5、MP4解复用---AAC+H264

• MP4
  MP4同样是一种容器格式，是由一个一个Box组成，每个Box又分为Header与Data，Data又包含很多子Box，具体的MP4文件结构也看过，内部Box结构比较复杂，一般不写MP4解释器的话，Box结构不用了解太细，对MP4封装有MP4V2库可以使用，当然也可以使用FFMPEG库。
  本文从代码的角度上来分析使用FFMPEG对MP4文件解复用。
  完整代码如下：

#include <stdio.h>#include "libavutil/log.h"
#include "libavformat/avformat.h"#define ERROR_STRING_SIZE 1024#define ADTS_HEADER_LEN  7;const int sampling_frequencies[] = {96000,88200,64000,48000,44100,32000,24000,22050,16000,12000,11025,8000
};int adts_header(char * const p_adts_header, const int data_length,const int profile, const int samplerate,const int channels)
{int sampling_frequency_index = 3;int adtsLen = data_length + 7;int frequencies_size = sizeof(sampling_frequencies) / sizeof(sampling_frequencies[0]);for(int i = 0; i < frequencies_size; i++){if(sampling_frequencies[i] == samplerate){sampling_frequency_index = i;break;}}p_adts_header[0] = 0xff;p_adts_header[1] = 0xf0;p_adts_header[1] |= (0 << 3);p_adts_header[1] |= (0 << 1);p_adts_header[1] |= 1;p_adts_header[2] = (profile)<<6;p_adts_header[2] |= (sampling_frequency_index & 0x0f)<<2;p_adts_header[2] |= (0 << 1);p_adts_header[2] |= (channels & 0x04)>>2;p_adts_header[3] = (channels & 0x03)<<6;p_adts_header[3] |= (0 << 5);p_adts_header[3] |= (0 << 4);p_adts_header[3] |= (0 << 3);p_adts_header[3] |= (0 << 2);p_adts_header[3] |= ((adtsLen & 0x1800) >> 11);p_adts_header[4] = (uint8_t)((adtsLen & 0x7f8) >> 3);p_adts_header[5] = (uint8_t)((adtsLen & 0x7) << 5);p_adts_header[5] |= 0x1f;p_adts_header[6] = 0xfc;return 0;
}int main(int argc, char **argv)
{char *in_filename = "/home/yx/media_file/believe.mp4";char *h264_filename = "/home/yx/media_file/believe.h264";char *aac_filename = "/home/yx/media_file/believe.aac";FILE *aac_fd = NULL;FILE *h264_fd = NULL;h264_fd = fopen(h264_filename, "wb");aac_fd = fopen(aac_filename, "wb");AVFormatContext *ifmt_ctx = NULL;int video_index = -1;int audio_index = -1;AVPacket *pkt = NULL;ifmt_ctx = avformat_alloc_context();avformat_open_input(&ifmt_ctx, in_filename, NULL, NULL);                                // 将媒体流与解复用器相关联video_index = av_find_best_stream(ifmt_ctx, AVMEDIA_TYPE_VIDEO, -1, -1, NULL, 0);       // 查找视频流标签audio_index = av_find_best_stream(ifmt_ctx, AVMEDIA_TYPE_AUDIO, -1, -1, NULL, 0);       // 查找音频流标签const AVBitStreamFilter *bsfilter = av_bsf_get_by_name("h264_mp4toannexb");             // 查找H264过滤器AVBSFContext *bsf_ctx = NULL;av_bsf_alloc(bsfilter, &bsf_ctx);avcodec_parameters_copy(bsf_ctx->par_in, ifmt_ctx->streams[video_index]->codecpar);av_bsf_init(bsf_ctx);                                                                   // 初始化过滤器pkt = av_packet_alloc();av_init_packet(pkt);while (1){if(av_read_frame(ifmt_ctx, pkt) < 0)                                                // 逐个包读取break;if(pkt->stream_index == video_index){av_bsf_send_packet(bsf_ctx, pkt);                                               // 视频包就将包发给过滤器while (1){if(av_bsf_receive_packet(bsf_ctx, pkt) != 0)                                // 从过滤器接收包break;size_t size = fwrite(pkt->data, 1, pkt->size, h264_fd);                     // 将包写到输出文件av_packet_unref(pkt);}}else if(pkt->stream_index == audio_index){char adts_header_buf[7] = {0};adts_header(adts_header_buf, pkt->size,ifmt_ctx->streams[audio_index]->codecpar->profile,ifmt_ctx->streams[audio_index]->codecpar->sample_rate,ifmt_ctx->streams[audio_index]->codecpar->channels);                // 音频包构建ADTS帧头部fwrite(adts_header_buf, 1, 7, aac_fd);                                          // 将头部写到输出文件size_t size = fwrite( pkt->data, 1, pkt->size, aac_fd);                         // 将AAC包写到输出文件av_packet_unref(pkt);                                                           // 引用计数--}else{av_packet_unref(pkt);}}printf("while finish\n");printf("Hello World!\n");return 0;
}