Pulse or digital communications – Bandwidth reduction or expansion – Television or motion video signal
Reexamination Certificate
1998-12-11
2002-03-12
Le, Vu (Department: 2613)
Pulse or digital communications
Bandwidth reduction or expansion
Television or motion video signal
Reexamination Certificate
active
06356592
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a moving image coding apparatus and, more particularly, to a moving image coding apparatus for performing coding operation by performing pre-processing using an intraframe filter.
In moving image compression coding, a moving image coding scheme based on a combination of motion compensation and orthogonal transformation is often used. Image data input to a coding apparatus contains high-frequency components due to noise. To prevent a deterioration in coding efficiency due to such high-frequency components, pre-processing is performed by using an intraframe filter. The above intraframe filter has the effect of removing high-frequency components that impair spatial correlation. The removal of such high-frequency components, however, leads to a decrease in resolution and a deterioration in image quality in a still area, in particular. A scheme for solving this problem is disclosed in, for example, Japanese Patent Laid-Open No. 2-154588. In this scheme, the filter strength is changed for the still and moving areas of a moving image such that a weak filter is used for the still area, and a strong filter is used for the moving area.
FIG. 5
shows a conventional moving image signal coding apparatus. An input image
1
is processed in units of pixel blocks (e.g., 16×16). A moving area determination circuit
4
compares each pixel of the input image
1
with a corresponding pixel of the previous frame stored in a frame memory
3
to determine a moving or still area. A filter coefficient
19
used by an intraframe filter circuit
2
is output on the basis of the determination result and coding control data
17
output from a coding control circuit
15
. The intraframe filter circuit
2
filters the input image
1
by using the filter coefficient
19
and sends the resultant data to a coding section
5
.
A subtracter
6
subtracts a predicted image
18
motion-compensated by a motion compensation circuit
14
from the pixel blocks sent to the coding section
5
. The resultant data is processed by a DCT (Discrete Cosine Transform) circuit
7
and a quantization circuit
8
and coded by a coding circuit
9
. The coded data is stored in the buffer
16
. The output from the quantization circuit
8
is processed by a de-quantization circuit
10
and an inverse DCT circuit
11
in the coding section
5
and is added to the predicted image
18
by an adder
12
. The resultant data is stored in a frame memory
13
.
In the above conventional apparatus, all the pixels even in an area having undergone accurate prediction based on motion compensation are filtered by the strong filter if the area is a moving area. This unnecessarily decreases the resolution of the image.
Furthermore, in the conventional apparatus, the strong filter sometimes works on even an accurately predicted area. The effect of this filter may increase the difference between an input signal and a predicted signal even in such an area where the difference should be sufficiently small, resulting in a decrease in coding efficiency.
SUMMARY OF THE INVENTION
The present invention has been made to solve the above problems in the prior art, and has as its object to provide a moving image signal coding apparatus which can improve the coding efficiency using an intraframe filter while minimizing a decrease in the resolution of an image.
In order to achieve the above object, according to the present invention, there is provided a moving image coding apparatus comprising motion compensation type moving image coding means for predicting a pixel block in a current frame from a preceding frame by a motion compensation scheme, and performing orthogonal transformation and quantization for a predicted error, thereby coding the predicted error, intraframe filter means for performing intraframe filtering for each pixel of an input frame, and means for adaptively determining and setting a filter characteristic of the intraframe filter means on the basis of a pixel of the input frame and a corresponding pixel of a predicted image or adaptively selecting an output from the intraframe filter means having an optimal filter characteristic.
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Takishima et al., “A Study of Adaptive Pre-filtering for Video Coding,” Proceedings from the 1992 (7th) Picture Coding Symposium of Japan.
Takishima et al., “Adaptive Coding Control by Means of Variable Characteristics Prefilter,” “1990 Autumn National Convention . . . The Institute of Electronics . . . ,” Oct. 1-4, 1990, Part 6, pp 6-230.
Kronander, T., “Post- and pre-processing in coding of image sequences using filters with motion compensated history”, ICASSP-88, Apr. 1988.
Foley & Lardner
Le Vu
NEC Corporation
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