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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
Drake Guan
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1 Answers1

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

n0p
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  • Thanks. It looks like those padding boxes (**free**) are designed for recording/capturing devices than playback? – Drake Guan Aug 20 '14 at 01:47
  • I don't think so although they are generated at file creation, while boxes are added, to guarantee (I guess) a fast navigation through the structures. – n0p Aug 20 '14 at 07:31
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    It has nothing to do with navigation. It's about the ability to change variables sized boxes without having to recalculate a bunch of offsets within other boxes – Sebastian Annies Feb 01 '15 at 19:03
  • Thanks for that correction, please feel free to add an answer if mine is not accurate – n0p Feb 01 '15 at 23:58