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mp4codecmpegmpeg-4

The reason for a 'free' box in an ISO BMFF or MP4 file?

It's easy to spot a free box in a .mp4 (or ISO BMFF) file. I'm wondering the reason for this box. Is it some sort of padding for file size? Sometimes, I even spotted two consecutive free boxes!

The following is a partial info by mp4dump

[ftyp] size=8+20
  major_brand = iso5
  minor_version = 0
  compatible_brand = avc1
  compatible_brand = iso5
  compatible_brand = dash
[free] size=8+17
[moov] size=8+637
  [mvhd] size=12+108, version=1
    timescale = 24000
    duration = 14315000
    duration(ms) = 596458
Solution

  • There are actually padding boxes, so that other sections can be correctly aligned (it is also ease updates).