Patent Publication Number: US-7587098-B2

Title: Pixel data generating method

Description:
BACKGROUND 
   The invention is related to an image processing method, and more particularly, to methods generating pixel data for a missing block in an image frame. 
     FIG. 1  shows a conventional method of generating a missing block pixel matrix  12  in an image frame  10 . The surrounding reference pixels  111 ,  112 ,  113  and  114  could easily acquired for a missing pixel  121  of the missing block pixel matrix  12 . These reference pixels  111 ,  112 ,  113  and  114  can be generated by finding pixels with matching vertical or horizontal position, on the neighboring image boundary  11  adjacent to missing block  12 . A reference pixel contributes the recovery of missing data depending on distance from the missing pixel. Combining contributions from all reference pixels regenerates a missing pixel data. The missing block is a linear color gradient filling of the neighboring image boundary, vertically and horizontally. 
   For images with non-vertical or non-horizontal patterns, the conventional method cannot efficiently regenerate missing block data with satisfaction.  FIG. 2A  shows an image before a block is lost.  FIG. 2B  shows a resulting image generated by the conventional method from the image of  FIG. 2A . The patterns of the image are cut vertically and horizontally. It is necessary to have a method that can regenerate patterns of images. 
   SUMMARY 
   A method of generating pixel data of a missing block in an image frame is disclosed. Edge points are detected from neighboring image sides adjacent to the missing block. A direction is calculated for each edge point. Edge lines are formed from edge points based on the direction thereof to partition the missing block into a plurality of missing regions. Data for missing pixels in each missing region are then calculated using reference pixels from neighboring image sides that adjacent to the missing region. 

   
     DESCRIPTION OF THE DRAWINGS 
     The accompanying drawings incorporated in and constitute a part of this specification, illustrate embodiments of the invention and, together with the description, serve to explain the features, advantages, and principles of the invention. 
       FIG. 1  shows a conventional method of generating a missing block pixel matrix  12  in an image frame  10 . 
       FIG. 2A  shows an image before a block is lost. 
       FIG. 2B  shows a resulting image generated by the conventional method from the image of  FIG. 2A . 
       FIG. 3A  is a flow chart of a method according to an embodiment of the invention. 
       FIG. 3B˜3H  illustrate the method shown in  FIG. 3A . 
   

   DETAILED DESCRIPTION 
   Patterns in images are distinguished by color change. Edges are formed between colors, and outline the patterns. By detecting edges from neighboring images of a missing block and extending edges into the missing block, patterns of an image can be rebuilt. 
     FIG. 3A  is a flow chart of a method according to an embodiment of the invention. First, edge points are detected from the neighboring image sides adjacent to the missing block (Step S 1 ). Second, a direction is calculated for each edge point (Step S 2 ). Third, edge lines are formed from edge points based on the direction thereof to partition the missing block into a plurality of missing regions (Step S 3 ). Data of missing pixels in each missing region is then calculated using reference pixels from neighboring image sides adjacent to the missing region (Step S 4 ). 
     FIG. 3B  shows an image  30  with missing block  31 . Edge points are detected from the neighboring image side adjacent to the missing block according to Step S 1  of  FIG. 3A .  FIG. 3C  illustrates steps of edge point detection from side  310  of  FIG. 3B . Illustration  310 A shows edge points  3101 ,  3102 ,  3103  and  3104  detected from side  310 , when the adjacent pixel difference exceeds maximum of 2*MAD (mean absolute difference) and QP/2 such as; 
   
     
       
         
           
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   Wherein X i  is pixel data and QP is a quantization parameter associated with the image frame. 
   In general, multiple edge points are detected around a true edge except sharp step edges, such as edge points  3102 ,  3103  and  3104  detected for just one real edge. In order to determine a true single edge point, edge thinning is performed such that one edge point with maximum edge magnitude is chosen. Illustration  310 B shows edge point  3103  chosen to represent a real edge point. 
     FIG. 3D  shows a k*k pixel matrix  33 , where k is odd number between 1 and 16, and k is chosen as 5 in this embodiment, containing the edge point  3101  chosen for direction calculation according to step  2  of  FIG. 3A . 3*3 Sobel Filter is chosen in this embodiment to calculate the direction of the edge point. Calculating row and column gradients, G R (n) and G C (n), of the k*k pixel matrix using 3*3 Sobel Filter results (k−2)*(k−2) set of G R (n) and G C (n). Edge angles and magnitudes can then be calculated from the G R (n) &amp; G C (n). For example, an edge angle can be calculated as 
               tan     -   1       ⁡     (         G   R     ⁡     (   n   )           G   C     ⁡     (   n   )         )       ,         
and an edge magnitude can be calculated as √{square root over ((G R (n)) 2 +(G C (n)) 2 )}{square root over ((G R (n)) 2 +(G C (n)) 2 )}. An edge angle is increased by π when the edge angle is lower than zero, making the edge angle positive. An average edge angle and magnitude of k*k pixel matrix are then calculated as the direction of the edge point. A set of edge angle and magnitude is discarded from calculating the average edge angle and magnitude when the edge magnitude is less than 2*MAD. Adaptive size of pixel matrix can provide more accurate edge direction property for the context adaptive edge points.
 
   An image with complicated information contains many small color patterns. Edge points may be found for small patterns on the sides of the missing block. Connecting edge points of small patterns with compatible direction of edges but different colors may crumble the missing block image. An edge point with statistic lower than a statistic threshold, which is 30% in this embodiment, then can be discarded. The statistic is calculated by: quantizing the (k−2)*(k−2) edge angles and the average angle of k*k pixel matrix with a quantizing parameter π/32, accumulating edge magnitudes with corresponding quantized edge angle equals to the quantized average edge angle, and dividing the accumulated edge magnitudes by sum of all edge magnitudes. An edge magnitude lower than MAD will be discarded from statistic calculation. Low statistic of an edge point means the neighboring image has low edge angel consistency and should not be connected to other side of the missing block. 
   A method of forming edge lines from edge points based on the direction thereof to partition the missing block into a plurality of missing regions according to step S 3  of  FIG. 3A  is described here. A linked edge line is defined for each pair of edge points with compatible directions, and an isolated edge line is defined from each unpaired edge point into the missing block along the direction of the unpaired edge point ending on a linked edge line. 
   A pair of edge points with compatible directions is chosen based on edge angles and magnitudes. All directions of edge points are normalized by dividing edge angles by Π and dividing edge magnitudes by a maximum storable value, such as 256 for an 8-bit image data system. Selecting edge point pairs with a normalized edge magnitude difference lower than 4*QP/N2 and normalized edge angle difference lower than Π/8 forms a linked edge line. Thresholds like 4*QP/N2 or Π/8 can be defined as other values to adapt to different image process requirements. 
     FIG. 3E  shows a linked edge line formed by connecting edge point A and B according to an embodiment of the invention. The linked edge line has a slope S and the edge points A and B have directions DA and DB respectively. The linked edge line is formed due to DA and DB compatibility. However, if angle difference of slope S to DA or DB is significant, inconsistent data is generated in the missing block. An angle difference of slope S to DA and DB is calculated. If the angle difference exceeds a threshold 0.125*Π, the linked edge line is discarded and both edge points are deemed unpaired. 
     FIG. 3F  shows detected edge points and formed edge lines for missing region  31  in image  30  of  FIG. 3B  according to an embodiment of the invention. Edge point pairs ( 3101 ,  3131 ) and ( 3103 ,  3122 ) form linked lines  1  and  2  respectively, and unpaired edge points  3111  and  3121  form the isolated edge lines  4  and  3  respectively. By treating the horizontal position as x and the vertical position as y and setting one corner of the missing block as (0, 0), a ax+by+c=0 function can represent each edge line. 
     FIG. 3G  shows a binary tree structure representing edge lines and edge regions of the missing block  31  of  FIG. 3F . Nodes  1 ,  2 ,  3  and  4  are created to represent edge lines  1 ,  2 ,  3  and  4 . Each node has a first branch to link a node in the positive region of the edge line (ax+by+c≧0) and a second branch to link a node in the negative region of the edge line (ax+by+c≦0). Linked nodes are connected first then isolated nodes. Unconnected branches A, B, C, D and E on the binary tree structure represent missing regions A, B, C, D and E, respectively. 
   A linked node of edge points with minimum of √{square root over ((P i -P j ) 2 +  ω {square root over ((P i -P j ) 2 +  ω   θ ·(θ i -θ j ) 2 )} can be chosen as a starting node of the binary tree structure; wherein P i  and P j  are normalized edge magnitudes of 2 edge points and θ i  and θ j  are normalized edge angles of the 2 edge points, and ω θ  is a predetermined weight coefficient. ω θ =2 is chosen to favor the edge angle contribution in this embodiment. 
     FIG. 3H  shows a method of locating reference pixels from neighboring image sides adjacent to the missing region H for missing pixel  340 , according to step S 4  of  FIG. 3A . Missing pixel  340  is located in the missing region associated to linked edge line  341  and isolated edge line  342 . Virtual line functions are generated past missing pixel  340  with slopes the same as line  342  and  341 , such as dashed lines  344  and  343  connecting neighboring image sides in pixels  3401 ,  3402 ,  3403  and  3404 . Pixels  3401  and  3403  are discarded not being adjacent to the missing region H. Pixels  3402  and  3403  are then chosen as reference pixels. All missing pixels have different numbers of reference pixels on 4 sides. 
   Data of a missing pixel is then calculated as the sum of the contributions of each reference pixel, wherein the contribution of each reference pixel being the pixel data times a weight coefficient. Missing pixel data {circumflex over (x)} j  can be calculated as: 
   
     
       
         
           
             
               
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   Wherein a i  is a weight coefficient equaling a normalized distance between the missing pixel and reference pixels. 
   The invention discloses methods of generating pixel data for a missing block in an image frame, providing smoothness and consistency applicable in macro-block lost error concealment in video communication or image manipulation. An unwanted block in an image can be deleted, and then regenerated using data from the neighborhood image by methods according to the invention. 
   While the invention has been described by way of example and in terms of preferred embodiment, it is to be understood that the invention is not limited thereto. Those skilled in this technology can still make various alterations and modifications without departing from the scope and spirit of this invention. Therefore, the scope of the present invention shall be defined and protected by the following claims and their equivalents.