How to determine duration from mp4 init segment

Question

So here I have an init segment: http://people.mozilla.org/~jyavenard/mediatest/fragmented/facebook-paper_dash.mp4

Now, all movie headers (mvhd atom), media header (mdhd) or track headers (tkhd atoms) indicate that the duration is 0.

(can check with this page: http://people.mozilla.org/~jyavenard/tests/mse_mp4/paper-init.html)

Yet, IE11 and Chrome reads the duration as 120.187s which makes sense as the original mp4 I extracted it from was 2 minutes long.

Here is the details of this mp4 init segment:

    [ftyp] size=8+28
      major_brand = isom
      minor_version = 200
      compatible_brand = isom
      compatible_brand = iso2
      compatible_brand = avc1
      compatible_brand = mp41
      compatible_brand = dash
    [free] size=8+47
    [moov] size=8+1168
      [mvhd] size=12+96
        timescale = 1000
        duration = 0
        duration(ms) = 0
      [mvex] size=8+64
        [trex] size=12+20
          track id = 1
          default sample description index = 1
          default sample duration = 1001
          default sample size = 0
          default sample flags = 10000
        [trex] size=12+20
          track id = 2
          default sample description index = 1
          default sample duration = 1024
          default sample size = 0
          default sample flags = 0
      [trak] size=8+467
        [tkhd] size=12+80, flags=f
          enabled = 1
          id = 1
          duration = 0
          width = 1920.000000
          height = 1080.000000
        [mdia] size=8+367
          [mdhd] size=12+20
            timescale = 30000
            duration = 0
            duration(ms) = 0
            language = eng
          [hdlr] size=12+33
            handler_type = vide
            handler_name = VideoHandler
          [minf] size=8+282
            [vmhd] size=12+8, flags=1
              graphics_mode = 0
              op_color = 0000,0000,0000
            [dinf] size=8+28
              [dref] size=12+16
                [url ] size=12+0, flags=1
                  location = [local to file]
            [stbl] size=8+218
              [stsd] size=12+138
                entry-count = 1
                [avc1] size=8+126
                  data_reference_index = 1
                  width = 1920
                  height = 1080
                  compressor = 
                  [avcC] size=8+40
                    Configuration Version = 1
                    Profile = Main
                    Profile Compatibility = 40
                    Level = 31
                    NALU Length Size = 4
                    Sequence Parameter = [67 4d 40 1f ec c0 3c 01 13 f2 e0 22 00 00 07 d2 00 01 d4 c0 1e 30 63 34]
                    Picture Parameter = [68 e9 78 f2 c8]
              [stts] size=12+4
                entry_count = 0
              [stsc] size=12+4
                entry_count = 0
              [stsz] size=12+8
                sample_size = 0
                sample_count = 0
              [stco] size=12+4
                entry_count = 0
      [trak] size=8+407
        [tkhd] size=12+80, flags=f
          enabled = 1
          id = 2
          duration = 0
          width = 0.000000
          height = 0.000000
        [mdia] size=8+307
          [mdhd] size=12+20
            timescale = 48000
            duration = 0
            duration(ms) = 0
            language = eng
          [hdlr] size=12+33
            handler_type = soun
            handler_name = SoundHandler
          [minf] size=8+222
            [smhd] size=12+4
              balance = 0
            [dinf] size=8+28
              [dref] size=12+16
                [url ] size=12+0, flags=1
                  location = [local to file]
            [stbl] size=8+162
              [stsd] size=12+82
                entry-count = 1
                [mp4a] size=8+70
                  data_reference_index = 1
                  channel_count = 2
                  sample_size = 16
                  sample_rate = 48000
                  [esds] size=12+30
                    [ESDescriptor] size=2+28
                      es_id = 2
                      stream_priority = 0
                      [DecoderConfig] size=2+20
                        stream_type = 5
                        object_type = 64
                        up_stream = 0
                        buffer_size = 0
                        max_bitrate = 192082
                        avg_bitrate = 192082
                        DecoderSpecificInfo = 11 90 56 e5 00 
                      [Descriptor:06] size=2+1
              [stts] size=12+4
                entry_count = 0
              [stsc] size=12+4
                entry_count = 0
              [stsz] size=12+8
                sample_size = 0
                sample_count = 0
              [stco] size=12+4
                entry_count = 0
      [udta] size=8+90
        [meta] size=12+78
          [hdlr] size=12+21
            handler_type = mdir
            handler_name = 
          [ilst] size=8+37
            [.too] size=8+29
              [data] size=8+21
                type = 1
                lang = 0
                value = Lavf55.12.100

I can't for the life of me figure out where it's getting that 120.187s duration...

Any ideas? thanks

Answer 1

You cannot determine the duration of an MP4 from the init segment only. More precisely, a fragmented MP4 (fMP4) does not have the required information about the entire presentation to compute the duration. You will need to read all the movie fragments, "moof boxes", and combine the durations to know the duration of the entire presentation.