Parsing H264 in mdat MP4

Question

I have a file that only contains the mdat atom in a MP4 container. The data in the mdat contains AVC data. I know the encoding parameters for the data. The format does not appear to be in the Annex B byte stream format. I am wondering how I would go about parsing this. I have tried searching for the slice header, but have not had much luck.

Is it possible to parse the slices without the NAL's?

score 19 · Answer 1 · answered Jul 13 '11 at 14:51

19

Be careful! The NAL Length is not required to be 4! The AvcConfigurationBox ('moov/trak/mdia/minf/stbl/stsd/avc1/avcC') contains a field 'lengthSizeMinusOne' specifying the length. But the default is 4.

answered Jul 13 '11 at 14:51

Sebastian Annies

2,438
1
20
38

I know this is a old discussion but can someone give me some idea how can i fetch the aac and h264 data from mdat element. – Dinesh Aug 26 '15 at 06:30

score 18 · Accepted Answer · answered Apr 08 '11 at 14:19

18

AVC NAL units are in the following format in MDAT section: [4 bytes] = NAL length, network order; [NAL bytes] Shortly, start codes are simply replaced by lengths.

answered Apr 08 '11 at 14:19

michael

216
2
2

1

Thank you very much. I have been searching the ISO documents, but coming up empty. Again. Thanks. – Steve McFarlin Apr 10 '11 at 06:15
The mdat doesn't always contain the nal length; normally you need the pps/sps to determine the positions and lengths of the chunks in the mdat. – Paul Gregoire Dec 23 '14 at 17:33

score 14 · Answer 3 · answered Dec 15 '11 at 22:59

14

I found what michael was talking about defined in section 5.2.3 of ISO 14496-15.

Sebastian's answer refers to section 5.2.4.1.1 and 5.3.4.1.2.

You will not be able to parse the slices in the 'mdat' box without copies of the SPS and PPS from the 'avcC' box (defined in section 5.2.4.1.1)

answered Dec 15 '11 at 22:59

Mutant Bob

3,121
2
27
52

1

Time flies. The 14496-15:2014 changed the section numbering, this reply refers to at least 14496-15:2004 standard. – FredG Oct 26 '18 at 20:20

Parsing H264 in mdat MP4

3 Answers3

Linked