Analysis of h264nalu head

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One NALU Head analysis

F： forbidden_zero_bit. stay H.264 It is stipulated in the specification that this person must be 0. NRI： nal_ref_idc. take 00~11, It seems to indicate this NALU Importance , Such as 00 Of NALU The decoder can discard it without affecting the playback of the image ,0～3, The larger the value , At present NAL The more important , Need to be protected first . If at present NAL It's a piece of reference frame , Or a sequence parameter set , Or the important units of the image parameter set , This syntactic element must be greater than 0. Type： identification NAL In the cell RBSP data type , among ,nal_unit_type by 1, 2, 3, 4, 5 Of NAL The unit is called VCL Of NAL unit , Other types NAL Unit is not VCL Of NAL unit . Two NALU Type analysis nalu type Calculation method of

At the beginning of each frame （00 00 00 01） And then I got nalu Head code A byte

Use this formula to calculate nalutype ：int type = code & 0x1fnalu type

nal_unit_type. This NALU The type of unit ,1～12 from H.264 Use ,24～31 from H.264 For applications other than , The brief is as follows :0 No definition

1-23 NAL unit Single NAL Unit package
1 No zoning , Not IDR The film of the image
2 Slice partition A
3 Slice partition B
4 Slice partition C
5 IDR The film in the image
6 Supplementary enhanced information unit （SEI）
7 SPS
8 PPS
9 End of sequence
10 End of sequence
11 End of stream
12 fill
13-23 Retain 24 STAP-A A single time combination package
25 STAP-B A single time combination package
26 MTAP16 Multiple time packages
27 MTAP24 Multiple time packages
28 FU-A Sliced units
29 FU-B Sliced units
30-31 No definition

Common data frames

Sequence parameter set SPS----7：

SPS namely Sequence Paramater Set SPS A set of encoded video sequences is stored in (Coded video sequence) Global parameters of . The so-called encoded video sequence is the sequence composed of the pixel data of the original video frame by frame after being encoded . The parameters of the encoded data of each frame are saved in the image parameter set . General situation SPS and PPS Of NAL Unit Usually at the beginning of the whole stream . But in some special cases , These two structures may also appear in the middle of the code stream , The main reason may be ： 1： The decoder needs to start decoding in the middle of the stream ; 2： The encoder changes the parameters of the code stream in the process of encoding （ Such as image resolution ）;

Image parameter set PPS----8：

Except for the sequence parameter set SPS outside ,H.264 Another important parameter set in is image parameter set Picture Paramater Set(PPS). Usually ,PPS Be similar to SPS, stay H.264 A single bitstream is stored in a single stream NAL Unit in , It's just PPS NAL Unit Of nal_unit_type The value is 8; In the package format ,PPS Usually with SPS Together , Save in the header of the video file .

Keyframes IDR frame ----5：

I Frames represent keyframes , You can understand it as a complete preservation of this frame ; Decoding only needs the frame data to complete （ Because it contains the whole picture ）

P frame ----1：

P The frame represents this frame and the previous keyframe （ or P frame ） The difference between , In decoding, the difference defined in this frame needs to be superimposed on the previous cached picture , Generate the final picture .（ That's the difference frame ,P Frame has no full picture data , Only data different from the previous frame ）

3、 ... and Reference code

h264nalu Head analysis can refer to ffmpeg Source code h264_parser.c Of parse_nal_units function