Abstract:
Artifacts of an incident digital image including pixels carrying information are reduced by determining, for certain pixels being considered from the image, displaced pixels. A displaced pixel associated with a pixel being considered is situated at a location that is displaced with respect to the location of the pixel being considered. Substitution information is determined by taking into account the variations between each piece of information carried by pixels situated at locations adjacent to the pixel being considered. The pixel being considered is then selectively replaced by a substitution pixel equal to the displaced pixel or to a combination of the displaced pixel and the pixel being considered, depending on the value of the substitution information.

Description:
PRIORITY CLAIM 
     The present application claims priority from French Application for Patent No. 05 00212 filed Jan. 10, 2005, the disclosure of which is hereby incorporated by reference. 
     BACKGROUND OF THE INVENTION 
     1. Technical Field of the Invention 
     The present invention relates to the processing of digital images, in particular the post-processing of digital images having been compressed by means of a compression standard requiring the division of the image into several blocks, such as for example the MPEG standard. 
     The invention can be advantageously applied to, but is not limited to, digital video applications using large-area screens, such as plasma screens. 
     2. Description of Related Art 
     In order to transmit or to store digital images for digital video applications, the image must be compressed by means of a compression standard. In order to facilitate the compression phase, the source of pixels is converted into coefficients by means of a transformation algorithm, such as for example the Discrete Cosine Transform (DCT) conventionally used in blocks of dimension 8*8. In order that the transmission or the storage of the image be optimal, the compression ratios can be very high, which leads to compression artifacts that are hardly visible on standard resolution screens, but that are significant on large-area and high-resolution screens. 
     Visually, the compression artifacts appear as an edge effect associated with the blocks generated during the transformation phase, together with the presence of noise generated by transient oscillations referred to as “ringing” or “mosquito noise.” 
     In addition, without suitable processing to reduce these artifacts, the use of normal decoders leads to the loss of fidelity in some colors. 
     A need accordingly exists to provide a solution to these problems. 
     SUMMARY OF THE INVENTION 
     According to an embodiment of the invention, a method is presented for reducing the artifacts of an incident digital image comprising pixels carrying information. 
     This information can, for example, correspond to three bytes for each of the components of the Red-Green-Blue (RGB), Color-luminance Color-red Color-blue (YCrCb) or Hue-Luminance-Saturation (HLS) storage formats. 
     According to a general feature of this embodiment of the invention, the method comprises determining, for certain pixels being considered from the image, displaced pixels. A displaced pixel associated with a pixel being considered is situated at a location that is displaced with respect to the location of the pixel being considered. Then, determining a substitution information taking into account the variations between each piece of information carried by pixels situated at locations adjacent to the pixel being considered. Next, replacing (or not) of the pixel being considered by a substitution pixel equal to the displaced pixel or to a combination of the displaced pixel and the pixel being considered, depending on the value of the substitution information. 
     In other words, certain pixels of the incident image are replaced by pixels called displaced pixels, in other words a pixel belonging to the incident image other than the pixel initially situated in this location. The substitution is carried out if the substitution information meets certain conditions, for example that the region neighboring the pixel that could be replaced by a displaced pixel be visually uniform. Indeed, if this neighboring region is visually uniform, the substitution of the original pixel by a displaced pixel will not be visible when the image is displayed. 
     One advantage of this method is the significant reduction in the artifacts, and especially the effects referred to as “block effects,” by breaking up the block structures created during the compression phase for the transfer or storage of the image. 
     Furthermore, the examination of the possible replacement of the pixel being considered advantageously comprises the comparison of the substitution information with a first and second threshold. In addition, the pixel being considered is preferably conserved if the substitution information is greater than or equal to the first threshold. On the other hand, the pixel being considered is replaced by the substitution pixel if the substitution information is less than the first threshold, the nature of the substitution pixel then depending on the result of the comparison between the substitution information and the second threshold. 
     According to one embodiment, the substitution pixel is preferably equal to the displaced pixel if the substitution information is less than the second threshold, and is equal to a weighted sum of the pixel being considered and of the displaced pixel if the substitution information is greater than the second threshold. 
     According to one embodiment of the invention, each displaced location is displaced from the corresponding location of the pixel being considered by a shift corresponding to a displacement along two orthogonal axes of the image, of amplitudes n and m, respectively, at least one of the amplitudes m or n being non-zero. 
     According to one embodiment, the determination of the substitution information relating to a pixel being considered can advantageously comprise a filtering of the information carried by the pixels situated at the locations adjacent to the pixel being considered, and the summing of the filtered information. 
     Preferably, displaced pixels are determined for one pixel in two of the incident image, and by the fact that the sum of the amplitudes of the displacement along the two orthogonal axes is even. 
     This checkered structure is that which allows the block structure to be maximally broken up. 
     Another subject of the invention is an embodiment of a device for reducing the artifacts of an incident digital image composed of pixels carrying information. 
     According to a general feature of this embodiment, the device comprises determination means capable of determining displaced pixels, for certain pixels being considered from the image, a displaced pixel associated with a pixel being considered being situated at a location that is displaced with respect to the location of the said pixel being considered, calculation means capable of determining a substitution information by taking into account the variations between each piece of information carried by pixels situated at locations adjacent to the said pixel being considered, and a substitution means capable of replacing or not the pixel being considered by a substitution pixel equal to the displaced pixel or to a combination of the displaced pixel and the pixel being considered, depending on the value of the said substitution information. 
     According to one embodiment of the invention, the device comprises a memory capable of storing the pixels of the incident image. In addition, the determination means can comprise means for addressing the said memory, capable of delivering a first address corresponding to the location of the pixel being considered and a second address corresponding to the displaced location of the displaced pixel associated with the said pixel being considered, such that each displaced location is displaced from the location corresponding to the pixel being considered by a shift corresponding to a displacement along two orthogonal axes of the image, of amplitudes n and m, respectively, at least one of the amplitudes m or n being non-zero. 
     Preferably, all the first addresses can correspond to one pixel in two of the incident image, and the sum of the amplitudes of the displacement along the two orthogonal axes is even. 
     Furthermore, according to one embodiment of the invention, the addressing means can comprise: a first pair of counters assigned to one axis of the image and mutually displaced as a function of the said displacement along this axis, a second pair of counters assigned to the other axis of the image and mutually displaced as a function of the said displacement along this other axis, a first multiplexer whose two inputs are respectively connected to the first pair of counters, a second multiplexer whose two inputs are respectively connected to the second pair of counters, and an adder connected to the outputs of the two multiplexers and delivering an address pointing to a memory location at the output. 
     According to one embodiment of the invention, the calculation means of the device can comprise: means capable of filtering information carried by the pixels situated at the locations adjacent to the pixel being considered, and summation means capable of summing filtering information, the said summation means delivering the said substitution information. 
     According to one embodiment of the invention, the substitution means advantageously comprises: comparison means capable of comparing substitution information with a first and a second threshold, and mixing means receiving the pixel from the incident image and, as the case may be, the corresponding displaced pixel, and delivering the incident pixel being considered or the substitution pixel for this incident pixel depending on the result of the comparison means. 
     Furthermore, another subject of the invention is a screen format system comprising a screen, for example a Plasma screen, designed to display a digital image and a device as is defined hereinabove. 
     In accordance with another embodiment, a method comprises receiving an incident digital image pixel matrix and generating two virtual sub-image pixel matrices from the incident digital image pixel matrix. An output digital image pixel matrix is then generated which comprises a first group of pixels taken from a first one of the virtual sub-image pixel matrices plus a second group of pixels selectively taken from one of the incident digital image pixel matrix and a second one of the virtual sub-image pixel matrices. 
     In accordance with another embodiment, a method for processing a received digital image which includes pixels comprises identifying certain pixels within the received digital image for direct inclusion in an output digital image. The received digital image is displaced by a certain distance to create a displaced digital image. For each certain pixel location in the output digital image not containing directly included pixels from the received digital image, a substitution factor is determined which takes into account variations between that certain pixel location and adjacent pixel locations. Based on the determined substitution factor, one of a pixel from the received digital image, a pixel from the displaced digital image or a pixel which is a combination of pixels from the received and displaced digital images is selectively chosen as the pixel at that certain pixel location. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A more complete understanding of the method and apparatus of the present invention may be acquired by reference to the following Detailed Description when taken in conjunction with the accompanying Drawings wherein: 
         FIG. 1  shows one embodiment of the device according to the invention; 
         FIG. 2  shows an exemplary embodiment of the artifact reduction method according to the invention; 
         FIG. 3  shows one embodiment of a part of the device according to the invention; and 
         FIGS. 4 and 5  illustrate an example of formation of the final digital image according to the invention, as a function of the value of the substitution information. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In  FIG. 1 , one embodiment of a device DIS according to the invention is shown. 
     The device DIS shown is situated at the end of a digital image transmission system, so as to perform a post-processing on the image that, prior to that, has been compressed, transmitted, then decompressed by the various processing modules of the transmission system (not shown). 
     The reference INI denotes an incident digital image delivered to the device DIS at the output of the decompression phase. The image INI is delivered pixel by pixel to determination means MD. These determination means MD will allow pixels, called “displaced” pixels, to be determined for certain pixels, for example one pixel in two, from the incident image. 
     In other words, the determination means MD will allow two virtual sub-images to be constructed, a first virtual sub-image being formed from one part of the incident image pixels, and a second virtual sub-image being formed from the pixels that are complementary to the first virtual sub-image and shifted by a given displacement. 
     This displacement can, for example, correspond to a translation along the orthogonal axes of the image, of respective amplitudes n and m, whose sum n+m is even and non-zero. A precise mode of operation of the determination means MD will be detailed hereinafter. 
     The pixels of the two virtual sub-images are respectively delivered to a delay unit DL 1  and to calculation means MC connected to the output of the determination means MD. 
     In parallel with the delay unit DL 1 , the device DIS also comprises a second delay unit DL 2  receiving at the input, pixel by pixel, the pixels of the incident digital image INI. 
     The delay units DL 1  and DL 2  are in this example used to allow the shifting of the pixels to be shifted, and the synchronization of the output. 
     The object of the calculation means MC is to detect whether the region of the pixels adjacent to the pixel being considered in the incident image is sufficiently uniform to authorize the substitution of the pixel being considered. For that purpose, the calculation means can for example use a filter of the gradient type, such as: 
     
       
         
           
             
               
                 
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     More precisely, if a pixel with coordinates (x, y) is considered, the contribution of the red component in the value of Gradout is:
 
 Abs (pixel( x,y )) r =absolute value( in ( x− 1, y− 1)− in ( x,y− 1)+ in ( x+ 1 ,y− 1)− in ( x− 1 ,y )− in ( x+ 1 ,y )+ in ( x− 1, y+ 1)− in ( x,y+ 1)+ in ( x+ 1, y+ 1))
 
where in(x,y) represents the value of the red component for the pixel of coordinates (x,y).
 
     In the example cited, a zero weighting is assigned to the central pixel of the filter, which is the pixel being considered, in order to study the variations within the region surrounding this pixel and avoid the multiplications in the calculation. 
     The region is considered as sufficiently uniform if the sum Abs(pixel (x,y)) of the coefficients at each filter for each RGB component is close to zero. 
     Of course, other digital filters may be used for analyzing the uniformity of the region surrounding the pixel being considered. 
     The calculation means MC therefore deliver a value α at the output that corresponds to the value delivered by the normalized gradient filter Gradout, in other words α=Gradout*norme where the value of the normalizing parameter norme depends on the filters used in the calculation means. 
     Thus, α is a factor varying from 0 to 1. Its variations will be described in more detail hereinafter. 
     The values supplied by the various units DL 1 , MC and DL 2 , respectively A, α and B, are delivered to a mixer MEL whose objective is to deliver a pixel forming part of the final digital image INF, depending on the substitution information α. 
     The information A received by the mixer MEL corresponds to a pixel of the incident digital image INI or else to a displaced pixel. As regards the information B, this corresponds to a pixel of the incident digital image INI. 
     In the case where the information A corresponds to a pixel of the incident digital image INI, the output OUT of the mixer MEL systematically corresponds to an initial pixel of the incident image INI. 
     In the case where the information A corresponds to a pixel displaced by the determination means MD, the information OUT can be equal to three different types of pixels depending on the value taken by the substitution information α, namely the pixel being considered, the displaced pixel or a sum weighted by a of these two pixels, according to the following equation:
 
OUT=(1−α)* A+α*B  
 
     The conditions for substitution of the pixel being considered from the incident digital image INI will be described in more detail hereinafter. 
     Reference is now made to  FIG. 2 , which illustrates in more detail, for certain pixels being considered from the image, the determination of the displaced pixels. 
     During a first step, the incident digital image INI, comprising in this example 8 rows and 8 columns of pixels numbered from 1 to 8 for the columns and from 100 to 800 for the rows, is considered. The reference INI could also represent an 8*8 block extracted from a incident image. 
     For this example, the incident digital image INI comprises four distinct regions represented by four different frames. 
     During a second step, two virtual sub-images INIF and INID are generated. The first virtual sub-image INIF comprises one pixel in two from the incident digital image INI. The second virtual sub-image INID is a sub-image that is complementary to the first sub-image INIF. In addition, each of the pixels of the second virtual sub-image INID is displaced by a given displacement with respect to its location within the incident image. This displacement comprises, in this example, in incrementing the abscissa of each pixel by n=2 and the ordinate of each pixel by m=200. 
     During a third step, the final digital image INIF is reconstituted by associating the two sub-images INIF and INID as a function of the value of the substitution information α. 
     Thus, the final digital image INF comprises the first virtual sub-image INIF, then, for each pixel missing from the first sub-image INIF, a substitution pixel belonging either to the second displaced sub-image INID, or to the initial digital image if the substitution has not been authorized by the substitution information α. Thus, for example, the final digital image INF comprises the initial pixel number  102 , then, in place of the initial pixel  103 , the pixel  301  from the second displaced virtual sub-image INID which is at the location of the pixel  103  following the displacement of the second sub-image INID. 
     For the pixels located at the edges of the final digital image INF, given that a substitution with pixels belonging to the second displaced virtual sub-image INID cannot be made, the pixels missing from the final digital image INF are completed by the initial pixels from the incident digital image INI. Thus, for example, the pixel  101  is conserved in the final digital image INF. 
     Reference is now made to  FIG. 3 , which represents one embodiment of the determination means. 
     In order to successively deliver the pixels belonging to the first virtual sub-image and to the second virtual sub-image, the determination means timed by a clock (not shown) comprise two multiplexers xmux and ymux, each being assigned to one axis of the image, the abscissa axis and the ordinate axis, respectively. 
     The outputs of the multiplexers xmux and ymux are connected to an adder add, which delivers at its output the address xyout corresponding to the number of the pixel delivered by the information A to the mixer MEL. 
     The inputs of the first multiplexer xmux are respectively connected to a pair of counters xcnt 1  and xcnt 2 , the second counter xcnt 2  being displaced with respect to the first counter xcnt 1  by the displacement along the abscissa axis, equal to 2 in this example. 
     The first pointer xmux also receives at its input a control variable xsel, alternatively taking the values “0” and “1”, so as to connect the output of the multiplexer xmux to one or the other of its inputs. 
     In the case where the edge of the image is processed, a variable EN controlling the value of xsel takes the value “0”, forcing the value of xsel to “0”. The multiplexer xmux then delivers the value received at its first input, so as to form the address of a pixel belonging to the initial image. 
     In a similar manner as for the first multiplexer, the inputs of the second multiplexer ymux are respectively connected to a second pair of counters ycnt 1  and ycnt 2 , the second counter ycnt 2  being displaced with respect to the first counter ycnt 1  by the displacement along the ordinate axis, equal to 200 in this example. 
     The second multiplexer ymux also receives at its input a control value ysel controlled by the variable EN such that the case of the edge of the image is processed in the same manner as for the first multiplexer xmux. 
     The adder add performs the sum of the values delivered by the first and the second multiplexer xmux and ymux, so as to form the address of the pixel to be delivered at the output. 
     This address will allow the determination means MD to go and point to the pixel designated by the address xyout stored in a memory (not shown) associated with the device DIS. 
     Another means for generating the two virtual sub-images would comprise using two masks by means of two filters that would mask one pixel in two from the incident digital image. 
     Reference is now made to  FIG. 4 , which describes more precisely the variation of the substitution information allowing the replacement or not of the pixel being considered by the substitution pixel. 
     The curve in  FIG. 4  shows the behavior of the output variable OUT of the mixer MEL as a function of the values taken by a which varies linearly with respect to Gradout. 
     As a variant, curves of the substitution information a that are not a linear function of Gradout could be considered. 
     If the information A contains a displaced pixel, capable of replacing a pixel being considered from the incident digital image, and the substitution information α is equal to 0, then the substitution is authorized and the pixel delivered at the output for forming the final digital image is equal to the displaced pixel. 
     In the case where α is in the range between 0 and 1, the pixel delivered by the mixer is equal to a sum weighted by a of the pixel contained in the information A and of the pixel contained in the information B. 
     Lastly, in the case where α is equal to 1, the substitution is then disallowed and the pixel being considered from the incident digital image is conserved. The pixel delivered at the output is then equal to the pixel contained in the information B. 
     The example shown in  FIG. 4  corresponds to an ideal case. In practice, as illustrated in  FIG. 5 , thresholds exist that are respectively a first threshold Gradmin and a second threshold Gradmax. If Gradout is less than Gradmin, then the pixel being considered from the incident digital image is replaced by the displaced pixel determined by the determination means MD. 
     If Gradout is greater than the threshold Gradmax, then the initial pixel from the incident digital image INI is conserved for generating the final digital image INF. 
     If Gradout is in the range between these two thresholds, then the pixel of the final digital image INF is equal to the weighted sum of the pixel being considered from the incident digital image and of the displaced pixel corresponding to the pixel being considered. 
     In the case where the permutation is not authorized, since the pixel is for example at the edge of the image, the information A and information B designate the same pixel and the value of the substitution information α is then irrelevant. 
     Furthermore, the values of the gradient thresholds Gradmin and Gradmax may be controlled as a function of the parameters of the decoders employed, if the capability exists. 
     Although preferred embodiments of the method and apparatus of the present invention have been illustrated in the accompanying Drawings and described in the foregoing Detailed Description, it will be understood that the invention is not limited to the embodiments disclosed, but is capable of numerous rearrangements, modifications and substitutions without departing from the spirit of the invention as set forth and defined by the following claims.