Block noise detector and block noise eliminator

Details Block noise detector and block noise eliminator Block noise detector and block noise eliminator

2000-01-21

2004-05-18

Mariam, Daniel (Department: 2624)

Image analysis

Image enhancement or restoration

Edge or contour enhancement

C382S232000, C382S233000, C358S426140

Reexamination Certificate

active

06738528

ABSTRACT:

TECHNICAL FIELD
The present invention relates to block noise detecting apparatuses and block noise eliminating apparatuses, and more particularly to a block noise detecting apparatus and a block noise eliminating apparatus of a type eliminating block noise that arises in digital images as a result of image encoding carried out by compressing the digital images for transferring and recording.
BACKGROUND ART
Data compression is conventionally done for digital images, for example, to store the digital images with a lower volume of data. Such data compression includes a lossless encoding method and a lossy encoding method. In the lossless encoding method, encoded data, after decoding, can be completely identical to data before encoding. On the other hand, in the lossy encoding method, encoded data, after decoding, cannot always be identical to data before encoding and may include some degree of error.
The lossless encoding method generally includes discrete cosine transform (hereinafter, referred to as DCT). After DCT is carried out, quantization often follows. In this manner, when data is encoded first through DCT and then quantization, for example, the data cannot be completely identical to data before encoding and includes noise (error). This means that an encoding operation first through DCT and then quantization is lossy encoding.
To carry out DCT, first of all, a one-frame image is regionally divided into a plurality of blocks. A block herein is a group of 8×8 two-dimensional pixel data, for example, and is regarded as a unit. Data encoded through DCT and quantization can be reconstituted by being subjected to inverse quantization and inverse DCT. Through the inverse quantization and inverse DCT, image data including block noise can be reconstituted.
Herein, block noise is described by referring to FIGS.
24
(
a
)-(
c
).
FIGS.
24
(
a
)-(
c
) are diagrams illustrating a conventional concept in eliminating the block noise. FIG.
24
(
a
) shows a one-frame image
701
, FIG.
24
(
b
) is an enlarged view of a partial boundary (hereinafter, referred to as block boundary)
706
between a block
704
and a block
705
adjacent thereto in FIG.
24
(
a
), and FIG.
24
(
c
) shows a state of pixels in FIG.
24
(
b
) after being smoothed.
It is now assumed, in FIG.
24
(
a
), that pixels are in line in specific blocks in the one-frame image
701
. In FIG.
24
(
b
), when a pixel a in the block
704
and pixel b in the block
705
are presumably bordering on the block boundary
706
, a difference in pixel level therebetween being larger than a difference in in-block pixel level between a pixel c and pixel d, for example, causes that part to be block noise. Accordingly, image quality in the part will be degraded to a greater degree.
As is known from this, block noise results from a level difference between pixels bordering on a block boundary in one-frame image.
The block noise is common in the lossy encoding method including DCT and quantization where processing is carried out on a block basis. To eliminate noise arisen in images, generally, the images are entirely subjected to smoothing. Smoothing is an operation of determining a pixel in average by using various pixels around a pixel to be processed. An operation of smoothing images with a low-pass filter (hereinafter, referred to as LPF) having a few taps is also referred to as smoothing. Such smoothing can eliminate not only block noise but noise, observed in images in their entirety as shown in FIG.
24
(
c
).
Although there is no doubt that smoothing done on pixels can advantageously eliminate block noise, edges of images other than the block noise are also smoothed. Therefore, the images will disadvantageously be blurred.
Further, in the above conventional technique, the block noise can only be eliminated in a case where a block size and a block boundary are perfectly identified.
Still further, in a case where edges of images are bordering on the block boundary, the block noise causes less influence at the edges of the images. In the conventional technique in the foregoing, however, every block boundary is subjected to smoothing. In this manner, the edges of images bordering on the block boundary will be blurred, and thus, smoothing done on the block boundary may degrade image quality to a greater degree.
Therefore, an object of the present invention is to provide a block noise detecting apparatus of a type correctly detecting a block boundary targeted to eliminate block noise even when it is not clearly identified.
Another object of the present invention is to provide block noise eliminating apparatus of a type eliminating block noise without blurring images, not smoothing images even on a block boundary, if block noise thereon is low, and best matching to visual scenes.
Still another object of the present invention is to provide a block noise eliminating apparatus of a type eliminating block noise observed in an input signal even if the input signal is an analog signal or external digital signal (DVD or STB, for example) in a multi-format (interlace system or progressive system, for example).
Further, a dot clock can be regenerated in a video processing system by using the block noise detecting apparatus of the present invention.
SUMMARY OF THE INVENTION
The present invention has the following features to attain the objects above.
A first aspect is directed to a block noise detecting apparatus of a type detecting, from a digital video signal subjected to lossy encoding on a predetermined image block basis, block noise caused by decoding the video signal, the apparatus comprising:
means for detecting a level of the block noise in the video signal; and
means for detecting a block boundary (where the block noise is generated) in the video signal.
As described above, in the first aspect, by correctly detecting a block boundary of an image to be regionally divided into a plurality of blocks and a block noise level thereon, block noise can be detected.
A second aspect is directed to a block noise detecting apparatus of a type detecting, from a digital video signal subjected to lossy encoding on a predetermined image block basis, block noise caused by decoding the video signal, the apparatus comprising:
signal extracting means for receiving the video signal and extracting only a high frequency component therefrom;
absolute value taking means for taking an absolute value of a high frequency component signal outputted from the signal extracting means;
accumulating/adding means for accumulating/adding the absolute-value-taken high frequency component signal outputted from the absolute value taking means in a predetermined period;
periodicity detecting means for detecting periodicity of the block noise in accordance with an accumulation/addition result outputted from the accumulating/adding means; and
block boundary determining means for determining a block boundary (where the block noise is generated) from a periodic signal detected by the periodicity detecting means.
As described above, in the second aspect, by detecting periodicity of block noise and by correctly detecting a block boundary of an image to be regionally divided into a plurality of blocks, the block noise can be detected.
According to a preferable and concrete third aspect, in the second aspect, the block boundary determining means distinguishes, in binary, between positional information on the block boundary and positional information a remainder of the blocks. In this manner, a block boundary can be easily provided.
According to a fourth aspect, in the second and third aspects, the block noise detecting apparatus further comprises:
frame difference taking means for receiving the video signal and determining a signal difference among a plurality of predetermined frames thereof;
region determining means for determining, by referring to the signal difference outputted from the frame difference taking means for whether or not the difference is more than a predetermined threshold value, a region where the block noise to be eliminated is observed (hereinafter, referre

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