视频流至少发从一帧SPS和一帧PPS来告诉客户端视频流信息

H.264原始码流（裸流）是由⼀个接⼀个NALU组成。它的功能分为两层，VCL (视频编码层) 和
NAL (⽹络提取层)。VCL：负责压缩视频；NAL：把VCL压缩的视频封装成NLU帧

1、NLU帧包括三个部分

[StartCode] [NALU Header] [NALU Payload]

1.4 H264 annexb模式

H264有两种封装

const AVBitStreamFilter *bsfilter = av_bsf_get_by_name("h264_mp4toannexb");
AVBSFContext *bsf_ctx = NULL;
// 2 初始化过滤器上下⽂
av_bsf_alloc(bsfilter, &bsf_ctx); //AVBSFContext;
// 3 添加解码器属性
avcodec_parameters_copy(bsf_ctx->par_in, ifmt_ctx->streams[videoindex]->codecpar);
av_bsf_init(bsf_ctx);

[4字节起始码] + [SPS NALU数据] 
[4字节起始码] + [PPS NALU数据] 
[3字节起始码] + [I帧NALU数据] 
[3字节起始码] + [P帧NALU数据] 
[3字节起始码] + [B帧NALU数据] 
...（后续NALU以此类推）

输出的文件是H.264视频文件

2. MP4直接的码流对应下图右边，开头记录的是NALU帧的长度，都是4字节

二、MP4->H.264代码

#include <iostream>extern "C"{#include <libavutil/log.h>#include <libavformat/avio.h>#include <libavformat/avformat.h>#include <libavcodec/bsf.h> 
}using namespace std;static char err_buf[128] = {0};
static char* av_get_err(int errnum)
{av_strerror(errnum, err_buf, 128);return err_buf;
}int main(int argc, char **argv) {string inputFile = "believe.mp4";string outputFile = "believe.h264"; FILE * outfp = fopen(outputFile.c_str(),"wb");printf("in:%s out:%s\n", inputFile.c_str(), outputFile.c_str());// 创建AVFormatContext上下文AVFormatContext * ifmt_ctx = avformat_alloc_context();if (!ifmt_ctx) {printf("[error] Could not allocate context.\n");return -1;}int ret = avformat_open_input(&ifmt_ctx, inputFile.c_str(), NULL, NULL);if (ret != 0) {printf("[error] avformat_open_input: %s\n", av_get_err(ret));return -1;}ret = avformat_find_stream_info(ifmt_ctx, NULL);if (ret < 0) {printf("[error] avformat_find_stream_info: %s\n", av_get_err(ret));avformat_close_input(&ifmt_ctx);return -1;}// 查找出哪个码流是video/audio/subtitlesint videoindex = -1;videoindex = av_find_best_stream(ifmt_ctx, AVMEDIA_TYPE_VIDEO, -1, -1, NULL, 0);if(videoindex == -1) {printf("[error] Didn't find a video stream.\n");avformat_close_input(&ifmt_ctx);return -1;}// 创建AVPacket包AVPacket * pkt = av_packet_alloc();av_init_packet(pkt);// 1 获取一个名为"h264_mp4toannexb"的比特流过滤器// 用于将MP4容器中的H.264视频流转换为Annex B格式的H.264流const AVBitStreamFilter * bsfilter = av_bsf_get_by_name("h264_mp4toannexb");AVBSFContext * bsf_ctx = NULL;// 2 初始化过滤器上下文av_bsf_alloc(bsfilter, &bsf_ctx); //AVBSFContext;// 3 复制视频流的编解码参数到过滤器上下文avcodec_parameters_copy(bsf_ctx->par_in, ifmt_ctx->streams[videoindex]->codecpar);av_bsf_init(bsf_ctx);int file_end = 0;while (0 == file_end) {if((ret = av_read_frame(ifmt_ctx, pkt)) < 0) {// 没有更多包可读file_end = 1;printf("read file end: ret:%d\n", ret);}if(ret == 0 && pkt->stream_index == videoindex) {int input_size = pkt->size;int out_pkt_count = 0;if (av_bsf_send_packet(bsf_ctx, pkt) != 0) // bitstreamfilter内部去维护内存空间{av_packet_unref(pkt);   // 你不用了就把资源释放掉continue;       // 继续送}av_packet_unref(pkt);   // 释放资源while(av_bsf_receive_packet(bsf_ctx, pkt) == 0) {out_pkt_count++;// printf("fwrite size:%d\n", pkt->size);size_t size = fwrite(pkt->data, 1, pkt->size, outfp);if(size != pkt->size){printf("fwrite failed-> write:%u, pkt_size:%u\n", size, pkt->size);}av_packet_unref(pkt);}if(out_pkt_count >= 2){printf("cur pkt(size:%d) only get 1 out pkt, it get %d pkts\n", input_size, out_pkt_count);}// 注释掉前面代码，可以直接保存mp4流// size_t size = fwrite(pkt->data, 1, pkt->size, outfp);// if(size != pkt->size)// {//     printf("fwrite failed-> write:%u, pkt_size:%u\n", size, pkt->size);// }// av_packet_unref(pkt);}else {if(ret == 0) av_packet_unref(pkt); // 释放内存}}if (outfp) fclose(outfp);if (bsf_ctx) av_bsf_free(&bsf_ctx);if (pkt) av_packet_free(&pkt);if (ifmt_ctx) avformat_close_input(&ifmt_ctx);printf("finish\n");return 0;
}