CODEWITHSUNDEEP
objective-cvideo-captureh.264pack
user2517182
user2517182

Reputation: 1299

What does packing nal packets mean?

I have been trying to use the information from this question to solve a similar problem.

However, from the answer; I am not sure what is meant by the following:

I was not packing the raw NAL data correctly (not sure where this is documented, if anywhere).

or even the solution to this packing issue.

To solve #2, through trial and error I found that giving my NAL units in the following form worked:

[7 8 5] [1] [1] [1]..... [7 8 5] [1] [1] [1]..... (repeating)

Where each NAL unit is prefixed by a 32-bit start code equaling 0x00000001.

I have seen similar expression concerning nal packets. The original post from the link above has a statement that says:

My stream of NALs contains only SPS/PPS/IDR/P NALs (1, 5, 7, 8)

Again, what does this mean? How would I pack raw NAL data correctly in objective-c? Any help would be greatly appreciated.

Upvotes: 0

Views: 1114

Answers (1)

9dan
9dan

Reputation: 4282

It is called H.264 Annex B byte stream format (defined in ISO/IEC 14496-12). http://wiki.multimedia.cx/?title=H.264

I think, below page has very good explanation: http://gentlelogic.blogspot.kr/2011/11/exploring-h264-part-2-h264-bitstream.html

There are many open source implementations about this (not easily reusable though).

NAL AU is the unit of data to encapsulate an encoded frame data. It is consist of header (start code), type, length, and body part.

SPS/PPS/IDR/P are frame types.

SPS : config information about overall stream (encoding method, parameter, etc)
PPS : config information about pictures (width, height, etc)
IDR : special frame/packet to setup decoder
P : usual encoded frame data

Order of frame in ordinary movie files: SPS (once) PPS (once) IDR (periodically) P (actual picture) P P P P P IDR P P P P P P P ...

For annex B byte stream processing, Intel IPP code samples are very good reference (umc_h264_nal_spl.cpp). Its currently free to download (latest version is free to evaluate 30 days).
https://software.intel.com/en-us/articles/code-samples-for-intel-integrated-performance-primitives-intel-ipp-library-71

Annex B byte stream format describes how to store H.264 encoded frame data in a media container (such as mp4, MPEG 2 TS). Handling the container format binary data also requires many hard works. Each container uses different mechanism to specify codec configuration. As mentioned related SO post (AVAssetWriterInput H.264 Passthrough to QuickTime (.mov) - Passing in SPS/PPS to create avcC atom?), mp4/mov uses avcc box format which is defined in other ISO/IEC document.

Upvotes: 2

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