Video encoding and decoding with improved error resilience

Abstract

A sequence of digital images is encoded into a bitstream, at least one portion of an image being encoded by motion compensation with respect to a reference image portion. A target number of motion information predictors is obtained. Using the target number a set of motion information predictors is generated having controlled diversity. A motion information predictor for the image portion to encode is selected from the generated set of motion information predictors. The target number is signaled in the bitstream, together with information relating to the selected motion information predictor.

1 . A method of decoding a bitstream comprising an encoded sequence of digital images, at least one portion of an image being encoded with respect to a reference image portion by a motion estimation process that determines motion information identifying the reference image portion, the method comprising decoding said motion information using a motion information prediction process comprising:
 obtaining, for an image portion of an image to be decoded one or more first motion information predictors having motion vectors obtained from motion vectors associated with image portions of said image to be decoded;   generating one or more second motion information predictors by adding an offset value to or subtracting an offset value from the first motion information predictors;   forming a set of motion information predictors from the first and second motion information predictors; and   determining a motion information predictor from the set of motion information predictors for said image portion of said image to be decoded.   
     
     
         2 . A method according to  claim 1 , wherein the offset value may be set to a predetermined value. 
     
     
         3 . A method according to  claim 1 , where the obtained one or more first motion information predictors have motion vectors with X and Y components and the offset value is added to or subtracted from one or both of the X and Y components of one of the motion vectors. 
     
     
         4 . A method according to  claim 3 , wherein the offset value for one or both of the X and Y components may be set to zero. 
     
     
         5 . A method according to  claim 1  wherein applying the offset comprises adding a predetermined motion vector to a motion vector of the one or more first motion information predictors to generate the one or more second motion information predictors 
     
     
         6 . A method according to  claim 1 , further comprising decoding an item of information representative of a selected motion information predictor for said image portion of said image to be decoded. 
     
     
         7 . A method according to  claim 6 , wherein said selected motion information predictor is determined from said generated set of motion information predictors using said decoded item of information. 
     
     
         8 . A device for decoding a bitstream comprising an encoded sequence of digital images, at least one portion of an image being encoded with respect to a reference image portion by a motion estimation process that determines motion information identifying the reference image portion, the apparatus comprising
 means for decoding said motion information using a motion information prediction process, the means for decoding comprising:   means for obtaining, for an image portion of an image to be decoded one or more first motion information predictors having motion vectors obtained from motion vectors associated with image portions of said image to be decoded;   means for generating one or more second motion information predictors by adding an offset value to or subtracting an offset value from the first motion information predictors;   means for forming a set of motion information predictors from the first and second motion information predictors; and   means for determining a motion information predictor from the set of motion information predictors for said image portion of said image to be decoded.   
     
     
         9 . A non-transitory computer readable carrier medium comprising processor executable code for a programmable apparatus, comprising a sequence of instructions for implementing a method of decoding a bitstream comprising an encoded sequence of digital images when loaded into and executed by the programmable apparatus, at least one portion of an image being encoded with respect to a reference image portion by a motion estimation process that determines motion information identifying the reference image portion, the method comprising decoding said motion information using a motion information prediction, the method comprising:
 obtaining, for an image portion of an image to be decoded one or more first motion information predictors having motion vectors obtained from motion vectors associated with image portions of said image to be decoded;   generating one or more second motion information predictors by adding an offset value to or subtracting an offset value from the first motion information predictors;   forming a set of motion information predictors from the first and second motion information predictors; and   determining a motion information predictor from the set of motion information predictors for said image portion of said image to be decoded.