Decoding apparatus including a filtering unit configured to filter an image based on averaging operation and difference

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 one embodiment, the filtering unit is configured to select at least four pixels of successive pixels according to a position of a pixel to be filtered. The four pixels include the pixel to be filtered. At least one of the four pixels may be in a first block, and three of the four pixels may be in a second block adjacent to the first block. The filtering unit is configured to determine a difference between two of the successive pixels. The filtering unit is configured to apply an averaging operation to the selected at least four pixels.

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 candiate 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 decoding apparatus, comprising:
   a filtering unit configured to,      select at least four pixels of successive pixels according to a position of a pixel to be filtered, the selected at least four pixels including the pixel to be filtered, at least one of the selected at least four pixels being in a first block, and three of the selected at least four pixels being in a second block adjacent to the first block,        determine a difference between two of the successive pixels, the two pixels including non - adjacent pixels, and        apply an averaging operation to the selected at least four pixels based on the difference, so as to filter the selected at least four pixels.        
     
     
       22. The apparatus of  claim 21 , wherein the difference is between two of the successive pixels in the second block.  
     
     
       23. The apparatus of  claim 22 , wherein one of the two successive pixels upon which the difference is based is the pixel to be filtered.  
     
     
       24. The apparatus of  claim 22 , wherein the difference is based on the pixel to be filtered and a nearby pixel.  
     
     
       25. The apparatus of  claim 21 , wherein one of the two successive pixels upon which the difference is based is the pixel to be filtered.  
     
     
       26. The apparatus of  claim 25 , wherein the pixel to be filtered is in the first block.  
     
     
       27. The apparatus of  claim 26 , wherein one of the two successive pixels upon which the difference is based is in the second block.  
     
     
       28. The apparatus of  claim 21 , wherein the filtering unit is configured to determine the difference between a pixel in the first block and a nearby one of the pixels in the second block.  
     
     
       29. The apparatus of  claim 28 , wherein the pixel in the first block is the pixel to be filtered.  
     
     
       30. The apparatus of  claim 28 , wherein one of the pixels in the second block is the pixel to be filtered.  
     
     
       31. The apparatus of  claim 21 , wherein the averaging operation includes applying a weight to a pixel being averaged.  
     
     
       32. The apparatus of  claim 31 , wherein the pixel to be filtered is in the second block.