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