Image analysis – Image enhancement or restoration – Edge or contour enhancement
Reexamination Certificate
2001-04-27
2004-02-10
Lee, Thomas D. (Department: 2624)
Image analysis
Image enhancement or restoration
Edge or contour enhancement
C382S199000
Reexamination Certificate
active
06690838
ABSTRACT:
TECHNICAL FIELD
The invention relates generally to electronic and computer circuits, and more particularly to an image processing circuit and a method for reducing the difference between the respective values of a first pixel on one side of an image boundary and a second pixel on the other side of the boundary. For example, such a circuit and method can be used to reduce blockiness in an image that has undergone block-based digital compression.
BACKGROUND OF THE INVENTION
To electronically transmit a relatively high-resolution image over a relatively low-band-width channel, or to electronically store such an image in a relatively small memory space, it is often necessary to compress the digital data that represents the image. For example, High-Definition-Television (HDTV) video images are compressed to allow their transmission over existing television channels. Without compression, HDTV video images would require transmission channels having bandwidths much greater than the bandwidths of existing television channels. Furthermore, to reduce data traffic and transmission time to acceptable levels, an image may be compressed before being sent over the internet. Or, to increase the image-storage capacity of a CD-ROM or server, an image may be compressed before being stored thereon.
Such image compression typically involves reducing the number of data bits necessary to represent an image. Unfortunately, many compression techniques are lossy. That is, visual information contained in the original image may be lost during compression. This loss of information may cause noticeable differences, often called visual artifacts, in the reconstructed image. In many cases, these artifacts are undesirable, and thus significantly reduce the visual quality of the reconstructed image as compared to the quality of the original image.
Referring to
FIGS. 1-3
, the basics of the popular block-based Moving Pictures Experts Group (MPEG) compression standards, which include MPEG-1 and MPEG-2, are discussed. For purposes of illustration, the discussion is based on using an MPEG 4:2:0 format to compress images represented in a Y, C
B
, C
R
color space, although the basic concepts discussed also apply to other MPEG formats and images represented in other color spaces, and to other block-based compression standards such as the Joint Photographic Experts Group (JPEG) standard, which is often used to compress still images. Furthermore, although many details of the MPEG standards and the Y, C
B
, C
R
color space are omitted for brevity, these details are well-known and are disclosed in a large number of available references.
Referring to
FIGS. 1-3
, the MPEG standards are often used to compress temporal sequences of images—which are also called video frames—such as found in a television broadcast. Each video frame is divided into areas called macro blocks, which each include one or more pixels.
FIG. 1A
is a 16-pixel-by-16-pixel macro block
10
having 256 pixels
12
. In the MPEG standards, a macro block is always 16×16 pixels, although other compression standards may use macro blocks having other dimensions. In the original video frame, i.e., the frame before compression, each pixel
12
has a respective luminance value Y and a respective pair of color-, i.e., chroma-, difference values C
B
and C
R
.
Referring to
FIGS. 1A-1D
, before compression of the frame, the digital luminance (Y) and chroma-difference (C
B
and C
R
) values that will be used for compression, i.e., the pre-compression values, are generated from the original Y, C
B
, and C
R
values of the original frame. In the MPEG 4:2:0 format, the pre-compression Y values are the same as the original Y values. Thus, each pixel
12
merely retains its original luminance value Y. But to reduce the amount of data to be compressed, the MPEG 4:2:0 format allows only one pre-compression C
B
value and one pre-compression C
R
value for each group
14
of four pixels
12
. Each of these pre-compression C
B
and C
R
values are respectively derived from the original C
B
and C
R
values of the four pixels
12
in the respective group
14
. Thus, referring to
FIGS. 1B-1D
, the pre-compression Y, C
B
, and C
R
values generated for the macro block
10
are arranged as one 16×16 matrix
16
of pre-compression Y values (equal to the original Y value for each pixel
12
), one 8×8 matrix
18
of pre-compression C
B
values (equal to one derived C
B
value for each group
14
of four pixels
12
), and one 8×8 matrix
20
of pre-compression C
R
values (equal to one derived C
R
value for each group
14
of four pixels
12
). It is, however, common in the industry to call the matrices
16
,
18
, and
20
“blocks” of values. Furthermore, because it is convenient to perform the compression transforms on 8×8 blocks of pixel values instead of 16×16 blocks, the block
16
of pre-compression Y values is subdivided into four 8×8 blocks
22
a
-
22
d,
which respectively correspond to the 8×8 blocks A-D of pixels in the macro block
10
. Thus, still referring to
FIGS. 1B-1D
, six 8×8 blocks of pre-compression pixel data are generated for each macro block
10
: four 8×8 blocks
22
a
-
22
d
of pre-compression Y values, one 8×8 block
18
of pre-compression C
B
values, and one 8×8 block
20
of pre-compression C
R
values.
FIG. 2
is a general block diagram of an MPEG compressor
30
, which is more commonly called an encoder
30
. Generally, the encoder
30
converts the pre-compression data for a frame or sequence of frames into encoded data that represent the same frame or frames with significantly fewer data bits than the pre-compression data. To perform this conversion, the encoder
30
reduces or eliminates redundancies in the pre-compression data and reformats the remaining data using efficient transform and coding techniques.
More specifically, the encoder
30
includes a frame-reorder buffer
32
, which receives the pre-compression data for a sequence of one or more frames and reorders the frames in an appropriate sequence for encoding. Thus, the reordered sequence is often different than the sequence in which the frames are generated. The encoder
30
assigns each of the stored frames to a respective group, called a Group Of Pictures (GOP), and labels each frame as either an intra (I) frame or a non-intra (non-I) frame. The encoder
30
always encodes an I-frame without reference to another frame, but can and often does encode a non-I frame with reference to one or more of the other frames in the GOP. The encoder
30
does not, however, encode a non-I frame with reference to a frame in a different GOP.
During the encoding of an I frame, the 8×8 blocks (
FIGS. 1B-1D
) of the pre-compression Y, C
B
, and C
R
values that represent the I frame pass through a summer
34
to a Discrete Cosine Transform (DCT) circuit
36
, which transforms these blocks of values into respective 8×8 blocks of one DC coefficient and sixty-three AC coefficients. That is, the summer
34
is not needed when the encoder
30
encodes an I frame, and thus the pre-compression values pass through the summer
34
without being summed with any other values. As discussed below, however, the summer
34
is often needed when the encoder
30
encodes a non-I frame. A quantizer
38
limits each of the coefficients to a respective maximum value, and provides the quantized AC (nonzero frequency) and DC (zero frequency) coefficients on respective paths
40
and
42
. A predictive encoder
44
predictively encodes the DC coefficients, and a variable-length coder
46
converts the quantized AC coefficients and the quantized and predictively encoded DC coefficients into variable-length codes, such as Huffman codes. These codes form the encoded data that represent the pixel values of the encoded I frame. A transmit buffer
48
then temporarily stores these codes to allow synchronized transmission of the encoded data to a decoder (discussed below in conjunction with FIG.
3
). Alternatively, if the encoded data is to be s
Brinich Stephen
Equator Technologies Inc.
Graybeal Jackson Haley LLP
Lee Thomas D.
LandOfFree
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