streaming may be realized through a tile-based encoding and streaming approaches. In one approach of tile-based encoding and streaming, encoding is performed in a manner that the resulting bitstream comprises motion-constrained tile sets. Several bitstreams of the same source content are encoded using motion-constrained tile sets. One or more motion- constrained tile set (MCTS) sequences are extracted from a bitstream, and each extracted motion-constrained tile set sequence is stored as a track (e.g. an HEVC tile track or a sub picture track) in a file. The client selectively receives tracks in a manner that better quality or higher resolution MCTS tracks may be received for the current viewport compared to the quality or resolution covering the remaining, currently non-visible viewports. The client merges the content of the received tracks, i.e. merges the received MCTSs, into a bitstream to be decoded. Since the viewing orientation may change over time, the selected tracks and MCTSs may also change over time and additionally the position of the respective MCTSs within the merged pictures may change from one merged picture to another. 

[00113] It is often desirable to reposition slices/tiles or extract a subset of slices/tiles. One typical instance is the situation where one may extract a tile set from an existing stream to create another stream. For example, one may extract a motion-constrained tile set from a bitstream into its own bitstream. Another typical instance is viewport switching in viewport- dependent streaming of 360°video. When slices/tiles can be repositioned in the bitstreams to be decoded without changing the slice or tile content, the repositioning can be done even for encrypted streams. 

[00114] Because the position of a slice within a picture is indicated through block indexing in raster scan order within a picture, traditionally, repositioning of a slice or extracting a subset of tiles or slices (other than picture-wide subsets starting from the top-left comer of the picture) requires rewriting of slice header(s). Similarly, when motion vector anchor positions are in use, syntax elements indicating locations such as tile ref x and tile ref y need to be rewritten when repositioning slices/tiles or extracting a subset of tiles or slices to their own bitstream. Rewriting slice headers for the purpose of repositioning a slice is inefficient as slice header rewriting affects the bitrate of the stream and/or requires special rewriting instructions. 

[00115] An example embodiment set forth herein enables repositioning of tiles in a bitstream without rewriting the slice headers by encoding tile identifiers indicative of the tile in raster scan order of tiles within the tile grid in a parameter set associated with the bitstream. 

[00116] Regardless of the file format of the video bitstream, the apparatus of an example embodiment may be provided by any of a wide variety of computing devices including, for example, a video encoder, a computer workstation, a server or the like, or by any of various 

