How to detect I/P/B frame from H264 RTP packet

Question

I got H264 RTP packet from RTSP stream. So I want to detect whether the frame is an I-frame or not.

Below is the first packet I got from the first time I open the stream. So I believe that it is an I-frame. Here are the first 160 bytes:

packet:
00 00 00 01 67 4D 00 1F : 95 A8 14 01 6E 40 00 00
00 01 68 EE 3C 80 00 00 : 00 01 06 E5 01 33 80 00
00 00 01 65 B8 00 00 08 : 52 90 9F F6 BE D6 C6 9C
3D F6 D4 2F 49 FB F7 13 : F2 A9 C7 27 2D A4 75 59
6C DB FF 35 27 A4 C7 B6 : E7 69 A2 E0 FB 0E FF 2D
0E E0 6F 25 43 78 BF B9 : 69 22 1B 24 E3 CA 60 56
44 16 6C 15 44 DA 55 29 : C2 39 24 86 CE D6 75 BB
E0 0C F4 F4 EC C5 76 E4 : 7B 59 B9 40 2D B3 ED 19
E4 1D 94 B7 54 9B B3 D0 : 8F 24 58 CD 3C F3 FA E0
D4 7D 88 70 0E 49 79 12 : B2 14 92 BA B6 9C 3A F7
8D 13 78 6B 4C CD C0 CC : C8 39 6A AC BE 3D AA 00
9A DB D2 68 70 5F C4 20 : B7 5C FC 45 93 DB 00 12
9F 87 5A 66 2C B2 B8 E7 : 63 C4 87 0B A4 AA 2E 6D
AB 42 3F 02 C2 A6 F9 41 : E5 FE 80 64 49 14 38 3D
52 4B F6 B2 E7 53 DD 3E : F6 BB A8 EB 13 23 BB 71
B1 C9 90 06 92 3E 5F 15 : F2 C0 39 43 EA 24 5A 86
AE 11 27 D4 C5 4B 5C CD : 6C 90 2B 44 80 18 76 95
6E 16 DF 5D 86 49 25 5A : B6 66 23 E6 40 D4 25 6B
CE A2 4C EE 13 DD 7B 88 : FF A0 64 EC 33 44 B1 DC
B7 0B 89 5B 8F 85 68 3C : 65 3E 55 0F 41 4B 32 C9
C8 56 78 1A 15 14 8C C7 : F5 17 40 D4 EC BC 5B 62
8A 24 66 6A C3 7E 3B DB : 44 A8 EC D8 EE 37 E0 DE
.. .. .. .. .. .. .. .. : .. .. .. .. .. .. .. ..

Then I used the below piece of code to determine the frame:

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;
   }

My problem is that I cannot know the exact value of RTPHeaderByte. In this case my packets always start with "00 00 00 01".

Answer 1

You will have to parse the payload. see the SO answer Possible Locations for Sequence/Picture Parameter Set(s) for H.264 Stream. For IDR, all VCL NALUs will be type 5. As for B/P you will need to parse out the exp-golmb encoded data to find the slice type.