Abstract:
Processing the pixel value of at least one image pixel contained in a current frame of a video sequence includes constructing an individual motion trajectory including motion-shifted versions of the at least one image pixel over a plurality of preceding and/or subsequent frames, and processing the pixel value based on the individual motion trajectory. Constructing the individual motion trajectory includes choosing the at least one image pixel of the current frame as a start pixel of the individual motion trajectory, and adding motion-shifted versions of the at least one image pixel of preceding and/or subsequent frames to the individual motion trajectory. For each of the plurality of preceding and/or subsequent frames, at least two motion-shifted versions of the at least one image pixel are determined, and one of the at least two motion-shifted versions of the at least one image pixel is selected and added to the individual motion trajectory.

Description:
The present invention relates to methods and devices for processing pixels contained in a video sequence. 
     BACKGROUND OF THE INVENTION 
     An in-loop filter for processing video sequences is described in [1]. This filter provides adaptive de-blocking and has shown to improve both the subjective and the objective quality of a decoded video sequence. 
     The concept of motion compensated temporal filtering was first introduced in [2]. The notion of a motion trajectory and its applications to video coding were first described in [3], which was extended to a temporal filtering approach in [4]. Approaches with a similar aim have been presented in [5] and [6]. 
     Even though most of the mentioned prior art filters provide quite good results, better performance is still desirable for many of today&#39;s video applications. 
     OBJECTIVE OF THE PRESENT INVENTION 
     An objective of the present invention is to provide a method for efficiently processing image pixels in order to increase the quality of a video sequence. 
     A further objective of the present invention is to provide a device for efficiently processing image pixels in order to increase the quality of a video sequence. 
     BRIEF SUMMARY OF THE INVENTION 
     An embodiment of the present invention relates to a method for processing the pixel value of at least one image pixel contained in a current frame of a video sequence, said method comprising the steps of constructing an individual motion trajectory comprising motion-shifted versions of the at least one image pixel over a plurality of preceding and/or subsequent frames, and processing the pixel value based on the individual motion trajectory, 
     wherein said step of constructing said individual motion trajectory comprises the steps of: 
     
         
         
           
             choosing the at least one image pixel of the current frame as a start pixel of the individual motion trajectory, and adding motion-shifted versions of the at least one image pixel of preceding and/or subsequent frames to the individual motion trajectory; 
             wherein for each of the plurality of preceding and/or subsequent frames, at least two motion-shifted versions of the at least one image pixel are determined, 
             wherein, for each of the plurality of preceding and/or subsequent frames, one of said at least two motion-shifted versions of the at least one image pixel is selected and 
             wherein the selected motion-shifted version is added to the individual motion trajectory. 
           
         
       
    
     According to this embodiment of the invention, two or more motion-shifted versions of the image pixel are determined for each preceding and/or subsequent frame of the trajectory. This allows picking the “best” motion-shifted version out of a plurality of versions for each frame. Thus, the estimation of the pixel motion can be optimized for each frame in view of predefined criteria such as the most accurate motion estimation (i.e. best video quality) and/or a minimum number of additional bits for describing the pixel motion (i.e. maximum data compression). 
     The step of selecting one of said at least two motion-shifted versions of the at least one image pixel preferably includes comparing said at least two motion-shifted versions with each other or with the at least one image pixel of the current frame, and choosing one of said at least two motion-shifted versions based on the comparison result. 
     The step of comparing said at least two motion-shifted versions with each other preferably includes determining the distance between said at least two motion-shifted versions and choosing one of said at least two motion-shifted versions based on said distance or distances. 
     A first motion-shifted version and a second motion-shifted version of the at least one image pixel may be determined, wherein for each of the plurality of preceding and/or subsequent frames, the first motion-shifted version is added to the individual motion trajectory if the distance between the pixel location of the first motion-shifted version and the pixel location of the second motion-shifted version is smaller than a predefined threshold, and wherein for each of the plurality of preceding and/or subsequent frames, the second motion-shifted version is added to the individual motion trajectory if the distance between the pixel location of the first motion-shifted version and the pixel location of the second motion-shifted version exceeds said predefined threshold. 
     The first motion-shifted version of the at least one image pixel may be determined based on a first motion model, and the second motion-shifted version of the at least one image pixel may be determined based on a second motion model. 
     The second motion model preferably requires more motion related parameters and provides a more accurate estimation of pixel motion than the first motion model. 
     Further, for each of the plurality of preceding and/or subsequent frames, the first motion-shifted version may be added to the individual motion trajectory if the difference between the pixel value of the first motion-shifted version and the pixel value of the second motion-shifted version is smaller than a predefined threshold; and for each of the plurality preceding and/or subsequent frames, the second motion-shifted version is added to the individual motion trajectory if the difference between the pixel value of the first motion-shifted version and the pixel value of the second motion-shifted version exceeds said predefined threshold. 
     Furthermore the step of comparing said at least two motion-shifted versions with the at least one image pixel of the current frame may include determining the distances between said at least two motion-shifted versions and the at least one image pixel of the current frame, respectively, wherein for each of the plurality of preceding and/or subsequent frames, the first motion-shifted version is added to the individual motion trajectory if the difference in the distances is smaller than a predefined threshold, and wherein for each of the plurality of preceding and/or subsequent frames, the second motion-shifted version is added to the individual motion trajectory if the difference in the distances exceeds the predefined threshold. 
     The step of comparing said at least two motion-shifted versions with the at least one image pixel of the current frame may also include comparing the pixel values of said at least two motion-shifted versions with the pixel value of the at least one image pixel of the current frame, respectively, and choosing the motion-shifted version which has the most similar pixel value compared to the at least one image pixel of the current frame. 
     Further, an individual stop marker for the processed pixel value may be added to the video data bit stream, said individual stop marker describing the individual length of the individual motion trajectory that was used for forming said processed pixel value. 
     The individual motion trajectory of said image pixel may be constructed by concatenating at least two block motion vectors between adjacent frames of the video sequence, said at least two block motion vectors describing the motion of image blocks, each of which comprises said image pixel. 
     Furthermore, at least two block motion vectors between adjacent frames of the video sequence may be coded in the video bit stream. 
     The frames of said video sequence may be filtered and/or predicted by incorporating said processed pixel value of said image pixel into the video sequence. 
     At least one flag bit may be generated to signal the presence of the processed pixel values in the video sequence. 
     Furthermore, further video sequences having spatially adjacent frames with the same image pixel may be taken into account to form said processed pixel value of said image pixel. 
     The processed pixel value of said block image pixel is preferably formed by recursive accumulation of the pixel values of the selected motion-shifted versions of said image pixel along the individual motion trajectory. 
     The method may comprise at least two different process modes, wherein, in each process mode, an individual maximum number of frames is considered for constructing the individual motion trajectory of the at least one image pixel, and wherein a flag is generated which indicates the process mode that has been carried out. 
     A further embodiment of the present invention relates to a device capable of carrying out the method as described above. 
     The device may comprise a processor and a memory storing an executable computer program capable of performing the method&#39;s steps as described above. 
     The device may be a part of a filter (e.g. in-loop-filter), a decoder and/or an encoder. 
     A preferred embodiment of such a device comprises:
         trajectory constructing means configured to construct an individual motion trajectory comprising motion-shifted versions of said block image pixel over a multiplicity of neighboring frames; and   combining means configured to combine the pixel values of the motion-shifted versions of said block image pixel along the individual motion trajectory using a weighting function, to form a processed pixel value of said block image pixel.       

     The trajectory constructing means and the combining means may be software modules stored in a memory and being run by a processor. 
     A preferred embodiment comprises a marker adder capable of adding at least one stop marker for at least one of the processed image pixels, said individual stop marker describing the individual length of the individual motion trajectory of said processed image pixel and/or a flag bit generator adapted to generate at least one flag bit to signal the presence of the processed pixel values in the video sequence. 
     The device may be an encoder for processing an incoming video sequence and generating an encoded video data bit stream, wherein the encoder comprises a data bit generator capable of generating said encoded video data bit stream that describes said video sequence, and wherein the encoder is configured to include said processed pixel values generated by said combining means into the encoded video data bit stream. 
     Alternatively, the device may be a decoder for decoding an encoded video data bit stream to generate a decoded video sequence, wherein the decoder comprises means for decoding data bits that describe said video sequence, and wherein the decoder is configured to include said processed pixel values generated by said combining means into the decoded video sequence. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In order that the manner in which the above-recited and other advantages of the invention are obtained will be readily understood, a more particular description of the invention briefly described above will be rendered by reference to specific embodiments thereof which are illustrated in the appended figures. Understanding that these figures depict only typical embodiments of the invention and are therefore not to be considered to be limiting of its scope, the invention will be described and explained with additional specificity and detail by the use of the accompanying drawings in which 
         FIGS. 1-4  show exemplary embodiments of a method for processing the pixel value of image pixels in frames of a video sequence, and 
         FIG. 5  shows an exemplary embodiment of a device capable of processing the pixel value of image pixels in frames of a video sequence. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The preferred embodiments of the present invention will be best understood by reference to the drawings, wherein identical or comparable parts are designated by the same reference signs throughout. 
     It will be readily understood that the present invention, as generally described herein, could vary in a wide range. Thus, the following more detailed description of the exemplary embodiments of the present invention, is not intended to limit the scope of the invention, as claimed, but is merely representative of presently preferred embodiments of the invention. 
       FIG. 1  shows a first exemplary embodiment of a method for processing a pixel value V(P) of an image pixel P contained in a current frame Fi of a video sequence. 
     First, an individual motion trajectory T is constructed for the image pixel P. The trajectory T extends from the current frame Fi to several preceding and/or subsequent frames. The preceding and/or subsequent frames may be frames which have been obtained by decoding previously encoded frames. 
     Regarding  FIG. 1 , the construction of the trajectory T is explained with reference to three preceding frames Fi- 1 , Fi- 2 , and Fi- 3 . Additionally, further preceding frames and subsequent frames might also be taken into account as will be more apparent from the explanations as set out below. 
     Starting from the image pixel P in the current frame Fi, at least two pixels P 1  and P 1 ′ are determined in the preceding frame Fi- 1 . The first motion-shifted version P 1  may be calculated based on a linear motion model and a linear motion vector which describes the movement of the image pixel from the preceding frame Fi- 1  to the current frame Fi. 
     The second motion-shifted version P 1 ′ may be calculated based on a higher order motion model which is supposed to provide a more accurate estimation of the pixel movement than the linear model applied to the first motion-shifted version P 1 . In order to provide a better estimation than the linear model, the higher order motion model preferably uses more estimation parameters than the linear model. 
     Then, the distance d 1  between both motion shifted versions P 1  and P 1 ′ is calculated and compared with a predefined threshold. Depending on the result of this comparison, the first motion-shifted version P 1  or the second motion-shifted version P 1 ′ will be added to the trajectory T: 
     a) If the distance d 1  between the pixel location of the first motion-shifted version P 1  and the pixel location of the second motion-shifted version P 1 ′ is smaller than the predefined threshold, the first motion-shifted version P 1  is added to the individual motion trajectory T.
 
b) If the distance d 1  between the pixel location of the first motion-shifted version P 1  and the pixel location of the second motion-shifted version P 1 ′ equals or exceeds the predefined threshold, the second motion-shifted version P 1 ′ is added to the individual motion trajectory T.
 
     Hereafter, it is assumed that the distance d 1  between the pixel location of the first motion-shifted version P 1  and the pixel location of the second motion-shifted version P 1 ′ is smaller than the predefined threshold. Therefore, the first motion-shifted version P 1  is added to the individual motion trajectory T, and the second motion-shifted version P 1 ′ is discarded. 
     In the same way, the other preceding frames Fi- 2  and Fi- 3  are analyzed, and motion-shifted versions of the image pixel P are added to the motion trajectory T: 
     Starting from the first motion-shifted version P 1  in frame Fi- 1 , two pixels P 2  and P 2 ′ are determined in the further preceding frame Fi- 2 . A first motion-shifted version P 2  is calculated based on the linear motion model. A second motion-shifted version P 2 ′ is calculated based on the higher order motion model. Then, the distance d 2  between both motion shifted versions P 2  and P 2 ′ is calculated and compared to the predefined threshold. Depending on the result of this comparison, the first motion-shifted version P 2  or the second motion-shifted version P 2 ′ will be added to the trajectory T. Here, it is assumed that the distance d 2  between the pixel location of the first motion-shifted version P 2  and the pixel location of the second motion-shifted version P 2 ′ exceeds the predefined threshold. Therefore, the second motion-shifted version P 2 ′ is added to the individual motion trajectory T, and the first motion-shifted version P 2  is discarded. 
     Starting from the second motion-shifted version P 2 ′ in frame Fi- 2 , two motion-shifted versions P 3  and P 3 ′ are determined in the further preceding frame Fi- 3 . The first motion-shifted version P 3  is calculated based on the linear motion model, and the second motion-shifted version P 3 ′ is calculated based on the higher order motion model. Then, the distance d 3  between both motion shifted versions P 3  and P 3 ′ is calculated and compared to the predefined threshold. Depending on the result of this comparison, the first motion-shifted version P 3  or the second motion-shifted version P 3 ′ will be added to the trajectory T as explained above. 
     The length of the trajectory may be predefined or flexibly adapted to further parameters such as parameters of the resulting trajectory. If for instance the pixel values (e.g. amplitude, color) of the motion shifted versions in previous or subsequent frames differ too much from the value of the image pixel P in the current frame Fi, the trajectory may be stopped. 
     After the trajectory T has been completed, the pixel values (e.g. pixel amplitudes) of the motion-shifted versions P 1 , P 2 ′, P 3  of the image pixel P along the individual motion trajectory T may be processed using a weighting function, and a processed pixel value (e.g. pixel amplitude) V′(P) of the image pixel P may be obtained. Furthermore, the difference ΔV(P) between the processed pixel value V′(P) and the pixel value V(P) of the image pixel P may be determined. 
     For encoding and/or decoding the image pixel P of the current frame Fi, the processed pixel value V′(P) and/or the difference ΔV(P) between the processed pixel value V′(P) and the unprocessed pixel value V(P) may be taken into account and added to the video sequence or to the bit stream which describes the video sequence. 
       FIG. 2  shows a second exemplary embodiment of a method for processing the pixel value V(P) of an image pixel P contained in a current frame Fi of a video sequence. 
     In contrast to the first embodiment of  FIG. 1 , the distances between the position of both motion shifted versions P 1  and P 1 ′ and the position of the image pixel P are calculated and compared to a predefined threshold. Depending on the result of the comparison, the first motion-shifted version or the second motion-shifted version will be added to the trajectory T: 
     Regarding frame Fi- 1 , the distance d 1  between the pixel location of the first motion-shifted version P 1  and the pixel location of the image pixel P in frame Fi is determined. Further, the distance d 1 ′ between the pixel location of the second motion-shifted version P 1 ′ and the pixel location of the image pixel P in frame Fi is determined. If the difference between the distances d 1  and d 1 ′ is smaller than a predefined threshold TH, the first motion-shifted version P 1  is added to the individual motion trajectory T; otherwise the second motion-shifted version P 1 ′ is added to the individual motion trajectory T:
 
| d 1− d 1′|&lt;TH             P 1 is added
 
| d 1− d 1′|&lt;TH           P 1′ is added

     In the same way, the other preceding frames Fi- 2  and Fi- 3  are analyzed, and motion-shifted versions P 2 ′ and P 3  of the image pixel P are added to the motion trajectory T. 
     After the trajectory T has been completed, the pixel values (e.g. pixel amplitudes) of the motion-shifted versions P 1 , P 2 ′, P 3  of the image pixel P along the individual motion trajectory T may be processed using a weighting function, and a processed pixel value (e.g. pixel amplitude) V′(P) of the image pixel P may be obtained. Furthermore, the difference ΔV(P) between the processed pixel value V′(P) and the pixel value V(P) of the image pixel P may be determined. For encoding and/or decoding the image pixel P of the current frame Fi, the processed pixel value V′(P) and/or the difference ΔV(P) between the processed pixel value V′(P) and the unprocessed pixel value V(P) may be taken into account and added to the video sequence or to the bit stream which describes the video sequence. 
       FIG. 3  shows a third exemplary embodiment of a method for processing the pixel value V(P) of an image pixel P contained in a current frame Fi of a video sequence. 
     In contrast to the first and second embodiments, the pixel values (e.g. pixel amplitudes, color values) between the motion shifted versions are calculated and the differences between the pixel values are compared to a predefined threshold. This will be explained in further detail below: 
     Starting from image pixel P in the current frame Fi, at least two pixels P 1  and P 1 ′ are determined in the preceding frame Fi- 1 . Again, the first motion-shifted version P 1  may be calculated based on a linear motion model, and the second motion-shifted version P 1 ′ may be calculated based on a higher order motion model. 
     Then, the pixel values V(P 1 ) and V(P 1 ′) of the first and second motion-shifted versions P 1  and P 1 ′ are compared, and a difference value ΔV 1  is calculated:
 
Δ V 1=| V ( P 1)− V ( P 1′)|
 
     If the difference value ΔV 1  is smaller than a predefined threshold, the first motion-shifted version P 1  is added to the individual motion trajectory T. 
     If the difference value ΔV 1  equals or exceeds the predefined threshold, the second motion-shifted version P 1 ′ is added to the individual motion trajectory T. 
     Hereafter, it is assumed that the difference value ΔV 1  is smaller than the predefined threshold. Therefore, the first motion-shifted version P 1  is added to the individual motion trajectory T, and the second motion-shifted version P 1 ′ is discarded. 
     In the same way, the other preceding frames Fi- 2  and Fi- 3  are analyzed and motion-shifted versions P 2 ′ and P 3  of the image pixel P are added to the motion trajectory T. 
     After the trajectory T has been completed, the pixel values (e.g. pixel amplitudes) of the motion-shifted versions P 1 , P 2 ′, and P 3  of the image pixel P along the individual motion trajectory T may be processed using a weighting function as explained above. 
       FIG. 4  shows a fourth exemplary embodiment of a method for processing the pixel value V(P) of an image pixel P contained in a current frame Fi of a video sequence. 
     In contrast to the third embodiment of  FIG. 3 , the pixel values (e.g. pixel amplitudes, color values) of the motion shifted versions are compared to the pixel value V(P) of the image pixel P in frame Fi. 
     Starting from image pixel P in the current frame Fi, at least two pixels P 1  and P 1 ′ are determined in the preceding frame Fi- 1 . The first motion-shifted version P 1  may be calculated based on a linear motion model, and the second motion-shifted version P 1 ′ may be calculated based on a higher order motion model. 
     Then, the pixel values V(P 1 ) and V(P 1 ′) of the first and second motion-shifted versions P 1  and P 1 ′ are compared to the pixel value V(P) of the image pixel P in frame Fi, and difference values ΔV 1  and ΔV 1 ′ are calculated:
 
Δ V 1 =|V ( P 1)− V ( P )|
 
Δ V 1′=| V ( P 1′)− V ( P )|
 
     If the difference value ΔV 1  is smaller than the difference value ΔV 1 ′, the first motion-shifted version P 1  is added to the individual motion trajectory T. 
     If the difference value ΔV 1  equals or exceeds the difference value ΔV 1 ′, the second motion-shifted version P 1 ′ is added to the individual motion trajectory T. The latter case is shown in  FIG. 4 . 
     In the same way, the other preceding frames Fi- 2  and Fi- 3  are analyzed, and motion-shifted versions P 2  and P 3  of the image pixel P are added to the motion trajectory T. 
     After the trajectory T has been completed, the pixel values (e.g. pixel amplitudes) of the motion-shifted versions P 1 ′, P 2 , and P 3  of the image pixel P along the individual motion trajectory T may be processed using a weighting function as explained above. 
       FIG. 5  shows an embodiment of a device  100  for processing the pixel value V(P) of block image pixels P contained in a video sequence. 
     The device  100  comprises a trajectory constructing unit  110  which constructs an individual motion trajectory T(P, P 1 ′,P 2 ,P 3 ) comprising motion-shifted versions of block image pixel P 1 ′, P 2 , and P 3  (see also  FIG. 4 ) over a multiplicity of neighboring frames Fi to Fi- 3  (see also  FIG. 4 ). 
     The device  100  further comprises a combining unit  120  which combines the pixel values V(P 1 ′), V(P 2 ) and V(P 3 ) of the motion-shifted versions P 1 ′, P 2 , and P 3  of the block image pixel P along the individual motion trajectory T using a predefined weighting function Fw in order to form a processed pixel value V′(P) of the said block image pixel P. 
     The weighting function Fw may be a function which averages the pixel values V(P 1 ′), V(P 2 ) and V(P 3 ) and generates a mean pixel value Vmean:
 
 V mean=( V ( P 1′)+ V ( P 2)+ V ( P 3)+ . . .  V ( Pn ))/ n  
 
wherein n defines the number of frames taken into account.
 
     REFERENCES 
     
         
         [1] P. List, A. Joch, J. Lainema, G. Bjøntegaard, and M. Karczewicz, “Adaptive deblocking filter,” IEEE Transactions on Circuits and Systems for Video Technology (TCSVT), 2003. 
         [2] E. Dubois and S. Sabri, “Noise reduction in image sequences using motion-compensated temporal filtering,” IEEE Transactions on Communications, vol. 32, no. 7, pp. 826-831, July 1984. 
         [3] J.-R. Ohm, “Three-dimensional subband coding with motion compensation,” IEEE Transactions on Image Processing, September 1994. 
         [4] X. Wang, M. Karczewicz, J. Ridge, and Y. Bao, “Simplified update step of motion compensated temporal filtering for video coding,” in Proceedings of the 24th Picture Coding Symposium, 2006, pp. 826-831. 
         [5] D. T. Vo and T. Q. Nguyen, “Optimal motion compensated spatio-temporal filter for quality enhancement of H.264/AVC compressed video sequences,” in Proceedings of the 16th International Conference on Image Processing (ICIP), November 2009, pp. 3173-3176. 
         [6] A. Glantz, A. Krutz, M. Haller, and T. Sikora, “Video coding using global motion temporal filtering,” in ICIP, November 2009, pp. 1053-1056. 
       
    
     REFERENCE SIGNS 
     
         
           100  device 
           110  trajectory constructing unit 
           120  combining unit 
         d 1  distance 
         d 1 ′ distance 
         d 2  distance 
         d 3  distance 
         Fi frame 
         Fi- 1  frame 
         Fi- 2  frame 
         Fi- 3  frame 
         Fw weighting function 
         P 1 , P 1 ′ pixels 
         P 2 , P 2 ′ pixels 
         P 3 , P 3 ′ pixels 
         V(P) pixel value 
         V′(P) processed pixel value 
         T motion trajectory