Publication number | USRE41526 E1 |

Publication type | Grant |

Application number | US 11/212,137 |

Publication date | Aug 17, 2010 |

Filing date | Aug 26, 2005 |

Priority date | Nov 13, 1998 |

Also published as | US6611361, USRE41089, USRE41400, USRE41835, USRE41907 |

Publication number | 11212137, 212137, US RE41526 E1, US RE41526E1, US-E1-RE41526, USRE41526 E1, USRE41526E1 |

Inventors | Min-Cheol Hong |

Original Assignee | Lg Electronics, Inc. |

Export Citation | BiBTeX, EndNote, RefMan |

Patent Citations (31), Non-Patent Citations (2), Classifications (25) | |

External Links: USPTO, USPTO Assignment, Espacenet | |

US RE41526 E1

Abstract

The present invention relates to an image processing technique, and in particular to a method for restoring a compressed image by using a hybrid motion compensation discrete cosine transform (hybrid MC/DCT) mechanism, including: a step of defining a smoothing functional having a smoothing degree of an image and reliability for an original image by pixels having an identical property in image block units; and a step of computing a restored image by performing a gradient operation on the smoothing functional in regard to the original image, thereby preventing the blocking artifacts and the ringing effects in regard to the pixels having an identical property in image blocks.

Claims(33)

1. A method for restoring a compressed image of an image processing system, comprising:

a step for defining a smoothing functional having a smoothing degree of an image and reliability for an original image by pixels having an identical property in image block units; and

a step for computing a restored image by performing a gradient operation on the smoothing functional in regard to the original image;

wherein the smoothing functional M(f) comprises a sum of a smoothing functional M_{VB}(f) for pixels positioned at the boundary of a block in a vertical direction, a smoothing functional M_{VW}(f) for pixels positioned inside the block in a horizontal direction, a smoothing functional M_{HB}(f) for pixels positioned at the boundary of a block in a horizontal direction, a smoothing functional M_{HW}(f) for pixels positioned inside the block in a horizontal direction, a smoothing functional M_{T}(f) for pixels moved and compensated in the temporal section, “f” indicating the original image.

2. The method according to claim 1 , wherein the step for defining the smoothing functional divides the pixels according to their position, horizontal direction, vertical direction and smoothing variation in a temporal section.

3. The method according to claim 1 , wherein the smoothing functionals M_{VB}(f), M_{HB}(f), M_{VW}(f), M_{HW}(f), M_{T}(f) are defined as;
$\begin{array}{c}{M}_{\mathrm{VB}}\left(f\right)=\parallel {Q}_{\mathrm{VB}}f{\parallel}^{2}+{\alpha}_{\mathrm{VB}}\parallel g-f{\parallel}_{W1}^{2}\\ {M}_{\mathrm{HB}}\left(f\right)=\parallel {Q}_{\mathrm{HB}}f{\parallel}^{2}+{\alpha}_{\mathrm{HB}}\parallel g-f{\parallel}_{W2}^{2}\\ {M}_{\mathrm{VW}}\left(f\right)=\parallel {Q}_{\mathrm{VW}}f{\parallel}^{2}+{\alpha}_{\mathrm{VW}}\parallel g-f{\parallel}_{W3}^{2}\\ {M}_{\mathrm{HW}}\left(f\right)=\parallel {Q}_{\mathrm{HW}}f{\parallel}^{2}+{\alpha}_{\mathrm{HW}}\parallel g-f{\parallel}_{W4}^{2}\\ {M}_{T}\left(f\right)=\parallel {Q}_{T}f{\parallel}^{2}+{\alpha}_{T}\parallel g-f{\parallel}_{W5}^{2}\end{array}$

Q_{VB}, Q_{VW}, Q_{HB}, Q_{HW}, Q_{T }indicating high pass filters for smoothing the respective pixels, α_{VB}, α_{VW}, α_{HB}, α_{HW}, α_{T }being regularization parameters, g being a reconstructed image, and W**1**, W**2**, W**3**, W**4**, W**5** indicating diagonal matrixes for determining whether each group has an element.

4. The method according to claim 1 , wherein the step for computing the restored image comprises a step for approximating the regularization parameter by applying a set theoretic, and it is presumed that the quantization variables of the DCT region regular in each macro block, and also presumed that the DCT quantization errors have the Gaussian distribution property in the spatial section.

5. The method according to claim 4 , wherein the regularization parameters are approximated as;
$\begin{array}{c}{\alpha}_{\mathrm{VB}}=\frac{\parallel {Q}_{\mathrm{VB}}f{\parallel}^{2}}{\parallel g-f{\parallel}_{W1}^{2}}=\frac{\parallel {Q}_{\mathrm{VB}}g{\parallel}^{2}}{\parallel g-f{\parallel}_{W1}^{2}}=\frac{\parallel {Q}_{\mathrm{VB}}g{\parallel}^{2}}{\sum _{n}\text{\hspace{1em}}\sum _{m}{w}_{1}\left(m,n\right){\mathrm{Qp}}^{2}\left(m,n\right)}\\ {\alpha}_{\mathrm{HB}}=\frac{\parallel {Q}_{\mathrm{HB}}f{\parallel}^{2}}{\parallel g-f{\parallel}_{W2}^{2}}=\frac{\parallel {Q}_{\mathrm{HB}}g{\parallel}^{2}}{\parallel g-f{\parallel}_{W2}^{2}}=\frac{\parallel {Q}_{\mathrm{HB}}g{\parallel}^{2}}{\sum _{n}\text{\hspace{1em}}\sum _{m}{w}_{2}\left(m,n\right){\mathrm{Qp}}^{2}\left(m,n\right)}\\ {\alpha}_{\mathrm{VW}}=\frac{\parallel {Q}_{\mathrm{VW}}f{\parallel}^{2}}{\parallel g-f{\parallel}_{W3}^{2}}=\frac{\parallel {Q}_{\mathrm{VW}}g{\parallel}^{2}}{\parallel g-f{\parallel}_{W3}^{2}}=\frac{\parallel {Q}_{\mathrm{VW}}g{\parallel}^{2}}{\sum _{n}\text{\hspace{1em}}\sum _{m}{w}_{3}\left(m,n\right){\mathrm{Qp}}^{2}\left(m,n\right)}\\ {\alpha}_{\mathrm{HW}}=\frac{\parallel {Q}_{\mathrm{HW}}f{\parallel}^{2}}{\parallel g-f{\parallel}_{W4}^{2}}=\frac{\parallel {Q}_{\mathrm{HW}}g{\parallel}^{2}}{\parallel g-f{\parallel}_{W4}^{2}}=\frac{\parallel {Q}_{\mathrm{HW}}g{\parallel}^{2}}{\sum _{n}\text{\hspace{1em}}\sum _{m}{w}_{4}\left(m,n\right){\mathrm{Qp}}^{2}\left(m,n\right)}\\ {\alpha}_{T}=\frac{\parallel {Q}_{T}f{\parallel}^{2}}{\parallel g-f{\parallel}_{W5}^{2}}=\frac{\parallel {Q}_{T}g{\parallel}^{2}}{\parallel g-f{\parallel}_{W5}^{2}}=\frac{\parallel {Q}_{T}g{\parallel}^{2}}{\sum _{n}\text{\hspace{1em}}\sum _{m}{w}_{5}\left(m,n\right){\mathrm{Qp}}^{2}\left(m,n\right)}\end{array}$

Q^{2} _{p}(m,n) indicating a quantization variable of a macro block including an (m,n)th pixel of a two-dimensional image.

6. The method according to claim 1 , wherein a local minimizer of the smoothing functional is a global minimizer.

7. The method according to claim 1 , wherein the regularization parameter indicates a ratio of a smoothing degree of the image and reliability for the original image.

8. The method according to claim 1 , further comprising a step for computing an iterative solution in regard to a restored image, after computing the restored image.

9. The method according to claim 8 , wherein the iterative solution f_{k+1 }is represented by;

f_{k+1}=f_{k}+β[Ag−Bf_{k}],

A=α_{VB}W_{1}+α_{HB}W_{2}+α_{VW}W_{3}+α_{HW}W_{4}+α_{T}W_{5 }

B=(Q^{T} _{VB}Q_{VB}+Q^{T} _{HB}Q_{HB}+Q^{T} _{VW}Q_{VW}+Q^{T} _{HW}Q_{HW}+Q^{T} _{T}Q_{T})+A

f

A=α

B=(Q

and, β is a relaxation parameter having a convergence property, and computed at the range of
$0<\beta <\frac{2}{1={\mathrm{max}}_{i}{\lambda}_{i}\left(A\right)},$

an eigen value λ(A) of the matrix A being replaced by a fixed value.

10. The method according to claim 8 , wherein a predetermined threshold value is set in computing an iterative solution, an image obtained after iteration is compared with the previously-set threshold value, and it is determined whether the iteration technique is continuously performed according to a comparison result, or the iteration is finished after the iteration technique is performed as many as a previously-set number.

11. The method according to claim 8 , further comprising a step for obtaining a mapped image by projecting a two-dimensional DCT coefficient of the restored image corresponding to a computed iterative solution, and for performing an inverse DCT on the mapped image.

12. The method according to claim 11 , wherein the step for obtaining the mapped image is mapping a projected restored image P(F_{k+1}(u,v)) to G(u,v)−Qp when the DCT coefficient of the restored image F_{k+1}(u,v) is smaller than G(u,v)−Qp, mapping the projected restored image P(F_{k+1}(u, v)) to G(u,v)+Qp when F_{k+1}(u,v) is greater than G(u,v)+Qp, and otherwise mapping the projected restored image P(F_{k+1}(u,v)) as it is, G(u,v) indicating a two-dimensional DCT coefficient obtained by performing the DCT on the reconstructed image, and Qp indicating quantization information.

13. The method according to claim 1 , wherein a predetermined threshold value is set in computing an iterative solution, an image obtained after iteration is compared with the previously-set threshold value, and it is determined whether the iteration technique is continuously performed according to a comparison result, or the iteration is finished after the iteration technique is performed as many as a previously-set number.

14. The method according to claim 1 , further comprising a step for obtaining a mapped image by projecting a two-dimensional DCT coefficient of the restored image corresponding to a computed iterative solution, and for performing an inverse DCT on the mapped image.

15. The method according to claim 14 , wherein the step for obtaining the mapped image is mapping a projected restored image P(F_{k+1}(u,v)) to G(u,v)−Qp when the DCT coefficient of the restored image F_{k+1}(u,v) is smaller than G(u,v)−Qp, mapping the projected restored image P(F_{k+1}(u, v)) to G(u,v)+Qp when F_{k+1}(u,v) is greater than G(u,v)+Qp, and otherwise mapping the projected restored image P(F_{k+1}(u,v)) as it is, G(u,v) indicating a two-dimensional DCT coefficient obtained by performing the DCT on the reconstructed image, and Qp indicating quantization information.

16. An apparatus for restoring a compressed image of an image processing system, comprising:

a decoder for decoding a coded image signal, and for outputting information of the restored image, such as the decoded image, a quantization variable, a macro block type and a motion vector; and

a post processing unit for including the information of the restored image inputted from the image decoder, for defining a smoothing functional including a sum of a smoothing functional M_{VB}(f) for pixels positioned at the boundary of a block in a vertical direction, a smoothing functional M_{VW}(f) for pixels positioned inside the block in a horizontal direction, a smoothing functional M_{HB}(f) for pixels positioned at the boundary of a block in a horizontal direction, a smoothing functional M_{HW}(f) for pixels positioned inside the block in a horizontal direction, a smoothing functional M_{T}(f) for pixels moved and compensated in the temporal section, “f” indicating the original image, and for performing a gradient operation on the smoothing functional in regard to the original image,

the smoothing functional including a regularization parameter having weight of reliability for the original image.

17. A method for restoring a compressed image of an image processing system, comprising:

a step for defining a smoothing functional having a smoothing degree of an image and reliability for an original image by pixels having an identical property in image blocks units;

a step for computing a restored image by performing a gradient operation on the smoothing functional in regard to the original image; and

a step for computing an iterative solution in regard to the restored image, after computing the restored image.

18. The method according to claim 17 , wherein the step for defining the smoothing functional divided the pixels according to their position, horizontal direction, vertical direction and smoothing variation in a temporal section.

19. The method according to claim 17 , wherein the smoothing functional M(f) comprises a sum of a smoothing functional M_{VB}(f) for pixels positioned at the boundary of a block in a vertical direction, a smoothing functional M_{VW}(f) for pixels positioned inside the block in a horizontal direction, a smoothing functional M_{HB}(f) for pixels positioned at the boundary of a block in a horizontal direction, a smoothing functional M_{HW}(f) for pixels positioned inside the block in a horizontal direction, a smoothing functional M_{T}(f) for pixels moved and compensated in the temporal section, “f” indicating the original image.

20. The method according to claim 19 , wherein the smoothing functionals M_{VB}(f), M_{HB}(f), M_{VW}(f), M_{HW}(f), M_{T}(f) are defined as;
$\begin{array}{c}{M}_{\mathrm{VB}}\left(f\right)=\parallel {Q}_{\mathrm{VB}}f{\parallel}^{2}+{\alpha}_{\mathrm{VB}}\parallel g-f{\parallel}_{W1}^{2}\\ {M}_{\mathrm{HB}}\left(f\right)=\parallel {Q}_{\mathrm{HB}}f{\parallel}^{2}+{\alpha}_{\mathrm{HB}}\parallel g-f{\parallel}_{W2}^{2}\\ {M}_{\mathrm{VW}}\left(f\right)=\parallel {Q}_{\mathrm{VW}}f{\parallel}^{2}+{\alpha}_{\mathrm{VW}}\parallel g-f{\parallel}_{W3}^{2}\\ {M}_{\mathrm{HW}}\left(f\right)=\parallel {Q}_{\mathrm{HW}}f{\parallel}^{2}+{\alpha}_{\mathrm{HW}}\parallel g-f{\parallel}_{W4}^{2}\\ {M}_{T}\left(f\right)=\parallel {Q}_{T}f{\parallel}^{2}+{\alpha}_{T}\parallel g-f{\parallel}_{W5}^{2}\end{array}$

Q_{VB}, Q_{VW}, Q_{HB}, Q_{HW}, Q_{T }indicating high pass filters for smoothing the respective pixels, α_{VB}, α_{VW}, α_{HB}, α_{HW}, α_{T }being regularization parameters, g being a reconstructed image, and W**1**, W**2**, W**3**, W**4**, W**5** indicating diagonal matrixes for determining whether each group has an element.

21. The method according to claim 17 , wherein the step for computing the restored image comprises a step for approximately the regularization parameter by applying a set theoretic, and it is presumed that the quantization variables of the DCT region are regular in each macro block, and also presumed that the DCT quantization errors have the Gaussian distribution property in the spatial section.

22. The method according to claim 21 , wherein the regularization parameters are approximated as;
$\begin{array}{c}{\alpha}_{\mathrm{VB}}=\frac{\parallel {Q}_{\mathrm{VB}}f{\parallel}^{2}}{\parallel g-f{\parallel}_{W1}^{2}}=\frac{\parallel {Q}_{\mathrm{VB}}g{\parallel}^{2}}{\parallel g-f{\parallel}_{W1}^{2}}=\frac{\parallel {Q}_{\mathrm{VB}}g{\parallel}^{2}}{\sum _{n}\text{\hspace{1em}}\sum _{m}{W}_{1}\left(m,n\right){\mathrm{Qp}}^{2}\left(m,n\right)}\\ {\alpha}_{\mathrm{HB}}=\frac{\parallel {Q}_{\mathrm{HB}}f{\parallel}^{2}}{\parallel g-f{\parallel}_{W2}^{2}}=\frac{\parallel {Q}_{\mathrm{HB}}g{\parallel}^{2}}{\parallel g-f{\parallel}_{W2}^{2}}=\frac{\parallel {Q}_{\mathrm{HB}}g{\parallel}^{2}}{\sum _{n}\text{\hspace{1em}}\sum _{m}{W}_{2}\left(m,n\right){\mathrm{Qp}}^{2}\left(m,n\right)}\\ {\alpha}_{\mathrm{VW}}=\frac{\parallel {Q}_{\mathrm{VW}}f{\parallel}^{2}}{\parallel g-f{\parallel}_{W3}^{2}}=\frac{\parallel {Q}_{\mathrm{VW}}g{\parallel}^{2}}{\parallel g-f{\parallel}_{W3}^{2}}=\frac{\parallel {Q}_{\mathrm{VW}}g{\parallel}^{2}}{\sum _{n}\text{\hspace{1em}}\sum _{m}{W}_{3}\left(m,n\right){\mathrm{Qp}}^{2}\left(m,n\right)}\\ {\alpha}_{\mathrm{HW}}=\frac{\parallel {Q}_{\mathrm{HW}}f{\parallel}^{2}}{\parallel g-f{\parallel}_{W4}^{2}}=\frac{\parallel {Q}_{\mathrm{HW}}g{\parallel}^{2}}{\parallel g-f{\parallel}_{W4}^{2}}=\frac{\parallel {Q}_{\mathrm{HW}}g{\parallel}^{2}}{\sum _{n}\text{\hspace{1em}}\sum _{m}{W}_{4}\left(m,n\right){\mathrm{Qp}}^{2}\left(m,n\right)}\\ {\alpha}_{T}=\frac{\parallel {Q}_{T}f{\parallel}^{2}}{\parallel g-f{\parallel}_{W5}^{2}}=\frac{\parallel {Q}_{T}g{\parallel}^{2}}{\parallel g-f{\parallel}_{W5}^{2}}=\frac{\parallel {Q}_{T}g{\parallel}^{2}}{\sum _{n}\text{\hspace{1em}}\sum _{m}{W}_{5}\left(m,n\right){\mathrm{Qp}}^{2}\left(m,n\right)}\end{array}$

Q^{2} _{p}(m,n) indicating a quantization variable of a macro block including an (m,n)th pixel of a two-dimensional image.

23. The method according to claim 17 , wherein a local minimizer of the smoothing functional is a global minimizer.

24. The method according to claim 17 , wherein the regularization parameter indicates a ratio of a smoothing degree of the image and reliability for the original image.

25. The method according to claim 17 , wherein the iterative solution f_{k+1 }is represented by;

f_{k+1}=f_{k}+β[Ag−Bf_{k}],

A=α_{VB}W_{1}+α_{HB}W_{2}+α_{VW}W_{3}+α_{HW}W_{4}+α_{T}W_{5 }

B=(Q^{T} _{VB}Q_{VB}+Q^{T} _{HB}Q_{HB}+Q^{T} _{VW}Q_{VW}+Q^{T} _{HW}Q_{HW}+Q^{T} _{T}Q_{T})+A

f

A=α

B=(Q

and, β is a relaxation parameter having a convergence property, and computed at the range of
$0<\beta <\frac{2}{1={\mathrm{max}}_{i}{\lambda}_{i}\left(A\right)},$

an eigen value λ(A) of the matrix A being replaced by a fixed value.

26. An apparatus for restoring a compressed image of an image processing system, comprising:

a decoder for decoding a coded image signal, and for outputting information of the restored image, such as the decoding image, a quantization variable, a macro block type and a motion vector; and

a post processing unit for including the information of the restored image inputted from the image decoder, for defining a smoothing functional including a smoothing degree of the image and reliability of an original image block unit, and for performing a gradient operation on the smoothing functional in regard to the original image,

the smoothing functional including a regularization parameter having weight of reliability for the original image.

27. A method of decoding a current image, comprising:
*obtaining a pixel value for a subject pixel in a current block and a pixel value for at least one pixel adjacent to the subject pixel; * *obtaining a difference between the obtained pixel value for the subject pixel and the obtained pixel value for the adjacent pixel; * *determining a pixel value for a pixel in the current block based on a smoothing function corresponding to a set of pixels determined based on a position of the subject pixel, the smoothing function having a smoothing degree based on the obtained difference; wherein * *the smoothing function includes one of a first sub*-*smoothing function for pixels positioned at a boundary of the current block in a vertical direction; a second sub*-*smoothing function for pixels positioned at a boundary of the current block in a horizontal direction; a third sub*-*smoothing function for pixels positioned inside the current block in the vertical direction; a fourth sub*-*smoothing function for pixels positioned inside the current block in the horizontal direction; and a fifth sub*-*smoothing function for temporally separated pixels.*

28. The method of claim 27 , wherein the smoothing function includes at least one term representing reliability of the determined pixel value with respect to an associated original pixel value.

29. The method of claim 27 , wherein the determining step includes performing a gradient operation on the smoothing function.

30. The method of claim 27 , wherein the set of pixels includes at least one of the pixels positioned at the boundary of the current block in the vertical direction, the pixels positioned at the boundary of the current block in the horizontal direction, the pixels positioned inside the current block in the vertical direction, the pixels positioned inside the current block in the horizontal direction, and the temporally separated pixels.

31. The method of claim 27 , wherein at least one of the first, second, third, fourth and fifth sub-*smoothing functions involves pixel values associated with one of an original image and the current image.*

32. The method of claim 27 , wherein the subject pixel and the adjacent pixel are in different blocks.

33. The method of claim 27 , wherein the determined pixel value is a restored pixel value for the subject pixel.

Description

Notice: More than one reissue application has been filed for the reissue of U.S. Pat. No. **6**,**611**,**361**. The other reissue applications are application Nos. **11**/**808**,**423**, **11**/**892**,**177**, **11**/**892**,**178**, **11**/**892**,**179** and **11**/**892**,**176**.

1. Field of the Invention

The present invention relates to an image process technique, and in particular to a method for restoring a compressed image by using a hybrid motion compensation discrete cosine transform (hybrid MC/DCT) mechanism, and an apparatus therefor.

2. Description of the Background Art

In general, image compression techniques, such as MPEG1 and MPEG2 employ a hybrid motion compensation discrete cosine transform (hereinafter, referred to as “hybrid MC/DCT”) mechanism in order to improve compression efficiency. The hybrid MC/DCT mechanism is roughly divided into an encoding process and a decoding process. In the encoding process, an original image is divided into a plurality of blocks in order to compress information in a spatial section, a second-dimensional discrete cosine transform is performed on each block, and redundancy information in the image or between the images is reduced by using the correlation on a time axis among the images in order to decrease information in a temporal section. In the decoding process, the steps of the encoding process are performed in a reverse order. An encoder and a decoder are necessary to carry out the hybrid MC/DCT mechanism.

**9**, passed through a first switching unit **2**, and inputted to a DCT unit **3**. The DCT unit **3** performs a discrete cosine transform on the inputted image signal. A quantization unit **4** quantizes the image signal, and outputs a DCT coefficient (q). An inverse quantization unit **6** inversely quantizes the DCT coefficient (q), and an inverse DCT unit **7** carries out an inverse discrete cosine transform thereon, thereby restoring the original image signal. The restored image signal is added to an image signal restored in a previous stage by an adder **8**, and inputted to an image memory **9**. A controller **5** controls switching of the first and second switching units **2**, **10**, and transmits INTRA/INTER information (p=mtype; flag for INTRA/INTER), transmission information (t; flag for transmitted or not), and quantization information (qz=Op; quantizer indication) to a decoder (not shown in FIG. **1**). The image memory **9** outputs a motion vector information (v=MV; motion vector) to the decoder. The DCT unit **3** outputs the DCT coefficient (q) to the decoder.

However, information of the original image signal is lost during the process of coding the image signal described above, especially during the quantization process, thereby causing blocking artifacts and ringing effects to the image which is reconstructed in the decoder. The blocking artifacts imply irregularity between the blocks generated due to information loss resulting from the quantization of the low-frequency DCT coefficients, and the ringing effects result from quantization errors of the high-frequency DCT coefficients.

That is, in accordance with a coding technique using the DCT in a coding system of a static image or dynamic image, an image is divided into a plurality of blocks, and the DCT is performed on each block. On the other hand, when the DCT is carried out on the original image, its important information is mainly included in low-frequency elements, and becomes lesser in high-frequency elements. Furthermore, the low-frequency elements include a lot of information relating to adjacent blocks. The DCT does not consider the correlation between the blocks, and quantizes the low-frequency elements by blocks, thereby destroying continuity of the adjacent blocks. It is called the blocking artifacts.

In addition, when the coefficients obtained by performing the DCT are quantized, as a quantization interval is increased, the elements to be coded are decreased, and thus the number of the bits to be processed is reduced. As a result, the information of the high-frequency element included in the original image is reduced, thereby generating distortion of the reconstructed image. It is called the ringing effects. The ringing effects generated by increasing the quantization interval are serious especially in a contour of an object in the reconstructed image.

As techniques for removing the blocking artifacts and the ringing effects, employed are a low pass filtering method and a regularized image restoration method.

According to the low pass filtering method, a plurality of pixels around a predetermined pixel are selected, and an average value thereof is computed. Here, a filter tap or filter coefficients are set by experience. For example, referring to

The regularized image restoration method adaptively deals with the blocking artifacts in accordance with statistical properties of the image. That is, irregular information around the boundary of the block or in the block is all computed. However, the computed values form a matrix shape, and thus a real time processing is difficult due to the great computation amount. In addition, an average value obtained by a computation result of the irregular information is equally applied to the pixels, regardless of a degree of irregularity. Accordingly, when a block has a high degree of irregularity, it can be reduced. However, in case of a block having a low degree of irregularity, it may be increased. Thus, the system is not adaptive. Also, the information in the temporal section is not processed, and thus irregularity between the images cannot be adaptively processed.

It is therefore an object of the present invention to provide a method for restoring a compressed image of an image processing system and an apparatus therefor which can reduce the blocking artifacts and ringing effects generated in a restored image signal.

It is another object of the present invention to provide a method for restoring a compressed image of an image processing system and an apparatus therefor which consider a smoothing degree of an image and reliability for an original image by pixels having an identical property in image block units, during a decoding process.

In order to achieve the above-described objects of the present invention, there is provided a method for restoring a compressed image of an image processing system including: a step of defining a smoothing functional having a degree of smoothing an image and reliability for an original image by pixels having an identical property in image block units; and a step of computing a restored image by performing a gradient operation on the smoothing functional in regard to the original image.

These and other objects of the present application will become more readily apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.

The present invention will become better understood with reference to the accompanying drawings which are given only by way of illustration and thus are not limitative of the present invention, wherein:

**210** receives INTRA/INTER information (p=mtype), transmission information (t), quantization information (qz=Qp), a discrete cosine transform (DCT) coefficient (q) and motion vector information (v=Mv; motion vector) from an encoder (as depicted in FIG. **1**), and performs decoding. The encoder and the decoder **210** are connected by a communication channel or network. A post processing unit **220** receives image signals Y, U, V, a quantization variable (qz=Qp), a macro block type (mtype) and a motion vector (v=MV) from the decoder **210**, and carries out an operation of restoring the compressed image in accordance with the present invention.

According to the present invention, a smoothing functional is defined in regard to pixels having an identical property by blocks, a regularization parameter is computed based on the smoothing functional, and available values are applied to the regularization parameter, thereby obtaining an image to be restored. Thereafter, an iterative technique, a discrete cosine transform (DCT), a projection and an inverse DCT are sequentially performed on the obtained image, thereby restoring a similar image to the original image. The whole processes will now be described in detail.

Definition of Smoothing Functional

When an original image (f) is compressed and transmitted, an image (g) reconstructed in the decoder **210** is represented by the following equation:

g=f+n (1)

Here, “g” and “f” indicate row vectors re-arranged in a stack-order, namely a scanning order, and “n” indicates a quantization error. When it is presumed that a size of the image is M×M, the original image (f), the reconstructed image (g) and (n) are column vectors having a size of M×1.

An original pixel for the original image (f) is represented by f(i,j). Here, “i” and “j” indicate a position of the pixel in the image.

The 8×8 pixels in the block are classified into the pixels having an identical property. That is, the pixels are divided in accordance with their position, vertical direction, horizontal direction and smoothing variation in the temporal section. Accordingly, it is defined that a set of the pixels positioned at a boundary of the block in a vertical direction is C_{VB}, a set of the pixels positioned inside the block in the vertical direction is C_{VW}, a set of the pixels positioned at a boundary of a block in a horizontal direction is C_{HB}, a set of the pixels positioned inside the block in the horizontal direction is C_{HW}, and a set of the pixels moved and compensated in the temporal section is C_{T}. The sets C_{VB}, C_{VW}, C_{HB}, C_{HW}, C_{T }are represented by the following expressions.

C_{VB}={f(i,j): i mod **8**=0,1, and j=0,1, . . . , M−1}

C_{VW}={f(i,f): i mod **8**≠0,1, and j=0,1, . . . , M−1} (2)

C_{HB}={f(i,j): j mod **8**=0,1, and i=0,1, . . . , M−1}

C_{HW}={f(i,j): j mod **8**≠0,1, and i=0,1, . . . , M−1}

C_{T}={f(i,j): f(i,j)εMB_{inter }or f(i,j)εMB_{not coded}}

Here, the set C_{T }is a set of the pixels having a macro block type of “inter” or “not coded” in order to remove temporal redundancy information.

The smoothing functional M(f) for using the regularization restoration method from the above-defined sets C_{VB}, C_{VW}, C_{HB}, C_{HW}, C_{T }is defined as follows.

M(f)=M_{VB}(f)+M_{HB}(f)+M_{VW}(f)+M_{HW}(f)+M_{T}(f) (3)

Here, M_{VB}(f) is a smoothing functional for the set C_{VB}, M_{HB}(f) is a smoothing functional for C_{HB}, M_{VW}(f) is a smoothing functional for the set C_{VW}, M_{HW}(f) is a smoothing functional for the set C_{HW}, and M_{T}(f) is a smoothing functional for the set C_{T}. The smoothing fuctionals are respectively defined as follows.

Here, first terms in each expression indicate a smoothing degree for the original pixel (reference pixel) and adjacent pixel, and second terms indicate reliability for the original pixel and the restored pixel. “∥.∥” indicates the Euclidean norm. Q_{VB}, Q_{VW}, Q_{HB}, Q_{HW}, Q_{T }indicate high pass filters for smoothing the pixels in the sets C_{VB}, C_{VW}, C_{HB}, C_{HW}, C_{T}.

The first term at the right side is represented by the following expression.

The smoothing functionals represented by Expression (4) are quadratic equations, respectively. Thus, local minimizers of each smoothing functional become global minimizers.

α_{VB}, α_{HB}, α_{VW}, α_{HW}, α_{T }included in the second terms of Expression (4) are regularization parameters in regard to each set, indicate a ratio of the smoothing degree and reliability, and imply an error element. W**1**, W**2**, W**3**, W**4**, W**5** indicate diagonal matrixes having a size of M×M in order to determine whether each set has an element, and have a value of “1”, or “0” according to whether each pixel is included in a corresponding set. That is, if the respective pixels are included in the corresponding sets, the value of the diagonal elements is “0”. If not, the value of the diagonal elements is

Thereafter, the regularization parameters, α_{VB}, α_{HB}, α_{VW}, α_{HW}, α_{T }are approximated as follows.

Approximation of Regularization Parameters

Approximation of the regularization parameters is a major element determining performance of the smoothing functional. In order to reduce the computation amount, presumptions are made as follows.

- (1) A maximum value of the quantization error generated in the quantization process of the DCT region is Qp, and thus it is presumed that the quantization variables Qp are regular in each macro block. For this, the maximum quantization error of the DCT coefficients of each macro block is regularly set to be Qp.
- (2) It is also presumed that the DCT quantization errors have the Gaussian distribution property in the spatial section.

Under the above presumptions, in case a set theoretic is applied, each regularization parameter is approximated as follows.

Here, Q^{2} _{p}(m,n) is a quantization variable of a macro block including a (m,n)th pixel of a two-dimensional image.

In Expression (6), denominator terms of the respective regularization parameters are a sum of the energy for the quantization noise of the elements included in each group. As described above, the values of the regularization parameters may be easily computed by applying the set theoretic under the two presumptions.

Computing Pixels to be Restored From Smoothing Functional

Only the original image needs to be computed. However, the smoothing functional includes a square term of the original image. Accordingly, in order to compute the original image, a gradient operation is carried out on the smoothing functional in regard to the original image. A result value thereof is “0”, and represented by the following expression.

∇_{f}M(f)=2Q^{T} _{VB}Q_{VB}+2Q^{T} _{HB}Q_{HB}+

2Q^{T} _{VW }

Q_{VW}+2Q^{T} _{HW}Q_{HW}+2Q^{T }

TQ_{T}−2α_{VB}W^{T} _{1}W_{1}(

g−f)−2α_{HB}W^{T} _{2}W_{2}(

g−f)−2α_{VW}W^{T} _{3}W_{3}(

g−f)−2α_{HW}W^{T} _{4}W_{4}(

g−f)−2α_{T}(g−f)=0 (7)

Here, a superscript “T” indicates a transposition of the matrix.

A restored image similar to the original image (f) can be obtained by Expression (7). However, operation of an inverse matrix must be performed, and thus the computation amount is increased. Thus, in accordance with the present invention, the restored image is computed by an iterative technique which will now be explained.

Iterative Technique

When Expression (7) is iterated k times, an iterative solution f_{k+1 }is represented by the following expression.

f_{k+1}=f_{k}+β[Ag−Bf_{k}],

A=α_{VB}W_{1}+α_{HB}W_{2}+α_{VW}W_{3}+α_{HW}W_{4}+α_{T}W_{5 } (8)

B=(Q^{T} _{VB}Q_{VB}+Q^{T} _{HB}Q_{HB}+Q^{T} _{VW}Q_{VW}+Q^{T} _{HW}Q_{HW}+Q^{T} _{T}Q_{T})+A

In Expression (8), “β” is a relaxation parameter having a convergence property. Expression (8) can be represented by the following expression by computing consecutive iterative solutions.

(f_{k+1}−f_{K})=(I−B)(f_{k}−f_{k−1}) (9)

Here, “I” is an identity matrix, and the matrix B has a positive definite property. Therefore, when the following condition is satisfied, the iterative solutions are converged.

∥I−B∥<1 (10)

Expression (10) can be summarized as follows.

In Expression (11), “λ(A)” depicts an eigen value of the matrix A. A considerable amount of computation is required to compute the eigen value λ(A). However, the high pass filters have a certain shape determined according to the positions of the respective pixels, regardless of the image. Accordingly, before computing Expression (8), the eigen value λ(A) can be replaced by a fixed value. The value may be computed by a power method which has been generally used in interpretation of numerical values.

For example, a computation process of an eigen value of an iterative solution will now be explained.

x_{k+1}=Kx_{k }

Here, “x_{k}” is a vector of M×1, and “K” is a positive-definite symmetric M×M matrix. The eigen value λ′ of the matrix K is approximated as follows.

In the above expression, if “k” is to infinity, the eigen value λ′ is approximated to a real value.

Thus, the iterative solution represented by Expression (8) is computed. The next thing to be considered is a time of finishing the iterative technique, in order to determine the number of iteration. Here, two standards are set as follows.

Firstly, a predetermined threshold value is set before starting iteration, an image obtained after iteration, namely a partially-restored image is compared with the previously-set threshold value, and it is determined whether the iteration technique is continuously performed according to a comparison result.

Secondly, the iteration technique is performed as many as a predetermined number, and then finished.

According to the first standard, a predetermined threshold value is set in performing iteration, and thus a wanted value is obtained. However, although the iteration number is increased, it may happen that the predetermined threshold value is not reached. On the other hand, the second standard is performed by experience, but can reduce a computation amount. Therefore, the two standards may be selectively used according to the design specification.

**1**, the quantization variable Qp and the image signals Y, U, V are inputted, and the regularization parameter is approximated as described above. In the step S**2**, the gradient operation is performed on the smoothing functional in regard to the original image. In the step S**3**, an iterative solution, namely a wanted restored image is obtained by the iteration technique. In this step, employed are the image signals Y, U, V and the motion vector MV which is moved and compensated.

In the step S**4**, the DCT is performed on the restored image corresponding to the iterative solution f_{k+1 }obtained in the step S**3**. An (u,v)th DCT coefficient of the two-dimensional restored image is expressed as F_{k+1}(u,v), and must exist in the following section in accordance with a property of the quantization process.

G(u,v)−Qp≦F_{k+1}(u,v)≦G(u,v)+Qp (12)

Here, “Qp” is a maximum quantization error as explained above, and “G(u,v)” is a two-dimensional DCT coefficient obtained by performing the DCT on the reconstructed image (g). The DCT coefficients F_{k+1}(u,v) and G(u,v) are represented as follows. In Expression (13), “B” indicates a block DCT.

F_{k+1}(u,v)=(Bf_{k+1})(u,v), and G(u,v)=(Bg)(u,v) (13)

In the step S**6**, a section of the DCT coefficient of the restored image is set as in Expression (12). Accordingly, in case the DCT coefficient F_{k+1}(u,v) of the restored image is not in the predetermined section, it must be projected as follows. A projection process is carried out in the step S**7**, and represented by Expression (14).

P(F_{k+1}(u,v))=G(u,v)−Qp, if F_{k+1}(u,v)<G(u,v)−Qp

P(F_{k+1}(u,v))=G(u,v)+Qp, if F_{k+1}(u,v)>G(u,v)−Qp 14

P(F_{k+1}(u,v))=F_{k+1}(u,v), otherwise.

Expression (14) will now be described.

When F_{k+1}(u,v) is smaller than G(u,v)−Qp, the projected restored image P(F_{k+1}(u,v)) is mapped to G(u,v)−Qp. In case F_{k+1}(u,v) is greater than G(u,v)+Qp, the projected restored image P(F_{k+1}(u,v)) is mapped to G(u,v)+Qp. Otherwise, the projected restored image P(F_{k+1}(u,v)) is mapped as it is.

In the step S**8**, the inverse DCT is performed on the mapped image P(F_{k+1}(u,v)) in the spatial section. The finally restored image is represented by Expression (14).

f_{k+1}=B^{T}PBf_{k+1 } (15)

Here, “B” indicates the DCT, “P” indicates mapping, and “B^{T}” indicates the inverse DCT.

The restored image is stored in a frame memory in the post processing unit **220** (Step S**9**). The post processing unit **220** performs motion compensation based on the motion vector MV (Step S**10**). The motion and compensation image is employed for generation of the regularization parameter for a succeeding image and the iteration technique.

The post processing unit **220** outputs the restored motion and compensation image as a video signal to a display (not shown) (Step S**11**).

As discussed earlier, the present invention can restrict a section of the restored image for the respective pixels by using the various regularization parameters. In addition, the present invention prevents flickering which may occur in the dynamic image compression technique.

Consequently, the present invention adaptively prevents the blocking artifacts and the ringing effects for the pixels having an identical property in image block units, and thus can be widely used for the products of the hybrid MC-DCT mechanism.

As the present invention may be embodied in several forms without departing from the spirit or essential characteristics thereof, it should also be understood that the above-described embodiment is not limited by any of the details of the foregoing description, unless otherwise specified, but rather should be construed broadly within its spirit and scope as defined in the appended claims, and therefore all changes and modifications that fall within the meets and bounds of the claims, or equivalences of such meets and bounds are therefore intended to be embraced by the appended claims.

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Classifications

U.S. Classification | 358/474, 375/E07.19, 375/240.16, 382/268, 358/426.03, 375/E07.241, 375/240.22, 382/232, 382/254, 358/426.07 |

International Classification | H04N19/80, H04N19/51, H04N19/44, H04N19/86, H04N19/423, H04N19/625, H03M7/30, G06T9/00, H04N7/24, H04N19/00, H04N1/04 |

Cooperative Classification | H04N19/192, H04N19/86 |

European Classification | H04N7/26A10T, H04N7/26P4 |

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