Method of image filtering based on comparison operation and averaging operation applied to selected successive pixels

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 of the present invention, a pixel of an image is filtered according to a filtering methodology. The filtering methodology includes selecting at least four pixels of successive pixels according to a position of a pixel to be filtered. The selected at least four pixels include the pixel to be filtered. The filtering methodology also includes applying an averaging operation to the selected at least four pixels, and making at least one comparison. With respect to the comparison, a determined value and a threshold are compared. The determined value is based on a difference value, and the difference value is based on two of the successive 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 of 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, with a filtering unit, a pixel of an image according to a filtering methodology, the filtering methodology including,      selecting 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,        comparing a first difference value and a threshold, the first difference value being based on the pixel to be filtered and a first pixel of the selected at least for pixels,        comparing a second difference value with the threshold, the second difference value being based on the pixel to be filtered and a second pixel of the selected at least four pixels, and        applying a mathematical operation including a difference determination to at least two of the selected at least four pixels based on the comparing steps, the at least two of the selected at least four pixels including non - adjacent pixels.       
     
     
       22. The method of  claim 21 , wherein one of the pixels upon which the difference is based is the pixel to be filtered. 
     
     
       23. The method of  claim 22 , wherein the threshold is based on a quantization information of at least a portion of the image including the pixel to be filtered. 
     
     
       24. The method of  claim 23 , wherein one of the pixels upon which the difference is based is one of the selected at least four pixels. 
     
     
       25. The method of  claim 21 , wherein the averaging operation includes applying a weight to a pixel being averaged. 
     
     
       26. The method of  claim 21 , wherein the selected at least four pixels lie in a horizontal direction of the image. 
     
     
       27. The method of  claim 26 , wherein the threshold is based on a quantization parameter of at least a portion of the image including the pixel to be filtered. 
     
     
       28. The method of  claim 21 , wherein the selected at least four pixels lie in a vertical direction of the image. 
     
     
       29. The method of  claim 28 , wherein the threshold is based on a quantization parameter of at least a portion of the image including the pixel to be filtered. 
     
     
       30. The method of  claim 21 , wherein the quantization information includes a quantization parameter.