Method and apparatus for filtering an image

Abstract

A method and apparatus to remove a block effect and a ring effect appearing in a compression-coded image is disclosed. The present invention is especially applicable to an image compression-coded at a low bit rate. In particular, the present invention includes a variety of masks for the removal of the block/ring effect. Thus, one mask is select for a pixel to be filtered. Moreover, candidate pixels to be averaged with the pixel to be filtered is selected from the pixels of the selected mask to better maintain the details of the image, and a weight is applied to improve the calculation speed of the averaging operation. More particularly, the masks according to the present invention have longer tabs toward adjacent blocks at boundaries of the given block to remove the block and ring effect.

Description

predetermined
where, k is the constant and q is the quantization step interval of a block.

In claims invention. Many alternatives, modifications, and variations will be apparent to those skilled in the art.

Claims

1. A filtering method for a pixel p of a block b in a reconstructed image, comprising:

selecting one of a plurality of filtering masks based upon a position of said pixel p in said block b; and averaging said pixel p and candidate pixels within the selected filtering mask.

2. A method of claim 1, wherein each of the plurality of filtering masks has 8 tabs.

3. A method of claim 1, wherein a filtering mask with more tabs toward a block adjacent said block b is selected.

4. A method of claim 1, further comprising selecting a pixel within the selected filtering mask p_m as a candidate pixel if the value of the pixel p_m meets a predetermined condition.

5. A method of claim 4, wherein the pixel p_m is selected as a candidate pixel if the absolute value of the difference between said pixel p and pixel p_m is less than a threshold value.

6. A method of claim 5, wherein the threshold value is calculated by an equation below,

δ=k×q

7. A method of claim 6, wherein the value of k is 1.0 for filtering boundary pixels of said block b and 0.6 for filtering pixels within the boundary pixels of said block b.

8. A method of claim 4, further comprising adding a weight value to said pixel p prior to the averaging, wherein said weight value is based upon a number of pixels p_m not selected as candidate pixels.

9. A method of claim 8, wherein said weight value is the number of pixels p_m not selected multiplied by the value of said pixel p.

10. A filtering apparatus to filter a pixel p of a block b in a reconstructed image, comprising:

a filtering masking unit selecting one of a plurality filtering masks based upon a position of said pixel p in said block b; and

an averaging unit averaging said pixel p and candidate pixels within the selected mask.

11. An apparatus of claim 10, wherein each of the plurality of filtering masks has 8 tabs.

12. An apparatus of claim 11, wherein the plurality of filtereing masks are modified 3×3 mask forms including:

a filtering mask in which eight tabs are selected from the 3×3 mask form, discarding one corner tab;

a filtering mask in which more tabs are selected in a vertical direction than a horizontal direction, and more vertical lower tabs are selected than vertical upper tabs;

a filtering mask in which more tabs are selected in the vertical direction than the horizontal direction, and more vertical upper tabs are selected than vertical lower tabs;

a filtering mask in which more tabs are selected in the horizontal direction than the vertical direction, and more horizontal left tabs are selected than horizontal right tabs; and

a filtering mask in which more tabs are selected in the horizontal direction than the vertical direction, and more horizontal right tabs are selected than horizontal left tabs.

13. An apparatus of claim 10, wherein a filtering mask with more tabs toward a block adjacent said block b is selected.

14. An apparatus of claim 10, further comprising:

a comparison unit selecting a pixel within the selected mask p_m as a candidate pixel if the value of the pixel p_m meets a predetermined condition.

15. An apparatus of claim 14, wherein the pixel p_m is selected as a candidate pixel if the absolute value of the difference between said pixel p and pixel p_m is less than a threshold value.

16. An apparatus of claim 15, wherein the threshold value is calculated by an equation below,

δ=k×q

17. An apparatus of claim 16, wherein the value of k is 1.0 for filtering boundary pixels of said block b and 0.6 for filtering pixels within the boundary pixels of said block.

18. An apparatus of claim 14, wherein the averaging unit adds a weight value to said pixel p prior to the averaging, wherein said weight value is based upon a number of pixels p_m not selected as candidate pixels.

19. An apparatus of claim 18, wherein said weight value is the number of pixels p_m not selected multiplied by the value of said pixel p.

20. A coding and decoding method comprising:

a discrete cosine transform (dct) unit performing a dct operation with respect to divided blocks of an image to generate dct coefficients;

a quantization unit quantizing the dct coefficients and transmitting the dct coefficients in a form of a bit stream through a transmission channel;

a dequantization unit dequantizing the dct coefficients received through the transmission channel; an inverse dct unit performing an inverse dct operation with respect to the dequantized dct coefficients to form a reconstructed image, and a filtering process unit filtering each pixel of each block of the reconstructed image by selecting one of a plurality of filtering masks based upon a position of said pixel in said block; and

averaging said pixel and candidate pixels within the selected filtering mask.

21. A method of filtering a pixel p of a block b in an image, comprising:

selecting a grouping of pixels from different groupings of pixels, the selected grouping of pixels including the pixel p to be filtered and at least one candidate pixel based on a position of the pixel p; and

filtering the pixel p based on the pixel p and the at least one candidate pixel.

22. The method of claim 21, wherein the method comprises:

performing at least one comparison, the comparison comparing a determined value and a threshold, the threshold being based on quantization information of at least the block b, the determined value being based on a difference value, and the difference value being based on the pixel p and the at least one candidate pixel.

23. The method of claim 22, wherein the at least one candidate pixel is in a neighborhood of the pixel p.

24. The method of claim 23, wherein at least one of the at least candidate pixel is adjacent to the pixel p.

25. The method of claim 24, wherein the difference value is based on a difference between the pixel p and the adjacent candidate pixel.

26. The method of claim 25, wherein the difference value is an absolute value of the difference.

27. The method of claim 25, wherein the determined value is an absolute value.

28. The method of claim 22, wherein the filtering step filters the pixel p based on a result of the comparison.

29. The method of claim 28, wherein the filtering step filters the pixel p using the pixel p and the at least one candidate pixel based on a result of the comparison.

30. The method of claim 29, wherein if the determined value is less than the threshold, the filtering step filters the pixel p using the pixel p and the at least one of the candidate pixels.

31. The method of claim 28, wherein if the determined value is less than the threshold, the filtering step filters the pixel p based on the difference value.

32. The method of claim 22, wherein the difference value is an absolute value.

33. The method of claim 22, wherein the determined value is the difference value.

34. The method of claim 22, wherein the determined value is an absolute value.

35. The method of claim 21, wherein the method comprises:

performing more than one comparison, each comparison comparing a respective determined value and a respective threshold, each respective determined value being based on a respective difference value, and at least one of the respective difference value being based on the pixel p and at least one candidate pixel.

36. The method of claim 35, wherein each respective difference value is based on a different difference.

37. The method of claim 35, wherein more than one of the respective difference values are based on the pixel p.

38. The method of claim 37, wherein if each of the comparisons results in a determination that the respective determined value is greater than or equal to the respective threshold, the filtering step does not filter the pixel p.

39. The method of claim 35, wherein if at least one of the comparisons results in a determination that the respective determined value is greater than or equal to the respective threshold, the filtering step does not filter the pixel p.

40. The method of claim 35, wherein the filtering step filters the pixel if each of the comparisons results in a determination that the respective determined value is less than the respective threshold.

41. The method of claim 35, wherein at least one of the respective thresholds is based on quantization information of at least the block b.

42. The method of claim 35, wherein each respective threshold is based on quantization information of the block b.

43. The method of claim 35, wherein at least two of the comparisons use a same respective threshold.

44. The method of claim 22, wherein the quantization information is a quantization parameter.

45. The method of claim 41, wherein the quantization information is a quantization parameter.

46. The method of claim 42, wherein the quantization information is a quantization parameter.