Abstract:
The invention proposes a motion compensated video interpolation that is not sensitive to the errors in the motion estimation vectors. The invention proposes a motion compensated video interpolation method in which the interpolation step varies according to the amplitude and/or a reliability index of the estimated motion vector for the considered pixel. The number of pixels to be taken into account for interpolation and the weighting factors associated with these pixels are made to vary according to the amplitude and/or reliability index of the estimated motion vector.

Description:
This application claims the benefit, under 35 U.S.C. §119 of EP Patent Application 0654213 filed 12 Oct. 2006. 
     FIELD OF THE INVENTION 
     The invention relates to the field of image or video processing and more specifically to the field of image interpolation with motion compensation. 
     TECHNOLOGICAL BACKGROUND OF THE INVENTION 
     A number of video processing operations call for motion compensation for improving the quality of the processing, for example in the case of a video interpolation, or for reducing data volumes, for instance in the case of a video compression. This invention relates more specifically to interpolation with motion compensation, also called motion compensated video interpolation. 
     A motion compensated video interpolation generally consists of a motion estimation step followed by an interpolation step. 
     The motion estimation consists in defining a motion vector for each of the points of the image to interpolate, the image being temporally positioned between two source images. Usually, the motion estimation step is carried out in 2 steps: a prediction step and a correction step. The prediction step consists in defining, for each one of the pixels of the image to interpolate, a motion vector from the motion vectors that have already been calculated for neighbouring pixels and from the projection of a motion vector calculated for the previous source image and including the considered pixel. The motion vectors already calculated are for example those of the n neighbouring pixels of the previous line of pixels. Prediction then consists in selecting, among the n+1 pre-calculated motion vectors, the motion vector generating the smallest DFD (Displacement Frame Difference) value. The correction step subsequently consists in rotating the selected motion vector around its axis in order to furthermore reduce, if possible, the DFD of the considered pixel. 
     The interpolation step, which follows the estimation step, is a bilinear interpolation in the majority of the cases. In the case of an image to interpolate I int  between a previous source image I prev  and a current source image I curr , the previously defined motion vector points to an image point of each one of the source images, this image point being either a pixel, or a point located between 4 pixels. In the case of an interpolation known as “single frame”, bilinear interpolation then consists in assigning to the considered P of the image I int  a value which is a bilinear function f of the values of the 4 pixels pointed to in one of the source images (generally, the current source image) by the estimated motion vector. For example, if V 1 , V 2 , V 3  and V 4  designate the values of the 4 pixels near the image point pointed to by the motion vector in the current source image and if α 1 , α 2 , α 3  and α 4  are weighting factors representative of the proximity of said pixels to the image point, the value assigned to the pixel of the image I int  is 
               (       ∑     i   =   1     4     ⁢       α   i     ·     V   i         )     /       ∑     i   =   1     4     ⁢       α   i     .             
If the motion vector points to a specific pixel among the 4 pixels, the weighting factor assigned to the other 3 pixels is nil and the value assigned to the considered pixel is the value of this specific pixel. In the case of an interpolation known as “double frame”, the bilinear interpolation is done in the same manner but with 8 pixels, i.e. 4 pixels of the previous source image and 4 pixels of the current source image.
 
     Given that motion estimation is never without errors, this type of compensated interpolation in movement contains visible defects in the image areas that disappear or appear between two successive source images since matching between pixels is not possible then. 
     SUMMARY OF THE INVENTION 
     The invention proposes a new motion compensated video interpolation that is less critical, especially for the appearance or disappearance areas of objects, than the one defined previously. 
     The invention proposes a motion compensated video interpolation method in which the interpolation step itself varies according to the amplitude of the estimated motion vector and/or a reliability index. The number of pixels to be taken into account for interpolation and the weighting factors associated with these pixels are made to vary according to the amplitude and/or reliability index of the estimated motion vector. 
     The invention therefore relates to a method for interpolating at least one pixel of a motion compensated image from the pixels of at least two source images, comprising the following steps:
         for the pixel of the image to interpolate, estimating a motion vector from the source images,   determining an information representative of the estimation of said vector,   selecting an interpolation pattern for the pixel of the image to interpolate in accordance with said information representative of the estimation of said vector, the interpolation pattern identifying pixels in at least one of the source images and associating to each one of the identified pixels a weighting factor, and   interpolating the pixel of the image from the pixels and the factors of the selected interpolation pattern.       

     The information representative of the estimation of the vector can be either the amplitude of the estimated vector, or a reliability index relating to the estimation of the vector, or else both of these. 
     According to a specific embodiment, only the amplitude of the estimated motion vector is taken into account for selecting the interpolation pattern. In this case, we can consider several levels of interpolation. 
     If the amplitude of the estimated motion vector of the pixel of the image to interpolate is less than or equal to a first non zero threshold, the interpolation pattern comprises the n pixels of one of the source images nearest to the spatial position of the pixel to interpolate, n being greater than or equal to 2, and the weighting factor assigned to each of the source image pixels is dependent on their proximity to the pixel to interpolate. In this case, this involves a single frame interpolation. In the case of a double frame interpolation, the interpolation pattern comprises the n pixels of each one of the source images nearest to the spatial position of the pixel to interpolate, n being greater than or equal to 2, and the weighting factor assigned to each of the source image pixels is dependent on their proximity to the pixel to interpolate and the temporal position of the image to interpolate with respect to both the source images. 
     If the estimated motion vector of a pixel of the image to interpolate is greater than the first threshold, the interpolation pattern consists, in the case of a single frame interpolation, of the n pixels of one of the source images nearest to the spatial position of the pixel to interpolate, n being greater than or equal to 2, and the weighting factors assigned to the source image pixels are all equal. In the case of a double frame interpolation, the interpolation pattern comprises the n pixels nearest to the spatial position of the pixel to interpolate in each one of the source images, n being greater than or equal to 2, and the weighting factor assigned to each of the source image pixels is dependent on their proximity to the pixel to interpolate and the temporal position of the image to interpolate with respect to both the source images. The number n of pixels increases appropriately with the amplitude of the estimated vector. 
     According to another specific embodiment, only a reliability index relating to the estimated motion vector is taken into account for selecting the interpolation pattern. This reliability index depends for instance on the variance of the motion vectors in a window of m×m pixels including the pixel to interpolate. In this case, we can also consider several levels of interpolation. 
     If the reliability index of the motion vector of the pixel of the image to interpolate is greater than a first reliability threshold, the interpolation pattern consists, in the case of a single frame interpolation, of the n pixels of one of the source images nearest to the spatial position of the pixel to interpolate, n being greater than or equal to 2, and the weighting factors assigned to each one of the source image pixels is dependent on their proximity to the pixel to interpolate. In the case of a double frame interpolation, the interpolation pattern comprises the n pixels of each one of the source images nearest to the spatial position of the pixel to interpolate, n being greater than or equal to 2, and the weighting factor assigned to each of the source image pixels is dependent on their proximity to the pixel to interpolate and the temporal position of the image to interpolate with respect to both the source images. 
     If the reliability index of the motion vector of the pixel of the image to interpolate is less than or equal to the first reliability threshold, the interpolation pattern consists, in the case of a single frame interpolation, of the n pixels of one of the source images nearest to the spatial position of the pixel to interpolate, n being greater than or equal to 2, and the weighting factors assigned to the source image pixels are equal. In the case of a double frame interpolation, the interpolation pattern comprises the n pixels nearest to the spatial position of the pixel to interpolate in each one of the source images, n being greater than or equal to 2, and the weighting factor assigned to each of the source image pixels is dependent on their proximity to the pixel to interpolate and the temporal position of the image to interpolate with respect to both the source images. The number n of pixels decreases advantageously with the value of the reliability index of the estimated vector. 
     According to another specific embodiment, the amplitude of the vector and the reliability index relating to the vector are taken into account for selecting the interpolation pattern. 
     Finally, the invention also relates to an interpolation device of at least one pixel of a motion compensated image from at least two source images, comprising:
         a memory for storing the source images;   a motion estimator for estimating, for the pixel of the image to interpolate, a motion vector from the source images,   a determination circuit for determining an information representative of the estimation of said vector, and   an interpolation circuit for selecting an interpolation pattern for the pixel of the image to interpolate in accordance with said information representative of the estimation of said vector, the interpolation pattern identifying pixels in at least one of the source images and associating a weighting factor with each one of the identified pixels and interpolating the pixel from the pixels and the factors of the selected interpolation pattern.       

    
    
     
       LIST OF FIGURES 
       The invention will be better understood upon reading the following description, provided for information only and referring to the annexed drawings wherein: 
         FIG. 1  is a flowchart representing the steps of the invention method in a first embodiment based on the amplitude of the estimated motion vector, 
         FIG. 2  is a diagram showing different interpolation patterns to be applied according to the amplitude of the estimated motion vector, 
         FIG. 3  more specifically illustrates the interpolation in the case wherein the estimated motion vector is less than a threshold SA 1 , 
         FIG. 4  more specifically illustrates the interpolation in the case wherein the estimated motion vector is between the threshold SA 1  and a threshold SA 2 , 
         FIG. 5  more specifically illustrates interpolation in the case wherein the estimated motion vector is greater than the threshold SA 2 , 
         FIG. 6  is a device capable of implementing the method of  FIG. 1 , 
         FIG. 7  is a flowchart representing the steps of the invention method in a second embodiment based on a reliability index of the estimated movement, 
         FIG. 8  is a diagram showing different interpolation patterns to be applied according to a reliability index of the estimated motion vector, 
         FIG. 9  is a device capable of implementing the method of  FIG. 7 , 
         FIG. 10  is a flowchart representing the steps of the invention method in a third embodiment based on the amplitude and a reliability index of the estimated motion vector, 
         FIG. 11  is a diagram showing different interpolation patterns to be applied according to the amplitude and a reliability index of the estimated motion vector, 
         FIG. 12  is a device capable of implementing the process of  FIG. 10 . 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     As previously indicated, the principle of the invention is to make the interpolation pattern vary, i.e. the number of pixels on which the interpolation and/or the associated weighting factors are based, according to the amplitude of the estimated motion vector for the considered pixel and or/a reliability index associated with the estimated motion vector. 
     Three embodiments are described hereinafter: a first embodiment based solely on the amplitude of the estimated motion vector, a second embodiment based solely on a reliability index of the estimated motion vector and a third embodiment combining both the above mentioned embodiments. 
       FIG. 1  is a flowchart representing the steps of the first embodiment of the invention method. It comprises:
         a motion estimation step  100  for estimating a motion vector (Vx,Vy) for each pixel of the image to interpolate,   a step  110  for determining the amplitude A of the estimated motion vector (A=√{square root over (Vx 2 +Vy 2 )}),   an interpolation pattern selection step  120  for each pixel of the image to interpolate according to the amplitude of the estimated motion vector for this pixel,   an interpolation step  130  for generating an interpolated image by using, for each pixel of this image, the interpolation pattern selected for this pixel.       

       FIG. 2  more specifically illustrates the interpolation step  130  and shows different types of interpolation (bilinear interpolation, interpolation by averaging) and various patterns to be applied according to the amplitude of the estimated motion vector. It can be noted that the direction of the motion vector (positive or negative motion vector) is not involved in the choice of the interpolation. In this embodiment, we use bilinear interpolations or averaging interpolations based on a plurality of neighbouring pixels. For each one of these types of interpolation, the higher the amplitude of the estimated motion vector, the greater is the increase in the number of pixels of the interpolation pattern. 
     If the amplitude of the motion vector is less than or equal to a first threshold SA 1 , the motion is considered as being low. The threshold SA 1  is for instance fixed at 2 pixels (per frame). The interpolation is then for example a bilinear interpolation based on 4 pixels, these 4 pixels being the neighbouring pixels of the pixel considered in the source images. Neighbouring pixels are understood to mean the source image pixels whose spatial coordinates are close to the coordinates of the considered pixel of the image to interpolate. In the case of a single frame interpolation, only the 4 neighbouring pixels of one of the source images are used. This case is illustrated by  FIG. 3 . The 4 neighbouring pixels are noted P 1 , P 2 , P 3  and P 4 , the video values associated with these 4 pixels are designated by V 1 , V 2 , V 3  and V 4  and the associated weighting factors are noted α 1 , α 2 , α 3  and α 4 . The value of the factor α i  is a function of the distance separating the associated pixel P i  and the pixel to interpolate. In the example of  FIG. 3 , the pixel P 1  is the nearest to the pixel to interpolate and its factor, α 1 , is therefore the highest. The pixel P 4  is the farthest from the pixel to interpolate and its factor, α 4 , is therefore the lowest. The value assigned to the pixel to interpolate is then equal to 
               (       ∑     i   =   1     4     ⁢       α   i     ·     V   i         )     /       ∑     i   =   1     4     ⁢       α   i     .             
Quite obviously and as indicated previously, if the motion vector points to a specific pixel among the 4 pixels, the weighting factor assigned to the other 3 pixels is nil. In the case of a double frame interpolation, bilinear interpolation is done on the 4 neighbouring pixels of the previous source image and the 4 pixels of the current source image. If V 1 , V 2 , V 3  and V 4  designate the values of the 4 neighbouring pixels of the current source image and α 1 , α 2 , α 3  and a designate the associated weighting factors and if V′ 1 , V′ 2 , V′ 3  and V′ 4  designate the values of the 4 neighbouring pixels of the previous source image and α′ 1 , α′ 2 , α′ 3  and α′ 4  designate the associated weighting factors, the value assigned to the pixel to interpolate is then equal to β.
 
               (       (       ∑     i   =   1     4     ⁢       α   i     ·     V   i         )     /     (       ∑     i   =   1     4     ⁢     α   i       )       )     +       (     1   -   β     )     ·       (       (       ∑     i   =   1     4     ⁢       α   i   ′     ·     V   i   ′         )     /     (       ∑     i   =   1     4     ⁢     α   i   ′       )       )     .             
In this formula, β is a factor that is dependent on the temporal position of the image to interpolate with respect to the source images. If the image to interpolate is temporally positioned at mid-distance between the source images, β=½. If the image to interpolate is more of the current source image source than the previous source image, β is greater than ½.
 
     If the amplitude of the estimated motion vector for a given pixel is nil, the motion vector then points to the pixel having the same spatial coordinates in one and the other of the previous and current source images. Bilinear interpolation amounts to recopying the value of the corresponding pixel, i.e. having the same spatial coordinates, from one of the previous or current source images. It may be noted that, if the previous and current source images are progressive images obtained by deinterleaving of an interleaved video sequence, it is possible that the values of the pixels which ought to be equal (since they are linked by a nil motion vector) are not so. In this case, we can still recopy one or the other of the values of the pixels pointed to by the motion vector or possibly work out the average. 
     If the amplitude of the motion vector is greater than the threshold SA 1  and less than or equal to a second threshold SA 2  greater than SA 1 , the motion is considered to be average. The threshold SA 2  is for instance fixed at 6 pixels (per frame). Interpolation is then an averaging step based on the 4 neighbouring pixels P 1 , P 2 , P 3  and P 4  of the pixel considered in the source images. This case is illustrated by  FIG. 4 . The weighting factors of the 4 neighbouring pixels are equal (α 1 =α 2 =α 3 =α 4 =α). In the case of a single frame interpolation, the value assigned to the pixel to interpolate is then equal to 
                 (       ∑     i   =   1     4     ⁢       α   i     ·     V   i         )     /       ∑     i   =   1     4     ⁢     α   i         =       (       ∑     i   =   1     4     ⁢       α   i     ·     V   i         )     /   4.           
In the case of a double frame interpolation where (α′ 1 =α′ 2 =α′ 3 =α′ 4 =α′), the value assigned to the pixel to interpolate is then equal to
 
     
       
         
           
             
               
                 β 
                 · 
                 
                   ( 
                   
                     
                       ( 
                       
                         
                           ∑ 
                           
                             i 
                             = 
                             1 
                           
                           4 
                         
                         ⁢ 
                         
                           
                             α 
                             i 
                           
                           · 
                           
                             V 
                             i 
                           
                         
                       
                       ) 
                     
                     / 
                     
                       ( 
                       
                         
                           ∑ 
                           
                             i 
                             = 
                             1 
                           
                           4 
                         
                         ⁢ 
                         
                           α 
                           i 
                         
                       
                       ) 
                     
                   
                   ) 
                 
               
               + 
               
                 
                   ( 
                   
                     1 
                     - 
                     β 
                   
                   ) 
                 
                 ⁢ 
                 
                   ( 
                   
                     
                       ( 
                       
                         
                           ∑ 
                           
                             i 
                             = 
                             1 
                           
                           4 
                         
                         ⁢ 
                         
                           
                             α 
                             i 
                             ′ 
                           
                           · 
                           
                             V 
                             i 
                             ′ 
                           
                         
                       
                       ) 
                     
                     / 
                     
                       ( 
                       
                         
                           ∑ 
                           
                             i 
                             = 
                             1 
                           
                           4 
                         
                         ⁢ 
                         
                           α 
                           i 
                           ′ 
                         
                       
                       ) 
                     
                   
                   ) 
                 
               
             
             = 
             
               
                 ( 
                 
                   
                     β 
                     · 
                     
                       ( 
                       
                         
                           ∑ 
                           
                             i 
                             = 
                             1 
                           
                           4 
                         
                         ⁢ 
                         
                           V 
                           i 
                         
                       
                       ) 
                     
                   
                   + 
                   
                     
                       ( 
                       
                         1 
                         - 
                         β 
                       
                       ) 
                     
                     · 
                     
                       ( 
                       
                         
                           ∑ 
                           
                             i 
                             = 
                             1 
                           
                           4 
                         
                         ⁢ 
                         
                           V 
                           i 
                           ′ 
                         
                       
                       ) 
                     
                   
                 
                 ) 
               
               / 
               4. 
             
           
         
       
     
     Finally, if the amplitude of the motion vector is greater than the threshold SA 2 , the movement is considered to be high. Interpolation is then an averaging step based on 8 neighbouring pixels, for example those shown in  FIG. 5 . The weighting factors of the 8 neighbouring pixels are equal (=α). In the case of a single frame interpolation, the value assigned to the pixel to interpolate is then equal to 
               (       ∑     i   =   1     8     ⁢     V   i       )     /   8.         
In the case of a double frame interpolation, the value assigned to the pixel to interpolate is then equal to
 
     
       
         
           
             
               ( 
               
                 
                   β 
                   · 
                   
                     ( 
                     
                       
                         ∑ 
                         
                           i 
                           = 
                           1 
                         
                         8 
                       
                       ⁢ 
                       
                         V 
                         i 
                       
                     
                     ) 
                   
                 
                 + 
                 
                   
                     ( 
                     
                       1 
                       - 
                       β 
                     
                     ) 
                   
                   · 
                   
                     ( 
                     
                       
                         ∑ 
                         
                           i 
                           = 
                           1 
                         
                         8 
                       
                       ⁢ 
                       
                         V 
                         i 
                         ′ 
                       
                     
                     ) 
                   
                 
               
               ) 
             
             / 
             8. 
           
         
       
     
       FIG. 6  represents a device capable of implementing the previously described process. This device comprises at least two image memories  200  each of which is capable of memorizing a source image, namely the current source image and the previous-source image, a motion estimator  210  for determining a motion vector (Vx,Vy) for each one of the pixels of the image to interpolate from the current source image at the input of the device and the previous source image stored in one of the image memories  200 , a circuit  220  for determining the amplitude A of the estimated motion vectors coming from the estimator  210  and an interpolator  230  for generating the interpolated image from the current and previous source images stored in the image memories  200  and the amplitudes A determined by the circuit  220 . The interpolator  230  compares, for each pixel to interpolate, the amplitude A of the estimated motion vector for this pixel at the thresholds SA 1  and SA 2  and the nil value and subsequently applies the appropriate interpolation pattern. 
       FIG. 7  is a flowchart showing the steps of the second embodiment of the invention. It comprises:
         a motion estimation step  300  for estimating a motion vector (Vx,Vy) for each pixel of the image to interpolate,   a step  310  for determining a reliability index C for the estimated motion vectors, and   an interpolation pattern selection step  320  for each pixel of the image to interpolate according to the reliability index of the estimated motion vector, and   an interpolation step  330  for generating an interpolated image by using, for each pixel of this image, the interpolation pattern selected for this pixel.       

     In this embodiment, the lower this reliability index, the higher is the number of pixels used by the interpolation pattern. In fact, the interpolated image can be made more blurred when we have less reliability in the value of the estimated motion vector. This reliability index is determined for instance by analyzing the dispersion of the value of the motion vectors in an image area including the considered pixel. The more dispersed the values of the motion vectors in this area are, the lower is the reliability index associated to the considered pixel. The reliability index C is for example inversely proportional to the variance of the motion vectors in a window of 5×5 pixels including the considered pixel: 
             C   =     1   -           ∑       i   =   1     ,     j   =   1         5   ,   5       ⁢           (       V   ⁢           ⁢     x     3   ,   3         -     V   ⁢           ⁢     x     i   ,   j           )     2     -       (       V   ⁢           ⁢     y     3   ,   3         -     V   ⁢           ⁢     y     i   ,   j           )     2             24   ⁢           (       2   ·   V     ⁢           ⁢     x   max       )     2     +       (       2   ·   V     ⁢           ⁢     y   max       )     2             .             
The second term of the equation is standardized in order to be contained between 0 and 1. The coordinate pixel (3,3) designates the current pixel. Vx i,j  and Vy i,j  designate respectively the horizontal and vertical components of the motion vector of the pixel (i,j). Vx max  and Vy max  designate the maximum horizontal and vertical components of the motion vectors. In the case of an 8-bit coding of each of these components, Vx max =Vy max =255.
 
       FIG. 8  more specifically illustrates the interpolation step  330  and shows examples of interpolation to apply according to the value of the reliability index associated with the estimated motion vector. 
     If the value of the reliability index is less than or equal to a first threshold SC 1 , the reliability index is considered as being low. Interpolation is then for example an averaging interpolation based on 8 pixels, these 8 pixels being the neighbouring pixels of the pixel considered in the source images. In the case of a single frame interpolation, only the 8 neighbouring pixels of one of the source images are used. If we reuse the notations defined for the first embodiment compared with  FIGS. 2 to 5 , the value assigned to the pixel to interpolate is then equal to 
                 (       ∑     i   =   1     8     ⁢     α   ·     V   i         )     /       ∑     i   =   1     8     ⁢   α       =       (       ∑     i   =   1     8     ⁢     V   i       )     /   8.           
In the case of a double frame interpolation, interpolation is done on the 8 neighbouring pixels of the previous source image and the 8 pixels of the current source image. The value assigned to the pixel to interpolate is then equal to
 
     
       
         
           
             
               ( 
               
                 
                   β 
                   · 
                   
                     ( 
                     
                       
                         ∑ 
                         
                           i 
                           = 
                           1 
                         
                         8 
                       
                       ⁢ 
                       
                         V 
                         i 
                       
                     
                     ) 
                   
                 
                 + 
                 
                   
                     ( 
                     
                       1 
                       - 
                       β 
                     
                     ) 
                   
                   · 
                   
                     ( 
                     
                       
                         ∑ 
                         
                           i 
                           = 
                           1 
                         
                         8 
                       
                       ⁢ 
                       
                         V 
                         i 
                         ′ 
                       
                     
                     ) 
                   
                 
               
               ) 
             
             / 
             8. 
           
         
       
     
     If the value of the reliability index is greater than the threshold SC 1  and less than or equal to a second threshold SC 2  greater than SC 1 , the reliability index is considered as being medium. Interpolation is then for example an averaging interpolation based on 4 pixels, these 4 pixels being the neighbouring pixels of the pixel considered in the source images. In the case of a single frame interpolation, only the 4 neighbouring pixels of one of the source images are used. If we reuse the notations defined for the first embodiment in relation to  FIGS. 2 to 5 , the value assigned to the pixel to interpolate is then equal to 
                 (       ∑     i   =   1     4     ⁢     α   ·     V   i         )     /       ∑     i   =   1     4     ⁢   α       =       (       ∑     i   =   1     4     ⁢     V   i       )     /   4.           
In the case of a double frame interpolation, interpolation is done on the 4 neighbouring pixels of the previous source image and the 4 pixels of the current source image. The value assigned to the pixel to interpolate is then equal to
 
     
       
         
           
             
               ( 
               
                 
                   β 
                   · 
                   
                     ( 
                     
                       
                         ∑ 
                         
                           i 
                           = 
                           1 
                         
                         4 
                       
                       ⁢ 
                       
                         V 
                         i 
                       
                     
                     ) 
                   
                 
                 + 
                 
                   
                     ( 
                     
                       1 
                       - 
                       β 
                     
                     ) 
                   
                   · 
                   
                     ( 
                     
                       
                         ∑ 
                         
                           i 
                           = 
                           1 
                         
                         4 
                       
                       ⁢ 
                       
                         V 
                         i 
                         ′ 
                       
                     
                     ) 
                   
                 
               
               ) 
             
             / 
             4. 
           
         
       
     
     Finally, if the value of the reliability index is greater than the threshold SC 2 , the reliability index is considered as being strong. Interpolation is then for example a bilinear interpolation based on 4 pixels, these 4 pixels being the neighbouring pixels of the pixel considered in the source images. In the case of a single frame interpolation, only the 4 neighbouring pixels of one of the source images are used. If we take up the notations defined for the first embodiment in relation to  FIGS. 2 to 5 , the value assigned to the pixel to interpolate is then equal to 
               (       ∑     i   =   1     4     ⁢       α   i     ·     V   i         )     /       ∑     i   =   1     4     ⁢       α   i     .             
In the case of a double frame interpolation, bilinear interpolation is done on the 4 neighbouring pixels of the previous source image and the 4 pixels of the current source image. The value assigned to the pixel to interpolate is then equal to
 
     
       
         
           
             
               β 
               · 
               
                 ( 
                 
                   
                     ( 
                     
                       
                         ∑ 
                         
                           i 
                           = 
                           1 
                         
                         4 
                       
                       ⁢ 
                       
                         
                           α 
                           i 
                         
                         · 
                         
                           V 
                           i 
                         
                       
                     
                     ) 
                   
                   / 
                   
                     ( 
                     
                       
                         ∑ 
                         
                           i 
                           = 
                           1 
                         
                         4 
                       
                       ⁢ 
                       
                         α 
                         i 
                       
                     
                     ) 
                   
                 
                 ) 
               
             
             + 
             
               
                 ( 
                 
                   1 
                   - 
                   β 
                 
                 ) 
               
               · 
               
                 
                   ( 
                   
                     
                       ( 
                       
                         
                           ∑ 
                           
                             i 
                             = 
                             1 
                           
                           4 
                         
                         ⁢ 
                         
                           
                             α 
                             i 
                             ′ 
                           
                           · 
                           
                             V 
                             i 
                             ′ 
                           
                         
                       
                       ) 
                     
                     / 
                     
                       ( 
                       
                         
                           ∑ 
                           
                             i 
                             = 
                             1 
                           
                           4 
                         
                         ⁢ 
                         
                           α 
                           i 
                           ′ 
                         
                       
                       ) 
                     
                   
                   ) 
                 
                 . 
               
             
           
         
       
     
       FIG. 9  represents a device capable of implementing the previously described method. This device comprises at least two image memories  400  each of which is capable of memorizing a source image, namely the current source image and the previous source image, a motion estimator  410  for determining a motion vector for each one of the pixels of the image to interpolate from the current source image at the input of the device and from the previous source image in one of the image memories  400 , a circuit  420  for determining a reliability index C for each of the estimated motion vectors coming from the estimator  410  and an interpolator  430  for generating the interpolated image from the current and previous source images stored in the image memories  400  and the reliability indices C determined by the circuit  420 . The interpolator  430 , compares, for each pixel to interpolate, the reliability index C of the estimated motion vector for this pixel at the thresholds SC 1  and SC 2  and subsequently applies the appropriate interpolation pattern. 
       FIG. 10  is a flowchart showing the steps of an embodiment of the invention method in which the interpolation pattern is selected according to the amplitude A and the reliability index C of the estimated motion vector. It comprises:
         a motion estimation step  500  for estimating a motion vector (Vx,Vy) for each pixel of the image to interpolate,   a step  510  for determining, for each one of the estimated motion vectors, its amplitude A (A=√{square root over (V/x 2 +Vy 2 )}) and a reliability index C (which is for instance the variance of the motion vectors in a window of pixels including the considered pixel as described for the second embodiment), and   an interpolation pattern selection step  520  for each pixel of the image to interpolate according to the amplitude and the reliability index of the estimated motion vector, and   an interpolation step  530  for generating an interpolated image by using, for each pixel of this image, the interpolation pattern selected for this pixel.       

     For the interpolation step  530 , we calculate for instance a value, called for example modified amplitude and noted A mod , according to which an interpolation pattern is selected. A mod  is for example equal to A mod =A×(1−C). The interpolation to be applied according to A mod  is for example illustrated by the  FIG. 10 . If A mod  is less than or equal to a first threshold SA′ 1 , the interpolation applied is a bilinear interpolation based on the 4 neighbouring pixels of the considered pixel in one or both the source images. If A mod  is greater than the threshold SA′ 1  but less than or equal to a threshold SA′ 2  greater than SA 1 , interpolation is an averaging step based on the 4 neighbouring pixels of the considered pixel in one or both the source images. Finally, if A mod  is greater than the threshold SA 2 , interpolation is an averaging step based on a greater number of neighbouring pixels, for example 8 pixels. 
       FIG. 11  represents a device capable of implementing the process illustrated by  FIG. 9 . This device comprises at least two image memories  600  each of which is capable of memorizing a source image, namely the current source image and the previous source image, a motion estimator  610  for determining a motion vector (Vx,Vy) for each one of the pixels of the image to interpolate from the current source image at the input of the device and from the previous source image stored in one of the image memories  600 , a circuit  620  for determining the amplitude A and the reliability index C of the estimated motion vectors coming from the estimator  610  and an interpolator  630  for generating the interpolated image from the current and previous source images stored in the image memories  600  and the amplitudes A and the reliability indices C determined by the circuit  620 . 
     Naturally, the invention is not restricted to the previously described embodiments. Those skilled in the art will be able to foresee the use of interpolation types and interpolation patterns other than the ones presented here, by using a different number of pixels or different weighting factors.