □. The motion vector obtained by the first-step search is assumed to be (uc,vc).
FIG. 2A
shows the state of matching when the motion vector is (uc,uv) in the first-step search. The prediction error is evaluated between each pair of ◯ and □ overlapped.
FIGS. 2B
, C and D show the case in which the motion vector is (uc+½,vc), (uc+½,vc+½), (uc−½,vc−½) in the second-step search. The prediction error is evaluated between each overlapped pair of X and □ in
FIGS. 2B
, C and D. As
Kimura Jun-ichi
Nakaya Yuichiro
Antonelli Terry Stout & Kraus LLP
Hitachi Ltd
Lee Young
LandOfFree
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