How to read a h264 encoded file.

[amulya]

Hi all,

I have downloaded a h264/avc file from elecard.com/download/clips.php. I am trying to stream this file using live555 libraries. I am struck on how to read the avc file and packetize it.

1. How to verify the NAL unit conetnts in the h264/avc file i.e does the NAL unit have one entire frame in it or the data is distributed in multiple NAL units.
2. To stream, I am planning to packetize one nal unit per RTP packet, (because it is the simplest way to packetize). How to extract each NAL unit from a H264 encoded file?

This question may sound naive, but I would really appreciate any guidance on how to go about this.

Thanks
Amulya

[amulya]

got it...identifying nal unts

found this after doing a little reading here and there..

"Basically, NAL units are separated by a simple (at least) 4-byte sequence containing a
big-endian 1, i.e. 00 00 00 01. Finding a NAL unit boundary is therefore as simple as searching for this byte sequence. The NAL unit separators themselves are discarded, only the following NAL unit content is processed further."

[amulya]

I have been working on studying the h.264 bit stream format. As far as I understand, we can identify the nal units containing sequence parameter sets by looking at the nal header octet after start_code_prefix(0x000001). I think the value should be x67 for a sequence parameter set. The 3 bytes after that give us profile_level_idc. However I dont see such pattern anywhere in a sample h.264 file. Please do correct me if I am going wrong somewhere.

[amulya]

Also, regarding identifying the end of access unit is by looking for a access unit delimiter nal unit which has nal header octet value of x69.

[amulya]

Hi all,

I am using a tool hexedit to view the 264 stream and trying to make some sense out of it according to the 264 spec. I am looking for nal_unit_type 67 and 68 to determine the sprop_paramter_set. I have some concerns
1. The SPS and PPS are not separated by 000001. Currenlty I am thinking the SPS ends until I encounter a 68 which indicates start of PPS. I know my understanding is wrong, but I am lost how else to read the stream.
2. How do I know the length of SPS? There are variable length parameters in the stream.
3. Are the sprop_paramter_set just a base 64 endoded version of these two, seprated by comma?
Here is the portion of the stream with the parametrs
00 17 67 4D 40 33 9A 73 80 A0 08 B4 20 00 32 E9 E0 09 89 68 11 E3 06 23 C0 01 00 04 68 EE 3C 80 00 00 00 14 62 74 72 74 00 04 4A 3E...
How do I know where the SPS ends when there are no NAL separators? Can anybody explan what this stream means?
Please help .

[Akshay]

Some inputs which may help you
The 4-byte sequence 0x 00 00 00 01 is the AnnexB header. It may or may not be present in the bit-stream. It is optional.
SPS starts with 67. PPS starts with 68. And the length of SPS is variable and dependent on the toolsets enabled. The length of SPS can be known by the no of bytes between 67 and 68. in the sample bit-stream shared , SPS size is 17 bytes.
I think using the elecard stream analyser would help you a lot

[kiruba]

How the separation between slice and MB??

Hi,
I am currently working on H.264 ip.

00000001 byte is a separator between SPS and PPS and SLice header.. Which is demarcation line(separator) between the SLice data and MB header???


Regards,
Kiruba

[amulya]

The 264 spec says
Nal unit synatx follows the following structure
"byte_stream_nal_unit( NumBytesInNALunit ) { C Descriptor
while( next_bits( 24 ) != 0x000001 &&
next_bits( 32 ) != 0x00000001 )
leading_zero_8bits /* equal to 0x00 */ f(8)
if( next_bits( 24 ) != 0x000001 )
zero_byte /* equal to 0x00 */ f(8)
start_code_prefix_one_3bytes /* equal to 0x000001 */ f(24)
nal_unit( NumBytesInNALunit )
while( more_data_in_byte_stream( ) &&
next_bits( 24 ) != 0x000001 &&
next_bits( 32 ) != 0x00000001 )

trailing_zero_8bits /* equal to 0x00 */ f(8)
}"

How do we know the NumBytesInNALunit? Also, how to identify the nal unit start in a packet oriented H264 encoded file. I read from the Annex B that in byte stream oriented files, the delimiter is used, but what about the packet oriented formats? I seem to have hit a road block here :'(

[ichandu]

Thanks
Friend;
discuss of this type of commitment......
According to this article.
nice article.......

[mbr]

Headers in H.264

Hi
I am working on H.264 FMO 2, ROI coding

I would like to know the header size for SPS and PPS in H.264,
and the cost of PPS in FMO type2 and FMO type6


Thankyou
Ram