H264 RTP payload 判斷 IDR (I frame)

RTP payload判斷為iframe:

1.single nal unit packet
=> nal_unit_type(first byte & 0x1f)=5(IDR)

2.fragmentation unit
=> 先看 fu indicator(first byte & 0x1f) = 28 or 29
=> nal_unit_type(second byte & 0x1f) = 5(IDR)
=> fu header start bit(second byte & 0x80) = 0x80

範例程式:

public static bool isH264iFrame(byte[] paket)
{
    int RTPHeaderBytes = 0;

    int fragment_type = paket[RTPHeaderBytes + 0] & 0x1F;
    int nal_type = paket[RTPHeaderBytes + 1] & 0x1F;
    int start_bit = paket[RTPHeaderBytes + 1] & 0x80;

    if (((fragment_type == 28 || fragment_type == 29) && nal_type == 5 && start_bit == 128) || fragment_type == 5)
    {
        return true;
    }

    return false;
}

參考

1. http://stackoverflow.com/questions/1957427/detect-mpeg4-h264-i-frame-idr-in-rtp-stream
2. http://tw.myblog.yahoo.com/stevegigijoe/article?mid=183&sc=1
3. http://www.rfc-editor.org/rfc/rfc3984.txt 
4. http://www.itu.int/rec/T-REC-H.264-201003-S