Pulse or digital communications – Bandwidth reduction or expansion – Television or motion video signal
Reexamination Certificate
1998-04-24
2001-05-29
Kelley, Chris (Department: 2613)
Pulse or digital communications
Bandwidth reduction or expansion
Television or motion video signal
C382S268000, C348S420100
Reexamination Certificate
active
06240135
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method of coding data, and more particularly, to a method of removing blocking artifacts when coding image signals such as in a moving picture at low-bit-rate.
2. Background of the Related Art
Generally, to efficiently compress a time variable video sequence, it is necessary to remove redundancy in the temporal domain as well as in the two-dimensional spatial domain. In moving picture experts group (MPEG), discrete cosine transform (DCT) is used to remove the redundancy in the two-dimensional spatial domain while a motion compensation method is used to remove the redundancy in the temporal domain.
The DCT is a method of removing the correlativity between data through a two-dimensional spatial transformation. Each block in a picture is spatially transformed using the DCT after the picture is divided into blocks. Data that has been spatially transformed tends to be driven to a certain direction. Only a group of the data driven in the certain direction is quantized and transmitted.
Pictures, which are consecutive in the temporal domain, tend to form motions of a human being or an object at the center of the frame. This property is used to reduce the redundancy of the temporal domain in the motion compensation method. A volume of data to be transmitted can be minimized by taking out a similar region from the preceding picture to fill a corresponding region, which has not been changed (or has very little change), in the present picture. The operation of finding the most similar blocks between pictures is called a motion estimation. The displacement representing a degree of motion is called a motion vector. MPEG uses a motion compensation-DCT method so that the two methods combine.
When a compression technique is combined with a DCT algorithm, the DCT transform is usually performed after input data is sampled in a unit size of 8×8, and the transform coefficients are quantized with respect to a visual property using quantization values from a quantization table. Then, the data is compressed through a run length coding (RLC). The data processed with the DCT is converted from a spatial domain to a frequency domain and compressed through the quantization with respect to the visual property of human beings, not to be visually recognized. For example, since eyes of human beings are insensitive to a high frequency, a high frequency coefficient is quantized in a large step size.
For the quantized data, the data having a relatively high frequency is coded with a short code word. The quantized data having a low frequency is coded with a long code word. Thus, the data is finally compressed.
In processing a moving picture as discussed above, blocks are individually processed to maximize the compression ratio and coding efficiency. However, the individual process causes blocking artifacts that disturb the eyes of human beings at boundaries between blocks.
A related art method of removing blocking artifacts will be described with reference to
FIGS. 1 and 2
.
FIG. 1
is a pixel matrix illustrating a method for removing blocking artifacts.
FIG. 2
is a pixel matrix illustrating block boundaries in horizontal and vertical directions.
Various algorithms have been presented for removing blocking artifacts that appear in a coding system, which individually processes blocks. For example, MPEG-4 used a deblocking filter by Telenor, which uses the following algorithm:
If B is replaced with B1 and C is replaced with C1,
B1=B+d1,
C1=C−d1, and
d1=sign(d)*(MAX(0,|d|−MAX(0,2*|d|−QP)))
where d=(3A−8B+8C−3D)/16 and QP denotes the quantization parameter of the macroblock where pixel C belongs.
In processing a MPEG-4 moving picture, blocking artifacts are removed using the above algorithm to improve picture quality. However, it is difficult to effectively remove the blocking artifacts with the above with a small operation capacity in a real time operation. For example, coding and decoding a moving picture is a real time operation. In other words, to completely remove the blocking artifacts, a large calculation amount is needed, which is undesirable in efficiency.
Alternatively, to remove the blocking artifacts, there is provided a method of changing processes of coding and decoding. This method increases the amount of bits to be transmitted.
Still another method for removing blocking artifacts is based on the theory of projection onto convex sets (POCS). However, this method is applied only to a still picture because of an iteration structure and long convergence time.
Thus, the related art methods for removing blocking artifacts in a coding system of a moving picture have several problems. First, in performing an algorithm for removing the blocking artifacts, a calculation is complicated and the calculation amount and time become correspondingly large. Further, the blocking artifacts are not removed in either complex regions or smooth regions in a picture. In addition, the amount of bits to be transmitted increases.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a method of removing blocking artifacts in a coding system that substantially obviates one or more of the problems due to limitations and disadvantages of the related art.
Another object of the present invention is to remove blocking artifacts when necessary in a smooth portion of a moving picture.
Yet another object of the present invention is to provide a method of removing blocking artifacts in a coding system of a moving picture where blocking artifacts of the moving picture are removed at real time using frequency features around a block boundary without increasing the amount of bits.
To achieve at least the above objects in a whole or in parts, a method of removing blocking artifacts in a coding system according to the present invention includes determining at least pixel sets S0, S1, S2 around a block boundary, selecting one of a default mode and a DC offset mode as a deblocking mode based on an amount of blocking artifacts, deblocking filtering pixels adjacent the block boundary if a default mode is selected, deblocking filtering of pixels adjacent the block boundary if a default mode is selected, and removing artifacts in the DC offset mode when the DC offset mode is selected and a DC offset mode condition is satisfied, where the artifacts are removed in the DC offset mode according to the following equation:
v
n
=
∑
k
=
-
4
4
b
k
·
p
n
+
k
,
1
≤
n
≤
8
P
m
=(|v
1
−v
0
|<QP)?v
0
:v
1
,
If m<1;
v
m
, if 1≦m≦8;
(|v
8
−v
9
|<QP) v
9
:v
8
, if m>8;
{b
k
:−4≦k≦4}={1,1,2,2,4,2,2,1,1}//16,
wherein v
0
−v
9
are boundary pixels, QP is the quanatation parameter of a block adjacent the block boundary, and v
n
is an adjusted pixel value.
To further achieve the above advantages and in accordance with the purpose of the present invention, as embodied and broadly described, a method of removing blocking artifacts in a coding system of a moving picture according to the present invention includes the steps of defining pixel sets S0, S1, S2 around block boundary, selectively determining a deblocking mode as a default mode or a DC offset mode depending on the degree of blocking artifacts after obtaining a mode decision value, obtaining frequency information around the block boundary per pixel using 4-point DCT kernel if the default mode is determined, replacing a magnitude of a discontinuous component belonging to the block boundary with a minimum magnitude of discontinuous components belonging to the surroundings of the block boundary in the frequency domain and applying this replacing step to the spatial domain, judging whether or not it is necessary to perform DC offset mode if the DC offset mode is determined, and removing the blocking artifacts in a smooth region when the judgment is to perform
Fleshner & Kim LLP
Kelley Chris
LG Electronics Inc
Tu Tung Vo
LandOfFree
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