Method of filtering an image including application of a weighted average operation

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 filtering mask is selected based on a position of a pixel to be filtered. The filtering mask includes the pixel to be filtered and at least one candidate pixel. The pixel to be filtered is filtered by applying a weighted averaging operation to the pixel to be filtered and the at least one candidate pixel. The weighted averaging operation includes applying weight to at least one of the pixel to be filtered and the at least one candidate pixel in producing a weighted sum value, and performing a shift operation on the weighted sum value to divide by a desired value.

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 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    
       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:
   selecting a filtering mask based on a position of a pixel to be filtered, the filtering mask including the pixel to be filtered and at least one candidate pixel; and        filtering, with a filtering unit, the pixel to be filtered by applying a weighted averaging operation to the pixel to be filtered and the at least one candidate pixel, the weighted averaging operation including,      applying weight to at least one of the pixel to be filtered and the at least one candidate pixel in producing a weighted sum value, and        performing a shift operation on the weighted sum value to divide by a desired value.       
     
     
       22. The method of  claim 21 , wherein the applied weight is based on the desired value. 
     
     
       23. The method of  claim 22 , wherein the desired value is eight. 
     
     
       24. The method of  claim 23 , wherein the applied weight adds up to the desired value. 
     
     
       25. The method of  claim 21 , wherein the filtering mask extends at least one of horizontally and vertically. 
     
     
       26. The method of  claim 25 , wherein the filtering mask extends at least one of horizontally and vertically based on a position of the pixel to be filtered. 
     
     
       27. The method of  claim 21 , wherein the filtering mask includes pixels in a first block and pixels in a second block, the second block being adjacent the first block. 
     
     
       28. The method of  claim 27 , wherein the filtering mask includes at least one pixel from the first block, and three pixels from the second block. 
     
     
       29. The method of  claim 27 , wherein the pixel to be filtered is in the second block. 
     
     
       30. The method of  claim 28 , wherein the pixel to be filtered is in the first block. 
     
     
       31. The method of  claim 21 , wherein the desired value is associated with a size of the filtering mask. 
     
     
       32. The method of  claim 21 , wherein the desired value equals a number of tabs in the filtering mask. 
     
     
       33. The method of  claim 32 , wherein the filtering mask has eight tabs. 
     
     
       34. The method of  claim 21 , wherein the desired value equals a number of pixels covered by the filtering mask. 
     
     
       35. The method of  claim 34 , wherein the number of pixels covered by the filtering mask is eight. 
     
     
       36. A method of filtering an image, comprising:
 selecting a filtering mask based on a position of a pixel to be filtered, the filtering mask including the pixel to be filtered and at least one candidate pixel; and  
   filtering, with a filtering unit, the pixel to be filtered by applying a weighted averaging operation to the pixel to be filtered and the at least one candidate pixel, the weighted averaging operation including,      applying weight to at least one of the pixel to be filtered and the at least one candidate pixel in producing a weighted sum value, and        dividing the weighted sum value by a desired value, the applied weight being associated with the desired value.