av_seek_frame

使用av_seek_frame截取视频，首先看一下av_seek_frame()的函数原型,如下：

int av_seek_frame(AVFormatContext *s, int stream_index, int64_t timestamp,int flags);

参数说明：

s: AVFormatContext类型的多媒体文件句柄

stream_index : int类型表示要进行操作的流索引

timestamp: int64_t类型的时间戳，表示要跳转到的时间位置

flags : 跳转方法，主要有一下几种

#define AVSEEK_FLAG_BACKWARD 1 ///< seek backward seek到timestamp之前的最近关键帧
#define AVSEEK_FLAG_BYTE 2 ///< seeking based on position in bytes 基于字节位置的跳转
#define AVSEEK_FLAG_ANY 4 ///< seek to any frame, even non-keyframes 跳转到任意帧，不一定是关键帧
#define AVSEEK_FLAG_FRAME 8 ///< seeking based on frame number 基于帧数量的跳转

函数执行成功 返回值>=0

在执行跳转之前使用AV_TIME_BASE进行一个时间戳的转换。

在代码中使用到了boost::program_options对命令行参数进行操作，如果要编译成功需要使用boost::program_options,如果不想使用可以直接将那部分代码删除，然后编译连接的时候要链接到ffmpeg lib即可。

示例代码如下：

#include <boost/program_options.hpp>
#include <iostream>
#include <string>
#ifdef __cplusplus
extern "C" {
#endif
#include <libavcodec/avcodec.h>
#include <libavformat/avformat.h>
#include <libavutil/avutil.h>
#ifdef __cplusplus
}
#endif
namespace opt = boost::program_options;
void Init(){av_register_all();av_log_set_level(AV_LOG_INFO);}
//parse command line
opt::variables_map getvm(int argc,const char *argv[]);
//find the media info in input file
void find_streaminfo(AVFormatContext* *s,const char *input);
//process the input time
int64_t getStartTime(const AVStream *in,int seconds);
//set output format context and return out stream
AVStream* set_output_format_context(const AVStream* in,AVFormatContext **s,const char *output);
// seek start time and save it to output file
void seekToAndSave(AVFormatContext *ifmt_ctx,AVStream *in_stream,
                   AVFormatContext *ofmt_ctx,AVStream *out_stream,int index,int64_t starttime);
int main(int argc,const char *argv[]){
    opt::variables_map vm = getvm(argc,argv);
    Init();
    const char *input = vm["input"].as<std::string>().c_str();
    const char *output = vm["output"].as<std::string>().c_str();
    AVFormatContext *ifmt_ctx = avformat_alloc_context();
    //find stream info
    find_streaminfo(&ifmt_ctx,input);
    //find video stream index
    int video_stream_index = -1;
    video_stream_index = av_find_best_stream(ifmt_ctx,AVMEDIA_TYPE_VIDEO,-1,-1,NULL,0);
    if (video_stream_index){
        av_log(NULL,AV_LOG_ERROR,"fail to find video stream");
        return -1;
    }
    AVStream *in_strem = ifmt_ctx->streams[video_stream_index];
    //find the seek time
    int seconds = vm.count("time") ? vm["time"].as<int>() : 0;
    int64_t startTime = getStartTime(in_strem,seconds);
    //the start time must less than the total length
    if (startTime > in_strem->duration){
        av_log(NULL,AV_LOG_ERROR,"start time must less than the total time");
        return -1;
    }
    //set output avformatcontext
    AVFormatContext *ofmt_ctx = avformat_alloc_context();
    AVStream *out_stream = set_output_format_context(in_strem,&ofmt_ctx,output);
    if (avio_open(&ofmt_ctx->pb,output,AVIO_FLAG_WRITE) < 0){
        av_log(NULL,AV_LOG_ERROR,"fail to open output file : %s",output);
        return -1;
    }
    //seek and  save
    seekToAndSave(ifmt_ctx,in_strem,ofmt_ctx,out_stream,video_stream_index,startTime);
    //
    avio_close(ofmt_ctx->pb);
    avformat_free_context(ofmt_ctx);
    avformat_close_input(&ifmt_ctx);
    return 0;
}

AVStream* set_output_format_context(const AVStream* in,AVFormatContext **s,const char *output){
    assert(in != NULL);
    if (*s == NULL){
        *s = avformat_alloc_context();
    }
    AVOutputFormat *oformat = av_guess_format(NULL,output,NULL);
    if (oformat == NULL){
        av_log(NULL,AV_LOG_ERROR,"fail to find the oformat");
        exit(-1);
    }
    (*s)->oformat = oformat;
    if (avformat_alloc_output_context2(s,oformat,oformat->name,output) < 0){
        av_log(NULL,AV_LOG_ERROR,"fail to alloc output context");
        exit(-1);
    }
    AVCodec *pCodec = avcodec_find_decoder(in->codecpar->codec_id);
    if (pCodec == NULL){
        av_log(NULL,AV_LOG_ERROR,"fail to find codec ");
        exit(-1);
    }
    AVStream *out_stream = avformat_new_stream(*s,pCodec);
    if (out_stream == NULL){
        av_log(NULL,AV_LOG_ERROR,"fail to create out stream");
        exit(-1);
    }
    if (avcodec_parameters_copy(out_stream->codecpar,in->codecpar) < 0){
        av_log(NULL,AV_LOG_ERROR,"fail to copy codec info");
        exit(-1);
    }
    out_stream->codec->codec_tag = 0;
    if (oformat->flags & AVFMT_GLOBALHEADER){
        out_stream->codec->flags |= AV_CODEC_FLAG_GLOBAL_HEADER;
    }
    //dump info
    av_dump_format(*s,0,output,1);
    return out_stream;
}

int64_t getStartTime(const AVStream *in,int seconds){
    int64_t startTime = seconds * AV_TIME_BASE;
    int64_t target_time = av_rescale_q(startTime,AV_TIME_BASE_Q,in->time_base);
    return target_time;
}
void find_streaminfo(AVFormatContext* *s,const char *input)
{
    if (avformat_open_input(s,input,NULL,NULL) < 0){
        av_log(NULL,AV_LOG_ERROR,"fail to open %s",input);
        exit(-1);
    }
    if (avformat_find_stream_info(*s,NULL) < 0){
        av_log(NULL,AV_LOG_ERROR,"fail to find stream info");
        exit(-1);
    }
    //dump the information to console
    av_dump_format(*s,0,input,0);
}
opt::variables_map getvm(int argc,const char *argv[]){
   //step 1 create a description
   opt::options_description descr("All options");
    //step 2 add options
   descr.add_options()
   ("time,t",opt::value<int>(),"start time")
   ("input,i",opt::value<std::string>(),"input file")
   ("output,o",opt::value<std::string>(),"output file")
   ("help,h","use guide");
    if (argc < 3){
    std::cout << descr << std::endl;
    exit(-1);
   }
   //step 3 create variables_map
   opt::variables_map vm;
   opt::store(opt::parse_command_line(argc,argv,descr),vm);
   opt::notify(vm);
   if (!vm.count("input") || !vm.count("output")){
    std::cout << " miss input file or output file" << std::endl;
    exit(-1);
   }
   if (vm.count("help")){
    std::cout << descr << std::endl;
    exit(-1);
   }
   return vm;
}
void seekToAndSave(AVFormatContext *ifmt_ctx,AVStream *in_stream,
                   AVFormatContext *ofmt_ctx,AVStream *out_stream,int index,int64_t starttime)
{
     assert(ifmt_ctx != NULL);
     assert(in_stream != NULL);
     assert(ofmt_ctx != NULL);
     assert(out_stream != NULL);
     if(avformat_write_header(ofmt_ctx,NULL) < 0){
        av_log(NULL,AV_LOG_ERROR,"fail to write header");
        exit(-1);
     }
     if (av_seek_frame(ifmt_ctx,index,starttime,AVSEEK_FLAG_BACKWARD) < 0){
        av_log(NULL,AV_LOG_ERROR,"fail to seek time");
     }
     AVFrame *pframe = av_frame_alloc();
     AVPacket *pkt = av_packet_alloc();
     av_init_packet(pkt);
     while(av_read_frame(ifmt_ctx,pkt) >=0){
        if (pkt->stream_index == index){
            pkt->pts = av_rescale_q_rnd(pkt->pts,in_stream->time_base,out_stream->time_base,static_cast<AVRounding>(AV_ROUND_NEAR_INF | AV_ROUND_PASS_MINMAX));
            pkt->dts = av_rescale_q_rnd(pkt->dts,in_stream->time_base,out_stream->time_base,static_cast<AVRounding>(AV_ROUND_NEAR_INF | AV_ROUND_PASS_MINMAX));
            pkt->duration = av_rescale_q(pkt->duration,in_stream->time_base,out_stream->time_base);
            pkt->pos = -1;
            if (av_interleaved_write_frame(ofmt_ctx,pkt) < 0){
                av_log(NULL,AV_LOG_ERROR,"fail to write frame");
                break;
            }
            av_packet_unref(pkt);
        }
     }
     if(av_write_trailer(ofmt_ctx) < 0){
        av_log(NULL,AV_LOG_ERROR,"fail to write trailer");
        exit(-1);
     }
      av_frame_free(&pframe);
      av_packet_free(&pkt);
}

赏

谢谢你请我吃糖果