ffmpeg audio and video decoding process (with code)

0 Preface

Recently I have been using and learning ffmpeg. Work needs to pull stream decoding, get audio and video data. These are processed using ffmpeg.

　　Because much of ffmpeg contact with not deep, use the process often encountered do not understand the place, you will spend a lot of time to access to information, so their own summary of the use of knowledge about facilitate their later You can easily find repeated use.

　　Environment: ubuntu16.04, dependent libraries have installed ffmpeg gcc compiler tool.

ffmpeg decoding process used in two very important structures, these two structures more complex, the number used very much, since I write separately to summarize.

AVPacket not save the decoded data.
AVFrame save decoded data.

1. decoding process

2. Code

  1 //***************************************************************
  2 // @file:    test.c
  3 // @author:  dingfang
  4 // @date    2019-07-24 18:55:16
  5 //***************************************************************
  6 
  7 #include <stdio.h>
  8 
  9 #ifdef __cplusplus
 10 extern "C"
 11 {
 12 #endif
 13 #include <libavcodec/avcodec.h>
 14 #include <libavformat/avformat.h>
 15 #ifdef __cplusplus
 16 };
 17 #endif
 18 
 19 int openCodecContext(const AVFormatContext *pFormatCtx, int *pStreamIndex, enum AVMediaType type, AVCodecContext **ppCodecCtx)
 20 {
 21     int streamIdx = -1;
 22     // 获取流下标
 23     for (int i = 0; i < pFormatCtx->nb_streams; i++) 
 24     {
 25         if (pFormatCtx->streams[i]->codec->codec_type == type) 
 26         {
 27             streamIdx = i;
 28             break;
 29         }
 30     }
 31     if (streamIdx == -1) 
 32     {
 33         printf("find video stream failed!\n");
 34         exit(-2);
 35     }
 36     // 寻找解码器
 37     AVCodecContext  *pCodecCtx = pFormatCtx->streams[streamIdx]->codec;
 38     AVCodec *pCodec = avcodec_find_decoder(pCodecCtx->codec_id);
 39     if (NULL == pCodec) 
 40     {
 41         printf("avcode find decoder failed!\n");
 42         exit(-2);
 43     }
 44 
 45     //打开解码器
 46     if (avcodec_open2(pCodecCtx, pCodec, NULL) < 0) 
 47     {
 48         printf("avcode open failed!\n");
 49         exit(-2);
 50     }
 51     *ppCodecCtx        = pCodecCtx;
 52     *pStreamIndex    = streamIdx;
 53 
 54     return 0;
 55 }
 56 
 57 int main(void)
 58 {
 59     AVFormatContext    *pInFormatCtx    = NULL;
 60     AVCodecContext  *pVideoCodecCtx    = NULL;
 61     AVCodecContext  *pAudioCodecCtx    = NULL;
 62     AVPacket *pPacket    = NULL;
 63     AVFrame *pFrame        = NULL; 
 64     int ret;
 65     /* 支持本地文件和网络url */
 66     const char streamUrl[] = "./test.flv";
 67 
 68     /* 1. 注册 */
 69     av_register_all();
 70     
 71     pInFormatCtx = avformat_alloc_context();
 72 
 73     /* 2. 打开流 */
 74     if(avformat_open_input(&pInFormatCtx, streamUrl, NULL, NULL) != 0)
 75     {
 76         printf("Couldn't open input stream.\n");
 77         return -1;
 78     }
 79 
 80     /* 3. 获取流的信息 */
 81     if(avformat_find_stream_info(pInFormatCtx, NULL) < 0)
 82     {
 83         printf("Couldn't find stream information.\n");
 84         return -1;
 85     }
 86 
 87     int videoStreamIdx = -1;
 88     int audioStreamIdx = -1;
 89     /* 4. 寻找并打开解码器 */
 90     openCodecContext(pInFormatCtx, &videoStreamIdx, AVMEDIA_TYPE_VIDEO, &pVideoCodecCtx);
 91     openCodecContext(pInFormatCtx, &audioStreamIdx, AVMEDIA_TYPE_AUDIO, &pAudioCodecCtx);
 92 
 93     pPacket    = av_packet_alloc();
 94     pFrame    = av_frame_alloc();
 95 
 96     int cnt = 30;
 97     while (cnt--)
 98     {
 99         /* 5. 读流数据, 未解码的数据存放于pPacket */
100         ret = av_read_frame(pInFormatCtx, pPacket);
101         if (ret < 0)
102         {
103             printf("av_read_frame error\n");
104             break;
105         }
106 
107         /* 6. 解码, 解码后的数据存放于pFrame */
108         /* 视频解码 */
109         if (pPacket->stream_index == videoStreamIdx)
110         {
111             avcodec_decode_video2(pVideoCodecCtx, pFrame, &ret, pPacket);
112             if (ret == 0)
113             {
114                 printf("video decodec error!\n");
115                 continue;
116             }
117             printf("* * * * * * video * * * * * * * * *\n");
118             printf("___height: [%d]\n", pFrame->height);
119             printf("____width: [%d]\n", pFrame->width);
120             printf("pict_type: [%d]\n", pFrame->pict_type);
121             printf("___format: [%d]\n", pFrame->format);
122             printf("* * * * * * * * * * * * * * * * * * *\n\n");
123         }
124 
125         /* 音频解码 */
126         if (pPacket->stream_index == audioStreamIdx)
127         {
128             avcodec_decode_audio4(pAudioCodecCtx, pFrame, &ret, pPacket);
129             if (ret < 0)
130             {
131                 printf("audio decodec error!\n");
132                 continue;
133             }
134             printf("* * * * * * audio * * * * * * * * * *\n");
135             printf("____nb_samples: [%d]\n", pFrame->nb_samples);
136             printf("__samples_rate: [%d]\n", pFrame->sample_rate);
137             printf("channel_layout: [%lu]\n", pFrame->channel_layout);
138             printf("________format: [%d]\n", pFrame->format);
139             printf("* * * * * * * * * * * * * * * * * * *\n\n");
140         }
141         av_packet_unref(pPacket);
142     }
143 
144     av_frame_free(&pFrame);
145     av_packet_free(&pPacket);
146     avcodec_close(pVideoCodecCtx);
147     avcodec_close(pAudioCodecCtx);
148     avformat_close_input(&pInFormatCtx);
149 
150     return 0;
151 }

该代码不能直接编译, 编译需要依赖ffmpeg库. 包含ffmpeg动态库和makefile文件的压缩包地址: 点我下载

解压后, 进入目录, 使用make命令即可编译.

3. 函数说明

这里就几个比较重要的函数简单介绍一下.

av_register_all() 　　　　　　 /* 使用ffmpeg几乎都要调用这一个函数, 注册ffmpeg各种编解码器, 复用器等. */

avformat_open_input() 　　　　/* 该函数用于打开本地多媒体文件或者网络流媒体url */

avformat_find_stream_info() /* 该函数用于读取一部分音视频数据并且获得一些相关的信息 */

avcodec_find_decoder()　　　　/* 由codec_id或者解码器名称来寻找对应的解码器 */

avcodec_open2()　　　　　　　 /* 初始化解码器 */

av_read_frame()　　　　　　　 /* 读流数据, 读出来的是压缩数据, 存放于AVPacket */

avcodec_decode_video2()　　　 /* 视频解码解码后数据为原始数据, 存放于AVFrame */

avcodec_decode_audio4()　　　 /* 音频解码解码后数据为原始数据, 存放于AVFrame */