Method of filtering an image based on comparisons

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.In the method, a pixel of an image is filtered according to a filtering methodology. The filtering methodology includes more than one comparison, and each comparison compares a respective determined value and a respective threshold. Each respective determined value may be based on a respective difference value, and each respective difference value may be based on a different difference. At least one the difference values may be based on the pixel being filtered and another pixel.

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    
       where k is a constant and q is a quantization step interval of said block B. 
     
     
       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 filtering 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 move 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    
       where k is a constant and q is a quantization step interval of said block B. 
     
     
       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 an image, comprising:
   filtering a pixel of an image according to a filtering methodology, the filtering methodology including 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 more than one of the respective difference values being based on the pixel being filtered and another pixel.      
     
     
       22. The method of  claim 21 , wherein at least one the another pixels is in a neighborhood of the pixel being filtered.  
     
     
       23. The method of  claim 22 , wherein at least one of the another pixels is adjacent to the pixel being filtered.  
     
     
       24. The method of  claim 23 , wherein at least one of the respective difference values is based on a difference between the pixel being filtered and the adjacent pixel.  
     
     
       25. The method of  claim 22 , wherein at least one of the respective difference values is based on a difference between the pixel being filtered and the neighborhood pixel.  
     
     
       26. The method of  claim 25 , wherein at least one of the respective difference values is an absolute value of the difference between the pixel being filtered and the neighborhood pixel.  
     
     
       27. The method of  claim 25 , wherein at least one of the determined values is an absolute value.  
     
     
       28. The method of  claim 21 , wherein at least one of the difference values is an absolute value of the difference between the pixel being filtered and another pixel.  
     
     
       29. The method of  claim 21 , wherein the filtering methodology filters the pixel based on a result of at least one of the comparisons.  
     
     
       30. The method of  claim 29 , wherein the filtering methodology filters the pixel using the pixel being filtered and another pixel based on a result of at least one of the comparisons.  
     
     
       31. The method of  claim 21 , 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 methodology does not filter the pixel.  
     
     
       32. The method of  claim 21 , wherein the filtering methodology filters the pixel if each of the comparisons results in a determination that the respective determined value is less than the respective threshold.  
     
     
       33. The method of  claim 32 , 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 methodology does not filter the pixel.  
     
     
       34. The method of  claim 21 , wherein each respective difference value is based on a different difference.  
     
     
       35. The method of  claim 21 , wherein at least one of the respective difference values is an absolute value.  
     
     
       36. The method of  claim 21 , wherein at least one of the respective determined values is the difference value.  
     
     
       37. The method of  claim 21 , wherein at least one of the respective determined values is an absolute value.  
     
     
       38. The method of  claim 21 , wherein at least two of the comparisons use a same respective threshold.