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?

Answer 1

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.

Answer 2

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.