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Media Source Extensions Notes

A collection of findings experimenting with Media Source Extensions on Chrome 36.

  • Specifying an audio and video codec when creating a source buffer but passing in an initialization segment with only a video track results in a decode error

ISO Base Media File Format (BMFF)

Init Segment

A working initialization segment is outlined below. It may be possible to trim this structure down further.

  • ftyp
  • moov
    • mvhd
    • trak
      • tkhd
      • mdia
        • mdhd
        • hdlr
        • minf
    • mvex

Media Segment

The structure of a minimal media segment that actually encapsulates movie data is outlined below:

  • moof
    • mfhd
    • traf
      • tfhd
      • tfdt
      • trun containing samples
  • mdat

Structure

sample: time {number}, data {array} chunk: samples {array} track: samples {array} segment: moov {box}, mdats {array} | moof {box}, mdats {array}, data {array}

track chunk sample

movie fragment -> track fragment -> [samples]

Sample Data Offsets

Movie-fragment Relative Addressing: all trun data offsets are relative to the containing moof (?).

Without default-base-is-moof, the base data offset for each trun in trafs after the first is the end of the previous traf.

iso5/DASH Style

moof |- traf (default-base-is-moof) | |- trun_0 + 0 | - trun_1 <size of moof> + 100 - traf (default-base-is-moof) - trun_2 <size of moof> + 300 mdat |- samples_for_trun_0 (100 bytes) |- samples_for_trun_1 (200 bytes) - samples_for_trun_2

Single Track Style

moof - traf - trun_0 + 0 mdat `- samples_for_trun_0