Method and Device for Encoding a Sequence of Images and Method and Device for Decoding a Sequence of Image

Abstract

A method and device for deriving motion information of an image portion of an image predicted by motion prediction with respect to at least one reference image, the method comprising for said image portion: obtaining a number i of motion information predictors 808 of a second set of motion information predictors 814 taken from a first set of motion information predictors for decoding the image portion; generating said second set of motion information predictors 814 consisting of said number i of motion information predictors, wherein there are no duplicates among the i motion information predictors of said second set. A method and device for of decoding a bitstream, and a method and device for encoding a sequence of image including a method for deriving motion information of an image portion of an image predicted by motion prediction with respect to at least one reference image as described above.

1 . A method of decoding a bitstream comprising an encoded sequence of images, at least one portion of an image having been encoded by motion compensation with respect to a reference image portion, the method comprising for at least one image portion to be decoded:
 obtaining an index i of a selected motion information predictor of a first set of motion information predictors for decoding the image portion;   generating a second set of motion information predictors consisting of said index number i of motion information predictors wherein there are no duplicates among the i motion information predictors of said second set; and   retrieving, for decoding of the image portion, the motion information predictor indexed by index i from the second set of motion information predictors.   
     
     
         2 . A method according to  claim 1  wherein the step of generating a second set of motion information predictors comprises:
 obtaining a motion information predictor indexed j for the second set of motion information predictors, 
 comparing the obtained motion information predictor to any previously obtained motion information predictor, 0 to j−1, of the second set of motion information predictors; 
 wherein in the case where the obtained motion information predictor j is different to any of the previously obtained motion information predictors 0 to j−1 the obtained motion information predictor is added to the second set of motion information predictors, otherwise the obtained motion information predictor is replaced with a replacement motion information predictor different to any of the previously derived predictors 0 to j−1. 
 
     
     
         3 . A method according to  claim 2  wherein the replacement motion information predictor is derived according to the value of the obtained motion information predictor j and data of a motion vector field storing encoded motion vectors of the current image and of previous images of the sequence. 
     
     
         4 . A method according to  claim 2  further comprising the step of comparing the replacement motion information predictor with each of the previously obtained motion information predictors 0 to j−1, of the second set of motion information predictors and in the case where the replacement motion information predictor is not different to each of the previously obtained motion information predictors 0 to j−1 the method further comprising deriving another replacement motion information predictor. 
     
     
         5 . A method according to  claim 1  comprising the step of determining the number of motion information predictors in the first set of motion information predictors and retrieving an information item to provide the index i of the selected motion information predictor based on the number of motion information predictors. 
     
     
         6 . A method of encoding a sequence of images into a bitstream, at least one portion of an image being encoded by motion compensation with respect to a reference image portion, the method comprising for at least one image portion to encode
 obtaining a number x, of motion information predictors taken from a first set of N motion information predictors wherein the number x is based on a threshold parameter; and   generating a second set of motion information predictors consisting of said number x of motion information predictors, wherein there are no duplicates among the x motion information predictors of said second set; and
 selecting, from among the second set of motion information predictors a motion information predictor for encoding the image portion; and 
   encoding an information item to provide an index i of the selected motion information predictor.   
     
     
         7 . A method according to  claim 6  wherein the step of generating a second set of motion information predictors comprises:
 obtaining a motion information predictor j for the second set of motion information predictors, 
 comparing the obtained motion information predictor to any previously obtained motion information predictor 0 to j−1 of the second set of motion information predictors; 
 wherein in the case where the obtained motion information predictor j is different to any of the previously obtained motion information predictors 0 to j−1 the obtained motion information predictor is added to the second set of motion information predictors, otherwise the obtained information predictor is replaced with a motion information predictor different to any of the previously derived predictors 0 to j−1. 
 
     
     
         8 . A method according to  claim 6  wherein the threshold number x is determined based on a maximum allowed number of motion information predictors. 
     
     
         9 . A method according to  claim 6  wherein the threshold number x is determined based on a rate distortion criterion. 
     
     
         10 . A method according to  claim 6  wherein the threshold number x is determined based on a threshold number of elementary operations for obtaining the second set of motion information predictors. 
     
     
         11 . A method according to  claim 10  wherein each of the following steps is counted as at least one elementary operation: the step of obtaining a motion information predictor j, the step of comparing the obtained motion information predictor to a previously obtained motion information predictor 0 to j−1 of the second set of motion information predictors, the step of replacing a motion information predictor j with a replacement motion information predictor, and the step of obtaining a replacement motion information predictor. 
     
     
         12 . A method according to  claim 6  wherein said item of information is encoded on a number of bits dependent upon the number N of motion information predictors. 
     
     
         13 . A method according to  claim 6  wherein the motion information predictor is selected based on a rate distortion criterion. 
     
     
         14 . A decoding device for decoding a bitstream comprising an encoded sequence of images, at least one portion of an image having been encoded by motion compensation with respect to a reference image portion, the decoding device comprising:
 index obtaining means for obtaining an index i of a selected motion information predictor of a first set of motion information predictors for decoding the image portion;   motion information set generation means for generating a second set of motion information predictors consisting of said index number i of motion information predictors, wherein there are no duplicates among the i motion information predictors of said second set; and   retrieval means for retrieving, for decoding of the image portion, the motion information predictor indexed by index i from the second set of motion information predictors.   
     
     
         15 . A device according to  claim 14  wherein the motion information set generation means comprises:
 means for obtaining a motion information predictor j for the second set of motion information predictors, 
 comparison means for comparing the obtained motion information predictor to any previously obtained motion information predictor, 0 to j−1, of the second set of motion information predictors; 
 wherein in the case where the obtained motion information predictor j is different to any of the previously obtained motion information predictors 0 to j−1 the motion information set generation means is operable to add the obtained motion information predictor to the second set of motion information predictors, otherwise the obtained motion information predictor is replaced with a replacement motion information predictor different to any of the previously derived predictors 0 to j−1. 
 
     
     
         16 . A device according to  claim 15  wherein the motion information set generation means is operable to derive the replacement motion information predictor according to the value of the obtained motion information predictor j and data of a motion vector field storing encoded motion vectors of the current image and of previous images of the sequence. 
     
     
         17 . A device according to  claim 15  wherein the comparison means is operable to compare the replacement motion information predictor with each of the previously obtained motion information predictors 0 to j−1 of the second set of motion information predictors and in the case where the replacement motion information predictor is not different to each of the previously obtained motion information predictors 0 to j−1, the motion information set generation means is operable to derive another replacement motion information predictor. 
     
     
         18 . A device according to  claim 14  further comprising means for determining the number of motion information predictors in the first set of motion information predictors and means for retrieving an information item to provide the index i of the selected motion information predictor based on the number of motion information predictors. 
     
     
         19 . An encoding device for encoding a sequence of images into a bitstream, at least one portion of an image being encoded by motion compensation with respect to a reference image portion, the encoding device comprising:
 means for obtaining a number x, of motion information predictors taken from a first set of N motion information predictors wherein the number x is based on a threshold parameter; and   motion information set generation means for generating a second set of motion information predictors consisting of said number x of motion information predictors, wherein there are no duplicates among the x motion information predictors of said second set; and   selection means for selecting, from among the second set of motion information predictors a motion information predictor for encoding the image portion; and   encoding means for encoding an information item to provide an index i of the selected motion information predictor.   
     
     
         20 . A device according to  claim 19  wherein motion information set generation means comprises:
 means for obtaining a motion information predictor j for the second set of motion information predictors, 
 comparison means for comparing the obtained motion information predictor to any previously obtained motion information predictor 0 to j−1 of the second set of motion information predictors; 
 wherein in the case where the obtained motion information predictor j is different to any of the previously obtained motion information predictors 0 to j−1 the motion information set generation means is operable to add the obtained motion information predictor to the second set of motion information predictors, otherwise to replace the obtained information predictor with a motion information predictor different to any of the previously derived predictors 0 to j−1. 
 
     
     
         21 . A device according to  claim 19  wherein the threshold parameter is based on a maximum allowed number of motion information predictors. 
     
     
         22 . A device according to  claim 19  wherein the threshold parameter is based on a rate distortion criterion. 
     
     
         23 . A device according to  claim 19  wherein the threshold parameter is based on a threshold number of elementary operations for obtaining the set of motion information predictors. 
     
     
         24 . A device according to  claim 23  wherein each of the following steps is counted as at least one elementary operation: the step of obtaining a motion information predictor j, the step of comparing the obtained motion information predictor to a previously obtained motion information predictor 0 to j−1 of the set of motion information predictors, the step of replacing a motion information predictor j with a replacement motion information predictor, and the step of obtaining a replacement motion information predictor. 
     
     
         25 . A device according to  claim 19  wherein the encoding means is operable to encode said item of information on a number of bits dependent upon the number N of motion information predictors. 
     
     
         26 . A device according to  claim 19  wherein the selection means is operable to select the motion information predictor based on a rate distortion criterion. 
     
     
         27 . A computer program product for a programmable apparatus, the computer program product comprising a sequence of instructions for implementing a method according to  claim 1  when loaded into and executed by the programmable apparatus. 
     
     
         28 . A computer-readable storage medium storing instructions of a computer program for implementing a method, according to  claim 1 . 
     
     
         29 - 30 . (canceled) 
     
     
         31 . A method of deriving motion information of an image portion of an image predicted by motion prediction with respect to at least one reference image, the method comprising for said image portion:
 obtaining, for a second set of motion information predictors, a number i of motion information predictors, said i motion information predictors taken from a first set of motion information predictors for decoding the image portion;   generating said second set of motion information predictors consisting of said number i of motion information predictors,   wherein there are no duplicates among the i motion information predictors of said second set.   
     
     
         32 . A method of decoding a bitstream comprising an encoded sequence of images, at least one portion of an image having been encoded by motion compensation with respect to a reference image portion, the method comprising for at least one image portion to be decoded:
 a step of deriving motion information of the image portion according to the method of  claim 31 , wherein the number i is an index of a selected motion information predictor in said first set of motion information predictors obtained during a encoding process applied to said image; and   retrieving, for decoding of the image portion, the motion information predictor indexed by index i from the second set of motion information predictors.   
     
     
         33 . A method of encoding a sequence of images into a bitstream, at least one portion of an image being encoded by motion compensation with respect to a reference image portion, the method comprising for at least one image portion to decode
 a step of deriving motion information of the image portion according to the method of  claim 31 , wherein the number i is based on a threshold parameter; and   selecting, from among the set of motion information predictors a motion information predictor for encoding the image portion; and   encoding an information item to provide an index i 2  of the selected motion information predictor.