Method and associated device for filtering digital video images

• US 20010017944A1
• Filed: 01/19/2001
• Published: 08/30/2001
• Est. Priority Date: 01/20/2000
• Status: Active Grant

• Current Assignee: Nokia Technologies Oy (Nokia Corporation)
• Original Assignee: Nokia Mobile Phones UK Limited (Nokia Corporation)
• Inventors: Karczewicz, Marta, Kalevo, Ossi, Aksu, Emre
• Application Number:
  US09/766,035
• Publication Number:
  US 20010017944A1
• Time in Patent Office: Days
• US Class Current: 382/268
• CPC Class Codes: H04N 19/527 Global motion vector estimation
• CPC Class Codes: H04N 19/86 involving reduction of coding artifacts, e.g. of blockiness

Abstract

The invention relates to a method for reducing visual artefacts in a frame of a digital video signal, which is coded by blocks and then decoded. A block type is defined according to the coding method for a block selected according to a predetermined set of coding types. In the method filtering is performed to reduce visual artefacts due to a block boundary. The filtering performed depends on block types of the frame in the environment of the block boundary (30).

Claims

1. A method for reducing visual artefacts in a frame of a digital video signal, which is coded by blocks and then decoded, a block type being defined according to the coding method for a block selected from a predetermined set of coding types, in which filtering is performed to reduce visual artefacts due to a block boundary, characterized in that the filtering performed on the block boundary depends on block types of the frame in the environment of the block boundary (30).

2. A method according to

claim 1

, characterized in that the frame comprises at least one region of blocks, each block within said region having a region type, and that the filtering performed on the block boundary depends on a region type of the blocks in the environment of the block boundary (30).

3. A method according to

claim 1

, characterised in that the filtering performed on the block boundary depends on a block type of a block on a first side of the block boundary (30) and on a block type of a block on a second side of the block boundary (30).

4. A method according to

claim 1

, characterised in that at least one parameter of the filtering performed to reduce visual artefacts due to a block boundary on at least one side of the block boundary is modified according to the block type of at least one block in the environment of the block boundary (30).

5. A method according to

claim 4

, characterised in that at least one parameter of the filtering performed to reduce visual artefacts due to a block boundary is modified according to the block type of a first block and a second block, the first and second block being located on opposite sides of the block boundary (30).

6. A method according to

claim 4

, characterised in that said at least one parameter is selected from a group comprising;

a number of pixels to be examined, a number of pixels to be filtered, an activity measure providing an indication of the difference between pixel values on one side of the block boundary, a filtering window.

7. A method according to

claim 1

, in which a number of pixels (n) is selected for examination from at least one side of the block boundary (30), characterized in that the number of pixels (n) selected for examination depends on the image content of the frame in the environment of the block boundary (30), and that the number of pixels (n) selected for examination further depends on the block type of a block in the environment of the block boundary (30).

8. A method according to

claim 7

, characterised in that the number of pixels (n) selected for examination depends on the difference in pixel value (Δ

) between pixels across the block boundary.

9. A method according to

claim 7

, characterized in that the number of pixels selected for examination depends on the size of the quantization step (QP) of the coefficients used in the coding of the blocks.

10. A method according to

claim 9

, characterized in that the number of pixels (n) selected for examination is determined by the formula;

$\begin{array}{cc}n=\{\begin{array}{cc}0& \mathrm{\Delta <br><br>}<br><br>\text{\hspace{1em}<br><br>}\ge <br><br>2.00<br><br>\text{\hspace{1em}<br><br>}<br><br>\mathrm{\alpha <br><br>}\\ 1& 1.50<br><br>\text{\hspace{1em}<br><br>}<br><br>\mathrm{\alpha <br><br>}\le <br><br>\mathrm{\Delta <br><br>}<br><br>\text{\hspace{1em}<br><br>}<<br><br>2.00<br><br>\text{\hspace{1em}<br><br>}<br><br>\mathrm{\alpha <br><br>}\\ 2& 1.00<br><br>\text{\hspace{1em}<br><br>}<br><br>\mathrm{\alpha <br><br>}\le <br><br>\mathrm{\Delta <br><br>}<br><br>\text{\hspace{1em}<br><br>}<<br><br>1.50<br><br>\text{\hspace{1em}<br><br>}<br><br>\mathrm{\alpha <br><br>}\\ 3& 0.66<br><br>\text{\hspace{1em}<br><br>}<br><br>\mathrm{\alpha <br><br>}\le <br><br>\mathrm{\Delta <br><br>}<br><br>\text{\hspace{1em}<br><br>}<<br><br>1.00<br><br>\text{\hspace{1em}<br><br>}<br><br>\mathrm{\alpha <br><br>}\\ 4& 0.40<br><br>\text{\hspace{1em}<br><br>}<br><br>\mathrm{\alpha <br><br>}\le <br><br>\mathrm{\Delta <br><br>}<br><br>\text{\hspace{1em}<br><br>}<<br><br>0.66<br><br>\text{\hspace{1em}<br><br>}<br><br>\mathrm{\alpha <br><br>}\\ 5& 0.25<br><br>\text{\hspace{1em}<br><br>}<br><br>\mathrm{\alpha <br><br>}\le <br><br>\mathrm{\Delta <br><br>}<br><br>\text{\hspace{1em}<br><br>}<<br><br>0.40<br><br>\text{\hspace{1em}<br><br>}<br><br>\mathrm{\alpha <br><br>}\\ 6& 0\le <br><br>\mathrm{\Delta <br><br>}<br><br>\text{\hspace{1em}<br><br>}<<br><br>0.25<br><br>\text{\hspace{1em}<br><br>}<br><br>\mathrm{\alpha <br><br>}\end{array},& \left(2\right)\end{array}$

wherein Δ

is the difference in value between pixels across the block boundary, α

=QP·

log(QP) and QP is the size of the quantization step of the coefficients used in the coding of the blocks.

11. A method according to

claim 8

, characterized in that the number of pixels (n) is first defined according to the image content of the frame in the environment of the block boundary (30), and the number of pixels (n) is further truncated according to the block type of a block in the environment of the block boundary (30) to give a truncated number of pixels (n_tr) for examination.

12. A method according to

claim 11

, characterized in that the truncated number of pixels (n_tr) is determined by selecting a truncation value (trval) according to the table Region typeof the Blockon the FirstRegion type of the Block on the Second sidesideINTRACOPYCODEDNOT_CODEDINTRAnn22n4n2COPY22222422CODED4n424442NOT_CODED2n222422

and using said selected truncation value (trval) with the formula

n_tr=Min(trval,n), 

 

(3).

13. A method according to

claim 1

, characterized in that certain pixels to be filtered are selected, and a new value is determined for each pixel to be filtered on the basis of pixels that appear in a filtering window set around the pixel.

14. A method according to

claim 1

, characterized in that pixels to be filtered are selected from the pixels selected for examination.

15. A method according to

claim 13

, characterized in that the new value of the pixel to be filtered is the mean value of the pixels that appear in the filtering window.

16. A method according to

claim 13

, characterized in that for determining a new value for the pixels to be filtered on the first side of the block boundary, said filtering window is used, and the size of the window is determined according to the table d_r (d_l >

17. A method according to

claim 16

, characterized in that said activity is determined on the basis of changes in pixel values.

18. A method according to

claim 16

, characterized in that

d_r=6, if |r₁−

r_j|≦

β

/j with all jε

[1,6],

otherwise;

d_r=i, where i meets the conditions

iε

[1,n_tr], |r₁−

r_i+1|>

β

/i, and |r₁−

r_j|≦

β

/j with all jε

[1,i],

wherein the auxiliary parameter β

=4·

log(QP) and QP is the size of the quantization step of the coefficients used in the coding of the blocks, and the value of the parameter d_l is determined similarly, with the exception that all r'"'"'s are replaced by l'"'"'s.

19. A device for reducing visual artefacts in a frame of a digital video signal, which is coded by blocks and then decoded, a block type being defined according to the coding method for a block selected according to a predetermined set of coding types, the device comprising a filter for reducing visual artefacts due to a block boundary, characterized in that the filter is arranged to operate adaptively according to the block types of the frame in the environment of the block boundary (30).

20. A device according to

claim 19

, characterised in that the filter is arranged such that the frame comprises at least one region of blocks, each block within said region having a region type, and that the filtering performed on the block boundary depends on a region type of the blocks in the environment of the block boundary (30).

21. A device according to

claim 19

, characterised in that the filter is arranged such that the filtering performed on the block boundary depends on a block type of a block on a first side of the block boundary (30) and on a block type of a block on a second side of the block boundary (30).

22. A device according to

claim 19

, characterised in that the filter is arranged such that at least one parameter of the filtering performed to reduce visual artefacts due to a block boundary on at least one side of the block boundary is modified according to the block type of at least one block in the environment of the block boundary (30).

23. A device according to

claim 22

, characterised in that the filter is arranged such that at least one parameter of the filtering performed to reduce visual artefacts due to a block boundary is modified according to the block type of a first block and a second block, the first and second block being located on opposite sides of the block boundary (30).

24. A device according to

claim 22

, characterised in that said at least one parameter is selected from a group comprising:

a number of pixels to be examined, a number of pixels to be filtered, an activity measure providing an indication of the difference between pixel values on one side of the block boundary, a filtering window.

25. A device according to

claim 21

, characterized in that it comprises means (42) that operate adaptively according to the image content of the frame, for selecting a number of pixels (n) for examination, and that means (42) for selecting a number of pixels (n) for examination comprises further means (42) for examining the block type of a block in the environment of the block boundary (30).

26. A device according to

claim 25

, characterised in that it comprises means (42) for selecting the number of pixels (n) for examination depending on the difference in pixel value (Δ

) between pixels across the block boundary.

27. A device according to

claim 19

, characterized in that it comprises means (42) for selecting a number of pixels (n) for examination depending on the size of the quantization step (QP) of the coefficients used in the coding of the blocks.

28. A device according to

claim 27

, characterized in that the means (42) for selecting the number of pixels (n) for examination comprises means (35) for determining said number of pixels according to the formula:

$\begin{array}{cc}n=\{\begin{array}{cc}0& \mathrm{\Delta <br><br>}<br><br>\text{\hspace{1em}<br><br>}\ge <br><br>2.00<br><br>\text{\hspace{1em}<br><br>}<br><br>\mathrm{\alpha <br><br>}\\ 1& 1.50<br><br>\text{\hspace{1em}<br><br>}<br><br>\mathrm{\alpha <br><br>}\le <br><br>\mathrm{\Delta <br><br>}<br><br>\text{\hspace{1em}<br><br>}<<br><br>2.00<br><br>\text{\hspace{1em}<br><br>}<br><br>\mathrm{\alpha <br><br>}\\ 2& 1.00<br><br>\text{\hspace{1em}<br><br>}<br><br>\mathrm{\alpha <br><br>}\le <br><br>\mathrm{\Delta <br><br>}<br><br>\text{\hspace{1em}<br><br>}<<br><br>1.50<br><br>\text{\hspace{1em}<br><br>}<br><br>\mathrm{\alpha <br><br>}\\ 3& 0.66<br><br>\text{\hspace{1em}<br><br>}<br><br>\mathrm{\alpha <br><br>}\le <br><br>\mathrm{\Delta <br><br>}<br><br>\text{\hspace{1em}<br><br>}<<br><br>1.00<br><br>\text{\hspace{1em}<br><br>}<br><br>\mathrm{\alpha <br><br>}\\ 4& 0.40<br><br>\text{\hspace{1em}<br><br>}<br><br>\mathrm{\alpha <br><br>}\le <br><br>\mathrm{\Delta <br><br>}<br><br>\text{\hspace{1em}<br><br>}<<br><br>0.66<br><br>\text{\hspace{1em}<br><br>}<br><br>\mathrm{\alpha <br><br>}\\ 5& 0.25<br><br>\text{\hspace{1em}<br><br>}<br><br>\mathrm{\alpha <br><br>}\le <br><br>\mathrm{\Delta <br><br>}<br><br>\text{\hspace{1em}<br><br>}<<br><br>0.40<br><br>\text{\hspace{1em}<br><br>}<br><br>\mathrm{\alpha <br><br>}\\ 6& 0\le <br><br>\mathrm{\Delta <br><br>}<br><br>\text{\hspace{1em}<br><br>}<<br><br>0.25<br><br>\text{\hspace{1em}<br><br>}<br><br>\mathrm{\alpha <br><br>}\end{array},& \left(2\right)\end{array}$

wherein Δ

is the difference in value between pixels across the block boundary, α

=QP·

log(QP) and OP is the size of the quantization step of the coefficients used in the coding of the blocks.

29. A device according to

claim 19

, characterized in that it comprises means (42) for truncating the number of pixels (n) selected for examination on the basis of the block type of a block in the environment of the block boundary.

30. A device according to

claim 19

, characterized in that the means (42) for selecting a number of pixels for examination comprises means for defining the number of pixels (n) according to the image content of the frame in the environment of the block boundary (30), and means for truncating the number of pixels (n) according to the block type of a block in the environment of the block boundary (30).

31. A device according to

claim 30

, characterized in that the means for truncating the number of pixels (n) comprises means for selecting a truncation value (trval) according to the table:

Type of theBlock on theType of the Block on the Second sideFirst sideINTRACOPYCODEDNOT_CODEDINTRAnn22n4n2COPY22222422CODED4n424442NOT_CODED2n222422

and using said selected truncation value (trval) with the formula;

n_tr=min(trval,n), 

 

(3).

32. A device according to

claim 19

, characterized in that it comprises means (42) for selecting certain pixels to be filtered, means (42) for defining a filtering window, and means for determining a new value for each pixel to be filtered on the basis of pixels that appear in a filtering window set around the pixel.

33. A device according to

claim 32

, characterized in that the means for determining a new value for each pixel comprises means for calculating a mean value of the pixels that appear in the filtering window.

34. A device according to

claim 28

, characterized in that the means for determining a new value for each pixel comprises means for using said filtering window for the pixels to be filtered on the first side of the block boundary, and means for determining the size of the window according to the table d_r (d_l >

35. A device according to

claim 34

, characterized in that

d_r=6, if |r₁−

r_l|≦

β

/j with all jε

[1,6],

otherwise;

d_r=i, where i meet the conditions

iε

[1,n_tr], |r₁−

r_i+1|>

β

/i, and |r₁−

r_j|≦

β

/j with all jε

[1,i],

wherein the auxiliary parameter β

=4·

log(QP) and OP is the size of the quantization step of the transformation coefficients used in transformation coding of the blocks, and the value of the parameter d_l being determined similarly, with the exception that all r'"'"'s are replaced by l'"'"'s.

36. A device according to

claim 19

, characterized in that it comprises programmable means (42) for selecting pixels from a saved frame as the pixels to be examined, programmable means (45) for selecting pixels to be filtered from among the pixels to be examined, and programmable means (44) for determining the new value of the pixels to be filtered.

37. A video encoder (10) comprising means (35-44) for coding and means for decoding a digital video signal by blocks, a block type being defined according to the coding method for a block selected according to a predetermined set of coding types, which encoder comprises a filter for reducing visual artefacts due to a block boundary, characterized in that the filter is arranged to operate adaptively according to the block types of the frame in the environment of the block boundary (30).

38. A video decoder (20) comprising means (35-44) for reducing visual artefacts in a frame of a digital video signal, which is coded by blocks and then decoded, a block type being defined according to the coding method for a block selected according to a predetermined set of coding types, which video decoder comprises a filter for reducing visual artefacts due to a block boundary, characterized in that the filter is arranged to operate adaptively according to the block types of the frame in the environment of the block boundary (30).

39. A video codec (10, 20) comprising means (35-44) for coding and decoding a digital video signal by blocks, a block type being defined according to the coding method for a block selected according to a predetermined set of coding types, which video codec comprises a filter for reducing visual artefacts due to a block boundary, characterized in that the filter is arranged to operate adaptively according to the the block types of the frame in the environment of the block boundary (30).

40. A mobile terminal (46) comprising a video codec (10, 20), which comprises means (35-44) for coding and decoding a digital video signal by blocks, a block type being defined according to the coding method for a block selected according to a predetermined set of coding types, which video codec comprises a filter for reducing visual artefacts due to a block boundary, characterized in that the filter is arranged to operate adaptively according to the the block types of the frame in the environment of the block boundary (30).

41. A storage medium for storing a software program comprising machine executable steps for coding and decoding a digital video signal by blocks, a block type being defined according to the coding method for a block selected according to a predetermined set of coding types, for reducing visual artefacts due to a block boundary by filtering, characterized in that the software program further comprises machine executable steps for filtering adaptively according to the the block types of the frame in the environment of the block boundary (30).