A method of decoding encoded video data from a first video stream that comprises video images encoded on the basis of a video codec, said video images having an image region and being divided into a spatial array of independently decodable slices, the spatial-array being associated with a first picture size, the first video stream comprising network abstraction layer units that each contain video content data for a respective one of the slices preceded by a header that comprises parameters relating to the respective one of the slices in relation to the first video stream, the method comprising - partially rewriting the network abstraction layer units for a sub-array of slices into rewritten network abstraction layer units, the network abstraction layer units being rewritten at a location in each of the respective network abstraction layer units, said location being in said header, wherein said location is selected on the basis of meta data, said partially rewriting comprising rewriting an indication of the spatial location of one or more slices for the sub-array of slices using a predetermined set of altered indications of the spatial locations of the slices whereby the altered indications distinguish the spatial locations of the slices within the sub-array, the rewritten network abstraction layer units forming a second video stream comprising encoded video data only for the sub-array of slices, said encoded video data representing partial video images that comprise a sub region of said image region, wherein said rewritten network abstraction layer units are decodable into said partial video images on the basis of said video codec wherein the method further comprises: - defining a mapping of values of the indication of the spatial location of the slices in the spatial array to target values indicating the spatial location of the slices in a first sub-array of slices, - changing the first sub-array into a second sub-array, said second sub-array comprising at least one slice that is different from the slices of said first sub-array; - keeping the target values of the indication of the spatial location of the slices in said mapping for slices that belong to an overlap between the second sub-array and the first sub-array; - redefining said mapping using a target value of the indication of the spatial location of the at least one slice that does not belong to the overlap, the rewriting being based on said redefined mapping, the target values being from said predetermined set of altered indications; - decoding the rewritten network abstraction layer units - post-processing the decoded partial video images to re-position image blocks that correspond to the slices of the second sub-array in a spatial sequence that corresponds to their spatial sequence in the full video images. - partially rewriting the network abstraction layer units for a sub-array of slices into rewritten network abstraction layer units, the network abstraction layer units being rewritten at a location in each of the respective network abstraction layer units, said location being in said header, wherein said location is selected on the basis of meta data, said partially rewriting comprising rewriting an indication of the spatial location of one or more slices for the sub-array of slices using a predetermined set of altered indications of the spatial locations of the slices whereby the altered indications distinguish the spatial locations of the slices within the sub-array, the rewritten network abstraction layer units forming a second video stream comprising encoded video data only for the sub-array of slices, said encoded video data representing partial video images that comprise a sub region of said image region, wherein said rewritten network abstraction layer units are decodable into said partial video images on the basis of said video codec wherein the method further comprises: - defining a mapping of values of the indication of the spatial location of the slices in the spatial array to target values indicating the spatial location of the slices in a first sub-array of slices, - changing the first sub-array into a second sub-array, said second sub-array comprising at least one slice that is different from the slices of said first sub-array; - keeping the target values of the indication of the spatial location of the slices in said mapping for slices that belong to an overlap between the second sub-array and the first sub-array; - redefining said mapping using a target value of the indication of the spatial location of the at least one slice that does not belong to the overlap, the rewriting being based on said redefined mapping, the target values being from said predetermined set of altered indications; - decoding the rewritten network abstraction layer units - post-processing the decoded partial video images to re-position image blocks that correspond to the slices of the second sub-array in a spatial sequence that corresponds to their spatial sequence in the full video images.