ffmpeg ts 关于av_seek_frame

最新推荐文章于 2025-05-06 11:08:01 发布

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最新推荐文章于 2025-05-06 11:08:01 发布 · 823 阅读

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#ffmpeg

博客主要围绕FFmpeg展开，介绍了用FFmpeg命令行裁剪视频文件的方法，如通过特定命令实现精准seek定位。还阐述了命令行入口、执行流程、输入上下文设置等内容，以及avformat_seek_file的调用过程和不同flags的查询规则。此外，对TS切割格式的分析也有所提及。

1 ffmpeg命令行

一般对视频文件的裁剪我们通过一行 ffmpeg命令行即可实现，比如

ffmpeg -ss 0.5 - t 3 - i a.mp4 vcodec copy b.mp4

其中 -ss 放置较前开启精准seek定位对于mp4而言 seek将从moov中相关索引表查找 0.5s时刻附近最近的关键帧（此描述可能不精确）

ffmpeg根据此命令是如何裁剪呢？

1.1 命令行入口

执行代码位置

fftools/ffmpeg.c

int main(int argc, char **argv)
{
    int i, ret;
    BenchmarkTimeStamps ti;

    init_dynload();

    register_exit(ffmpeg_cleanup);

    setvbuf(stderr,NULL,_IONBF,0); /* win32 runtime needs this */

    av_log_set_flags(AV_LOG_SKIP_REPEATED);
    parse_loglevel(argc, argv, options);

    if(argc>1 && !strcmp(argv[1], "-d")){
        run_as_daemon=1;
        av_log_set_callback(log_callback_null);
        argc--;
        argv++;
    }

#if CONFIG_AVDEVICE
    avdevice_register_all();
#endif
    avformat_network_init();

    show_banner(argc, argv, options);

    /* parse options and open all input/output files */
    ret = ffmpeg_parse_options(argc, argv);
    if (ret < 0)
        exit_program(1);

    if (nb_output_files <= 0 && nb_input_files == 0) {
        show_usage();
        av_log(NULL, AV_LOG_WARNING, "Use -h to get full help or, even better, run 'man %s'\n", program_name);
        exit_program(1);
    }

    /* file converter / grab */
    if (nb_output_files <= 0) {
        av_log(NULL, AV_LOG_FATAL, "At least one output file must be specified\n");
        exit_program(1);
    }

    for (i = 0; i < nb_output_files; i++) {
        if (strcmp(output_files[i]->ctx->oformat->name, "rtp"))
            want_sdp = 0;
    }

    current_time = ti = get_benchmark_time_stamps();
    if (transcode() < 0)
        exit_program(1);
    if (do_benchmark) {
        int64_t utime, stime, rtime;
        current_time = get_benchmark_time_stamps();
        utime = current_time.user_usec - ti.user_usec;
        stime = current_time.sys_usec  - ti.sys_usec;
        rtime = current_time.real_usec - ti.real_usec;
        av_log(NULL, AV_LOG_INFO,
               "bench: utime=%0.3fs stime=%0.3fs rtime=%0.3fs\n",
               utime / 1000000.0, stime / 1000000.0, rtime / 1000000.0);
    }
    av_log(NULL, AV_LOG_DEBUG, "%"PRIu64" frames successfully decoded, %"PRIu64" decoding errors\n",
           decode_error_stat[0], decode_error_stat[1]);
    if ((decode_error_stat[0] + decode_error_stat[1]) * max_error_rate < decode_error_stat[1])
        exit_program(69);

    exit_program(received_nb_signals ? 255 : main_return_code);
    return main_return_code;
}

1.2 执行流程

代码流程比较清晰主要流程如下：

1.3 输入上下文设置

ffmpeg将解析的参数存放到OptionParseContext中包含输入及输出参数设置，然后进入输入上下文

根据之前解析的数据OptionPareseContext作为参数传入，确定好seek点，然后调用 avformat_seek_file

static int open_input_file(OptionsContext *o, const char *filename)
{
    InputFile *f;
    AVFormatContext *ic;
    AVInputFormat *file_iformat = NULL;
    int err, i, ret;
    int64_t timestamp;
    AVDictionary *unused_opts = NULL;
    AVDictionaryEntry *e = NULL;
    char *   video_codec_name = NULL;
    char *   audio_codec_name = NULL;
    char *subtitle_codec_name = NULL;
    char *    data_codec_name = NULL;
    int scan_all_pmts_set = 0;

    if (o->stop_time != INT64_MAX && o->recording_time != INT64_MAX) {
        o->stop_time = INT64_MAX;
        av_log(NULL, AV_LOG_WARNING, "-t and -to cannot be used together; using -t.\n");
    }

    if (o->stop_time != INT64_MAX && o->recording_time == INT64_MAX) {
        int64_t start_time = o->start_time == AV_NOPTS_VALUE ? 0 : o->start_time;
        if (o->stop_time <= start_time) {
            av_log(NULL, AV_LOG_ERROR, "-to value smaller than -ss; aborting.\n");
            exit_program(1);
        } else {
            o->recording_time = o->stop_time - start_time;
        }
    }

    if (o->format) {
        if (!(file_iformat = av_find_input_format(o->format))) {
            av_log(NULL, AV_LOG_FATAL, "Unknown input format: '%s'\n", o->format);
            exit_program(1);
        }
    }

    if (!strcmp(filename, "-"))
        filename = "pipe:";

    stdin_interaction &= strncmp(filename, "pipe:", 5) &&
                         strcmp(filename, "/dev/stdin");

    /* get default parameters from command line */
    ic = avformat_alloc_context();
    if (!ic) {
        print_error(filename, AVERROR(ENOMEM));
        exit_program(1);
    }
    if (o->nb_audio_sample_rate) {
        av_dict_set_int(&o->g->format_opts, "sample_rate", o->audio_sample_rate[o->nb_audio_sample_rate - 1].u.i, 0);
    }
    if (o->nb_audio_channels) {

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