Image analysis – Image enhancement or restoration – Edge or contour enhancement
Reexamination Certificate
1997-10-22
2001-07-10
Lee, Thomas D. (Department: 2624)
Image analysis
Image enhancement or restoration
Edge or contour enhancement
C382S199000
Reexamination Certificate
active
06259823
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to data filtering, and more particularly, to a signal adaptive filtering method for reducing a blocking effect and ringing noise, and a signal adaptive filter suitable for the method.
2. Description of the Related Art
Generally, picture encoding standards such as the Motion Picture Expert Groups (MPEG) of the International Organization for Standardization (ISO) and H.263 recommended by the International Telecommunication Union (ITU) adopt block-based motion estimation and discrete cosine transform (DCT) compression of blocks. When an image is highly compressed the block-based coding may cause a blocking effect and ringing noise, as is well known. A typical blocking effect is grid noise in a homogeneous area in which adjacent pixels have relatively similar pixel values. Another blocking effect is staircase noise which has the shape of a staircase and is generated along an edge of the image. Also, the ringing noise is due to the typical Gibb's phenomenon which results from truncation of a DCT coefficient during quantization when the image is highly compressed.
In the case of grid noise, traces caused by the process being performed on each block can be seen at the boundary between blocks when the compressed data is restored to be displayed on a screen. Accordingly, a border between blocks can be noticed by a user. In the case of staircase noise, an edge of the image is shaped like a staircase, so that a jagged edge of the image can be noticed by a user. The ringing noise causes a problem in that an object in the image is displayed as multiple overlapping objects.
SUMMARY OF THE INVENTION
To solve the problems above, an object of the present invention is to provide a signal adaptive filtering method for reducing the blocking effect and ringing noise in a high compression encoding system, and to provide a signal adaptive filter for implementing that method.
In a signal adaptive filtering method according to the present invention, a gradient of the image data is calculated for each pixel of the image data. Then, the gradient data of each pixel is compared with a global threshold value (T
g
) which is determined based on a predetermined quantization step (Q), and a global edge map information of the pixel is generated. Meanwhile, the gradient data of each pixel is compared with a local threshold value (T
n
) which is determined for each block having a predetermined size, and a local edge map information of the pixel is generated. An OR operation is performed with respect to the global edge map information and the local edge map information to generate binary edge map information. Then, a filter window of a predetermined size is applied to determine whether edges are present in the filter window based on the binary edge map information within the filter window. Afterwards, the pixel values of the corresponding filter window are filtered pixel by pixel by using predetermined first weighted values to generate a new pixel value if it is determined that edges are not present. Also, the pixel values of the corresponding filter window are filtered pixel by pixel by using predetermined second weighted values to generate a new pixel value if it is determined that edges are present. However, the filtering is not performed if the pixel located at the center of the filter window represents an edge.
Preferably, the global threshold value (Tg) is determined by:
T
g
=
{
4
Q
+
60
,
6
≤
Q
≤
18
84
,
Q
≤
5
132
,
Q
≥
19
where Q is the quantization step of a quantizer.
Meanwhile, a signal adaptive filter of the present invention comprises an image storing unit for temporarily storing decompressed image data; a gradient operation unit for receiving the image data from the image storing unit in units of blocks having a predetermined size and calculating a gradient of the image data in the horizontal and vertical directions by using gradient operators to find edge pixels; a global edge map generator for comparing the gradient data of each pixel output by the gradient operation unit with a global threshold value (T
g
) determined based on a quantization step (Q) to generate binary global edge map information; a local edge map generator for comparing pixel by pixel the gradient data output from the gradient operation unit with a local threshold value which is individually determined for each predetermined size block, to generate binary local edge map information; an OR-gate for OR-operating, pixel by pixel, the global edge map information from the global edge map generator and the local edge map information from the local edge map generator to generate binary edge map information; a filter selector for storing the binary edge map information output by the OR-gate and classifying the input image data into an edge area including information of at least one edge and a homogeneous area having no edge information, according to the binary edge map information; an average filter for performing an average filtering on a central pixel within a filtering window of a filtering area when the filtering area is classified as a homogeneous area by the filter selector, to generate a new pixel value; and a weighted filter for performing a weighted filtering on the central pixel within a filtering window of a filtering area when the filtering area is classified as an edge area by the filter selector, to generate a new pixel value.
REFERENCES:
patent: 4847682 (1989-07-01), Tsinberg et al.
patent: 5148809 (1992-09-01), Biegeleisen-Knight et al.
patent: 5363210 (1994-11-01), Sasaki et al.
patent: 5430497 (1995-07-01), Strolle et al.
patent: 5818964 (1998-10-01), Itoh
patent: 5920356 (1999-07-01), Gupta et al.
patent: 6185320 (2001-02-01), Bick et al.
patent: 0 817 497 A2 (1998-01-01), None
ISO/IEC JTC1/SC29/WG11, MPEG96/M1824, Feb. 1997 Coding of Moving Pictures and Associated Audio Information.
Lee Yung-Lyul
Park Hyun-Wook
Brinich Stephen
Lee Thomas D.
Samsung Electronics Co,. Ltd.
Sughrue Mion Zinn Macpeak & Seas, PLLC
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