A method implemented in a video coding device, the method comprising: mapping, by a processor of the video coding device, a spherical video signal, stitched from multiple directional video signals, into a plurality of sub-picture video signals each containing a sequence of sub-pictures; encoding, by the processor, the plurality of sub-picture video signals as a plurality of sub-picture bitstreams, and composing, by the processor, the plurality of sub-picture bitstreams into a plurality of sub-picture tracks to be compatible with a multi-bitstream merge function for reconstruction of the spherical video signal or a part thereof when each sub-picture track is suitable to be merged with other sub-picture tracks to generate a single video bitstream using a lightweight bitstream rewriting process that does not involve changing of block-level coding results; characterized by encoding, by the processor, one or more track level mergable indications within a track level box for each of the sub-picture tracks, the track level mergable indications indicating that a video bitstream carried in a current sub-picture track can be merged with a sub-picture bitstream carried in the sub-picture tracks in the same track group, wherein the track level mergable indications includes a track_mergable_with_other_video_tracks flag that, when set, indicates that the sub-picture bitstream carried in a corresponding sub-picture track can be merged with one or more other of the plurality of sub-picture bitstreams carried in one or more other of the plurality of sub-picture tracks to generate a single video bitstream without decoding mismatch by rewriting only header data of the bitstreams; and transmitting, by a transmitter of the video coding device, a set of the plurality of sub-picture tracks containing a set of the plurality of sub-picture bitstreams and the track level mergable indications toward the decoder to support decoding and displaying a virtual reality video viewport. mapping, by a processor of the video coding device, a spherical video signal, stitched from multiple directional video signals, into a plurality of sub-picture video signals each containing a sequence of sub-pictures; encoding, by the processor, the plurality of sub-picture video signals as a plurality of sub-picture bitstreams, and composing, by the processor, the plurality of sub-picture bitstreams into a plurality of sub-picture tracks to be compatible with a multi-bitstream merge function for reconstruction of the spherical video signal or a part thereof when each sub-picture track is suitable to be merged with other sub-picture tracks to generate a single video bitstream using a lightweight bitstream rewriting process that does not involve changing of block-level coding results; characterized by encoding, by the processor, one or more track level mergable indications within a track level box for each of the sub-picture tracks, the track level mergable indications indicating that a video bitstream carried in a current sub-picture track can be merged with a sub-picture bitstream carried in the sub-picture tracks in the same track group, wherein the track level mergable indications includes a track_mergable_with_other_video_tracks flag that, when set, indicates that the sub-picture bitstream carried in a corresponding sub-picture track can be merged with one or more other of the plurality of sub-picture bitstreams carried in one or more other of the plurality of sub-picture tracks to generate a single video bitstream without decoding mismatch by rewriting only header data of the bitstreams; and transmitting, by a transmitter of the video coding device, a set of the plurality of sub-picture tracks containing a set of the plurality of sub-picture bitstreams and the track level mergable indications toward the decoder to support decoding and displaying a virtual reality video viewport.