h. 264 code stream explanation

H264 The code stream can be divided into two layers ,VCL Layer and the NAL layer ,NAL The full name is Network abstraction layer, It is called network abstraction layer , It has been saved. H264 Relevant parameter information and image information ,NAL The layer consists of multiple units NALU form ,NALU By NALU head （00 00 00 01 perhaps 00 00 01）、sps( Sequence parameter set )、pps( Image parameter set )、slice、sei、IDR frame 、I frame （ When the image motion changes less ,I After the frame is 7 individual P frame , If the image motion changes greatly , One sequence is short ,I The frame may be followed by 3 A or 4 individual P frame ）、P frame 、B Frame and other data .

sps、pps、I frame 、P Frame in NALU And nalu type Judge

A complete NALU The unit structure diagram is as follows ：

         00 00 00 01 yes NALU head , Is the beginning of the sequence identifier ,0x27 To binary is 100111,00111 To convert to decimal is 7, that 7 Corresponding NALU type=sps,0x28 To binary is 101000, To convert to decimal is 8,8 Corresponding NALU type=pps,0x25 To binary is 100101,00101 To convert to decimal is 5,5 Corresponding NALU type=IDR frame （ Use FFMPEG,sps and pps Is stored in the AVCodecContext Of extradata.data in , Extract in decoding sps and pps when , Judge NALU type It can be used extradata.data[ 4 ]&0x1f( The result is 7 yes sps,8 yes pps, The calculation method is to convert to binary ,0x27&0x1f=11111&00111=00111=7,pps The calculation is similar to )）,NALU type=1 yes Slice,splice There are three coding modes ,I_slice、P_slice、B_slice,I The frame is divided and saved in Slice in .