Method of removing blocking artifacts in a coding system of a moving picture

Abstract

A method of coding a moving picture reduces blocking artifacts. The method includes defining pixel sets S0, S1, S2 around a block boundary, selectively determining a deblocking mode as a default mode or a dc offset mode depending on the degree of blocking artifacts. If the default mode is selected, frequency information is obtained around the block boundary per pixel using a 4-point DCT kernel, for example, a magnitude of a discontinuous component belonging to the block boundary is replaced with a minimum magnitude of discontinuous components belonging to the surroundings of the block boundary in the frequency domain and the replacing step is applied to the spatial domain. If the dc offset mode is selected and a determination is made to perform dc offset mode, the blocking artifacts in a smooth region are removed in the dc offset mode.

Description

(|v₈−v₉|<QP)v₉:v₈, if m>8;

• • where φ(γ)=1 if |γ|≦THR1(first threshold value) and φ(γ)=0 otherwise.

If the mode decision value eq_cnt≧THR2(i.e., a second threshold value), the DC offset mode is applied. In the remaining cases, default mode is applied.

A method for removing the blocking artifacts to code a moving picture at low-rate-bit according to the preferred embodiment of the present invention will be described with reference to FIG. 4. After beginning in FIG. 4, control continues to step 401S. In step 401S, three pixel sets S0, S1, S2 are defined based on the horizontal block boundary. From step 401S, control continues to step 402S.

In step 402S, the mode decision value (e.g., eq_cnt) is determined and control continues to step 403S. In step 403S, the mode decision value is compared with a decision value (e.g., a second threshold value THR2 preferably set by a user) to perform deblocking filtering process by selecting the mode depending on the degree of the blocking artifacts in the picture.

If the determination in step 403S is negative, control continues to step 404S where the default mode is set. From step 404S, control continues to step 405S where frequency information around the block boundary on each of the pixel is determined, for example, using the 4-point DCT kernel. From step 405S, control continues to step 406S.

In step 406S, the magnitude of the discontinuous component belonging to the block boundary is replaced with the minimum magnitude of the discontinuous components belonging to the surroundings of the block boundary in the frequency domain. This adjusting operation is applied to the spatial domain. That is, the magnitude of the discontinuous component belonging to the block boundary is replaced with the minimum magnitude of the discontinuous components belonging to the surroundings of the block boundary in the spatial domain.

In the default mode of the preferred embodiment, the blocking artifacts are removed in step 406S using the method as described below:

• • v₄′=v₄−d;
  • v₅′=v₅+d; and
  • d=CLIP(c₂.(a_3,0′−a_3,0)//c₃,0,(v₄−v₅)/2)*δ(|a₃|<QP),
    where a_3,0′=SIGN(a_3,0)*MIN(|a_3,0|,|a_3,1|,|a_3,2|),
  • a_3,0=([c₁−c₂ c₂−c₁]*[v₃v₄v₅v₆]^T)//c₃,
  • a_3,1=([c₁−c₂ c₂−c₁]*[v₁v₂v₃v₄]^T)//c₃,
  • a_3,2=([c₁−c₂ c₂−c₁]*[v₅v₆v₇v₈]^T)//c₃. In the default mode, the blocking artifacts are effectively removed in a complex region. However, the default mode does not sufficiently remove blocking artifacts in a smooth region.

If the determination in step 403S is affirmative, control continues to step 407S where the DC offset mode is set to remove the blocking artifacts. From step 407S, control continues to step 408S where the minimum and maximum data values (min, max) are determined. From step 408S, control continues to step 409S where a determination is made to remove the blocking artifacts in the default mode. If the determination in step 409S is negative, the process ends. If the determination in step 409S is affirmative, control continues to step 410S.

In the DC offset mode according to the preferred embodiment, in step 410S, the blocking artifacts are removed using the following algorithm.

• • max=MAX(v₁, v₂, v₃, v₄, v₅, v₆, v₇, v₈),
  • min=MIN(v₁, v₂, v₃, v₄, v₅, v₆, v₇, v₈),
  • if(|max−min|<2.QP), /*low pass filtering*/

$v_n=\sum _{k=-4}^4{b}_k·p_{n+k},1\le n\le 8$ $P_m=\left(v_1-v_0<\mathrm{QP}\right)?v_0\text{:}{v}_1,$

• • if m<1;
  • v_m, if 1≦m≦8;
  • (|v₈−v₉|<QP)v′₉:v₈, if m>8; (|v₈−v₉|<QP)?v₉: v₈, if m>8;
  • {b_k: −4≦k≦4}={1,1,2,2,4,2,2,1,1}//16.

The maximum data value and the minimum data value in the block boundary pixels are obtained in step 408S. Then, if the absolute value of the maximum data value minus the minimum data value is smaller than 2QP (i.e., if deblocking is required), the blocking artifacts in the smooth region are removed by the DC offset mode in steps 409S and 410S.

From step 406S and 410S, control continues to step 411S. If the deblocking filtering process around the horizontal block boundary is completed, the deblocking filtering process around the vertical block boundary is performed in step 411S. From step 411S, control continues to step 412S.

In step 412S, the deblocking filtering processes around the horizontal and vertical block boundaries repeat over the whole frame. From step 412S, the process ends.

FIG. 5 is a table illustrating exemplary PSNR properties according to the method of removing the blocking artifacts of the preferred embodiment. The conditions yielding the exemplary results of FIG. 5 are as follows:

• • 300 frames (only the initial frame was coded in intra.);
  • Fixed QP;
  • H.263 quantization;
  • F_code=1;
  • Enable DC/AC prediction; and
  • Rectangular shape VOP.
    As shown in FIG. 5, the method for removing the blocking artifacts of the preferred embodiment improves results relative to VM (no filtering) of MPEG-4.

As described above, the method for removing the blocking artifacts according to the preferred embodiments of the present invention has various advantages. The deblocking filtering process is performed using features of the frequency domain so that the blocking artifacts are effectively removed. Further, the blocking artifacts are removed in both the complex and smooth regions. Thus, an excellent image or picture quality is provided. In addition, amount of bits does not increase.

The foregoing embodiments are merely exemplary and are not to be construed as limiting the present invention. The present teaching can be readily applied to other types of apparatuses. The description of the present invention is intended to be illustrative, and not to limit the scope of the claims. Many alternatives, modifications, and variations will be apparent to those skilled in the art.

Claims

1. A method for removing blocking artifacts in a coding system of a moving picture comprising the steps of:

determining a plurality of pixel sets around a block boundary;

selecting one of a first mode and a second mode as a deblocking mode based on a degree of blocking artifacts;

performing an analysis, if the first mode is selected, comprising,

obtaining frequency information for each of the plurality of pixel sets,

replacing a magnitude of at least one discontinuous component in the frequency domain of a selected pixel set of the plurality of pixel sets belonging to the block boundary with a magnitude of at least one corresponding discontinuous component belonging to a replacement pixel set of the plurality of pixel sets near the block boundary, and

applying the replaced frequency information of the selected pixel set to the spatial domain to remove the blocking artifacts; and

removing the blocking artifacts in the second mode, if the second mode is selected and a second mode condition is satisfied.

2. The method as claimed in claim 1, wherein the magnitude of the discontinuous component of the selected pixel set is replaced with a minimum value of a magnitude of discontinuous components of one of a first pixel set and a second pixel set when the selected pixel set is located across the block boundary and the first and second pixel sets are located within a block adjacent the block boundary.

3. The method of claim 1, further comprising determining a mode decision value, wherein the second mode is selected if the mode decision value is greater than a first threshold value.

4. The method as claimed in claim 3, wherein mode decision value is determined based on the following equation:

mode decision value=φ(v₀−v₁)+φ(v₁−v₂)+φ(v₂−v₃)+φ(v₃−v₄)+φ(v₄−v₅)+φ(v₅−v₁)+φ(v₇−v₈)+φ(v₈−v₉),

wherein φ(γ)=1 when |γ|≦ a second threshold value and φ(γ)=0 otherwise, and wherein v₀-v₉ are boundary pixel values.

5. The method as claimed in claim 1, wherein the second mode condition is satisfied when an absolute value of a maximum data value minus a minimum data value in block boundary pixels is smaller than 2QP, wherein the maximum data value=MAX(v₁, v₂, v₃, v₄, v₅, v₆, v₇, v₈), the minimum data value=MIN(v₁, v₂, v₃, v₄, v₅, v₆, v₇, v₈), QP is the quantization parameter of a block adjacent the block boundary and v₁-v₈ are pixels.

6. The method as claimed in claim 1, wherein the second mode performs low pass filtering to remove the blocking artifacts.

7. The method as claimed in claim 1, wherein the deblocking filtering in the default mode is performed by replacing the magnitude of the discontinuous component of pixels v₄ and v₅ that sandwich the block boundary with v₄′ and v₅′, according to the following equation:

v₄′=v₄−d

v₅′=v₅+d

d=CLIP(c₂.(a_3,0′−a_3,0)//c₃,0,(v₄−v₅)/2)*δ(|a_3,0|<QP)

a_3,0′=SIGN(a_3,0)*MIN(|a_3,0|,|a_3,1|,|a_3,2|)

a_3,0=([c₁−c₂ c₂−c₁]*[v₃v₄v₅v₆]^T)//c₃

a_3,1=([c₁−c₂ c₂−c₁]*[v₁v₂v₃v₄]^T)//c₃

a_3,2=([c₁−c₂ c₂−c₁]*[v₅v₆v₇v₈]^T)//c₃,

8. The method as claimed in claim 7, wherein c₁ and c₂ are approximated to an integer and c₃ is approximated to a multiple of 2, wherein the DCT is a 4-point DCT used to determine the frequency information, and wherein a_3,0, a_3,1, a_3,2 are evaluated from an inner product of the DCT kernel and the selected pixel set being S0, a first pixel set S1 and a second pixel set S2.

9. The method as claimed in claim 7, wherein |a_3,0|<QP prevents over-smoothing.

10. The method as claimed in claim 1, further comprising performing the deblocking filtering process around horizontal and vertical block boundaries in a frame.

11. The method of claim 1, wherein the removing blocking artifacts in the second mode satisfies the following equation:

12. The method of claim 1, wherein the replacement pixel set contains a minimum magnitude of the at least one corresponding discontinuous component.

13. The method of claim 1, wherein the first mode is the default mode and the second mode is the dc offset mode, and wherein each of the plurality of pixel sets has four pixels.

14. A method for removing blocking artifacts in a coding system comprising:

determining at least pixel sets S0, S1, S2 around a block boundary;

selecting one of a default mode and a dc offset mode as a deblocking mode based on an amount of blocking artifacts;

deblocking filtering of pixels adjacent the block boundary if the default mode is selected; and

removing artifacts in the dc offset mode, if the dc offset mode is selected and a dc offset mode condition is satisfied, wherein the artifacts are removed in the dc offset mode according to the following equation:

15. The method of claim 14, wherein the deblocking filtering step comprises:

obtaining frequency information for each of the plurality of pixel sets S0, S1, S2;

replacing a magnitude of at least one discontinuous component in the frequency domain of a selected pixel set S0 of the plurality of pixel sets belonging to the block boundary with a magnitude of at least one corresponding discontinuous component belonging to a replacement pixel set S1, S2 of the plurality of pixel sets near the block boundary; and

applying the replaced frequency information of the selected pixel set S0 to the spatial domain to remove the blocking artifacts.

16. The method of claim 14, further comprising determining a mode decision value, wherein the dc offset mode is selected if the mode decision value is greater than a first threshold value, wherein mode decision value is determined based on the following equation:

mode decision value=φ(v₀−v₁)+φ(v₁−v₂)+φ(v₂−v₃)+φ(v₃−v₄)+φ(v₄−v₅)+φ(v₅−v₁)+φ(v₇−v₈)+φ(v₈−v₉),

wherein φ(γ)=1 when |γ|≦ a second threshold value and φ(γ)=0 otherwise, and wherein v₀-v₉ are boundary pixel values.

17. The method of claim 14, wherein the dc offset mode condition is satisfied when an absolute value of a maximum data value minus a minimum data value in block boundary pixels is smaller than 2QP, wherein the maximum data value=MAX(v₁, v₂, v₃, v₄, v₅, v₆, v₇, v₈), the minimum data value=MIN(v₁, v₂, v₃, v₄, v₅, v₆, v₇, v₈), QP is the quantization parameter of a block adjacent the block boundary and v₁-v₈ are the block boundary pixels.

18. The method of claim 14, wherein the deblocking filtering in the default mode is performed by replacing the magnitude of the discontinuous component of pixels v₄ and v₅ sandwiching the block boundary with v₄′ and v₅′, according to the following equation:

v₄′=v₄−d

v₅′=v₅+d

d=CLIP(c₂.(a_3,0′−a_3,0)//c₃,0,(v₄−v₅)/2)*δ(|a_3,0|<QP)

a_3,0′=SIGN(a_3,0)*MIN(|a_3,0|,|a_3,1|,|a_3,2|)

a_3,0=([c₁−c₂ c₂−c₁]*[v₃v₄v₅v₆]^T)//c₃

a_3,1=([c₁−c₂ c₂−c₁]*[v₁v₂v₃v₄]^T)//c₃

a_3,2=([c₁−c₂ c₂−c₁]*[v₅v₆v₇v₈]^T)//c₃,

19. The method of claim 14, further comprising performing the determining through removing steps for each horizontal and vertical block boundaries in a frame.

20. A method for removing blocking artifacts in a coding system comprising:

determining at least pixel sets S0, S1, S2 around a block boundary;

selecting one of a default mode and a dc offset mode as a deblocking mode based on an amount of blocking artifacts;

deblocking filtering of pixels adjacent the block boundary based on frequency information of the pixels adjacent to the bock boundary, if the default mode is selected; and

removing artifacts in the dc offset mode, if the dc offset mode is selected and a dc offset mode condition is satisfied, wherein the artifacts are removed in the dc offset mode according to the following equation:

21. The method of claim 20, wherein the deblocking filtering step comprises:

obtaining frequency information for each of the plurality of pixel sets S0, S1, S2;

replacing a magnitude of at least one discontinuous component in the frequency domain of a selected pixel set S0 of the plurality of pixel sets belonging to the block boundary with a magnitude of at least one corresponding discontinuous component belonging to a replacement pixel set S1, S2 of the plurality of pixel sets near the block boundary; and

applying the replaced frequency information of the selected pixel set S0 to the spatial domain to remove the blocking artifacts.

22. The method of claim 20, futher comprising determining a mode decision value, wherein the dc offset mode is selected if the mode decision value is greater than a first threshold value, wherein mode decision value is determined based on the following equation:

mode decision value=φ(v₀−v₁)+φ(v₁−v₂)+φ(v₂−v₃)+φ(v₃−v₄)+φ(v₄−v₅)+φ(v₅−v₁)+φ(v₇−v₈)+φ(v₈−v₉),

wherein φ(γ)=1 when |γ|≦ a second threshold value and φ(γ)=0 otherwise,

wherein v₀-v₉ are boundary pixel values, and

wherein φ(γ) is a function generating 1 or 0 according to |γ| and γ is a variable denoting a result of substraction of two given boundary pixel values.

23. The method of claim 20, wherein the dc offset mode condition is satisfied when an absolute value of a maximum data value minus a minimum data value in block boundary pixels is smaller than 2QP, wherein the maximum data value=MAX(v₁, v₂, v₃, v₄, v₅, v₆, v₇, v₈), the minimum data value=MIN(v₁, v₂, v₃, v₄, v₅, v₆, v₇, v₈), QP is the quantization parameter of a block adjacent the block boundary, and v₁-v₈ are the block boundary pixels.

24. The method of claim 20, wherein the deblocking filtering in the default mode is performed by replacing the magnitude of the discontinuous component of pixels v₄ and v₅ sandwiching the block boundary with v₄′ and v₅′, according to the following equation:

v₄′=v₄−d

v₅′=v₅+d

d=CLIP(c₂·(a_3,0′−a_3,0)//c₃,0,(v₄−v₅)/2)*δ(|a_3,0|<QP)

a_3,0′=SIGN(a_3,0)*MIN(|a_3,0|,|a_3,1|,|a_3,2|)

a_3,0=([c₁−c₂ c₂−c₁]*[v₃v₄v₅v₆]^T)//c₃

a_3,1=([c₁−c₂ c₂−c₁]*[v₁v₂v₃v₄]^T)//c₃

a_3,2=([c₁−c₂ c₂−c₁]*[v₅v₆v₇v₈]^T)//c₃,

where QP is the quantization parameter of the block containing the pixel v₅, values c₁, c₂, c₃ are kernel constants used in a 4-point DCT, and values of a_3,0, a_3,1, a_3,2, are based on a simple inner product of the DCT kernel and the selected pixel set S0, a first pixel set S1 and the second pixel set S2,

wherein a_3,0, a_3,1, and a_3,2 represent DCT coefficients of corresponding pixel sets,

wherein d is a variable used to calculate v₄′ and v₅′ and denotes a result of a function CLIP (x, p, q) where x, p, and q are integers,

wherein the function CLIP (x, p, q) clips x to a value between p and q,

wherein δ(condition) is a function generating 1 if the “condition” is true and generating 0 if the “condition” is not true,

wherein T denotes a transpose of vector matrix, and

wherein v₁, v₂, v₃, v₄, v₅, v₆, v₇, and v₈ denote boundary pixel values.

25. The method of claim 20, further comprising performing the determining through removing steps for each horizontal and vertical block boundaries in a frame.

26. The method of claim 20, further comprising dividing a picture into blocks that each include multiple pixels, wherein the pixels of a first block are separated from pixels of a neighboring second block by the block boundary,

wherein deblocking filtering of the pixels adjacent to the block boundary further comprises adjusting a first pixel of the first block based upon a neighboring second pixel of the second block.

27. The method of claim 26,

wherein adjusting the first pixel further comprises replacing a pixel value v₃ of the first pixel with a value v₃′,

wherein the value v₃′ is expressed as:

v₃′=v₃−d,

wherein d is expressed as:

28. The method of claim 26, wherein δ(|a_3,0|<QP)=1 if |a_3,0|<QP, and wherein δ(|a_3,0|<QP)=0 if |a_3,0|≧QP,

wherein a_3,0 represents a DCT coefficient of a corresponding pixel set, and

wherein δ(condition) is a function generating 1 if the “condition” is true and generating 0 if the “condition” is not true.

29. The method of claim 26, wherein c₂ is an interger and c₃ is a multiple of 2.

30. The method of claim 26, wherein c₂=5 and c₃=8.

31. The method of claim 26, further comprising applying the adjusting operation to a spatial domain of the first pixel.

32. The method of claim 26, wherein the block boundary is a vertical or horizontal block boundary.

33. The method of claim 26, further comprising adjusting the neighboring second pixel based upon frequency information of the first pixel.

34. The method of claim 26, wherein the magnitude of the adjusting operation is constrained to be no greater than half of the difference between a value of the first pixel and a value of the second pixel, using a clipping function.

35. The method of claim 26, wherein the magnitude of the adjusting operation is based upon a simple inner product of a DCT kernel and the first and second pixels.

36. The method of claim 26, wherein the magnitude of the adjusting operation is based upon a minimum magnitude of frequency information of the first and second pixels.

37. The method of claim 26, wherein the magnitude of the adjusting operation is based upon frequency information of the first pixel and the neighboring second pixel.

38. The method of claim 26, further comprising obtaining the frequency information for the neighboring second pixel, wherein the first pixel is adjusted if a magnitude of the blocking artificat is less than a quantization parameter of the second block.