音视频开发工程师：FFmpeg时间戳详解

think@opensuse> ffprobe tnmil3.flv 
Input #0, flv, from 'tnmil3.flv':
  Metadata:
    encoder         : Lavf58.20.100
  Duration: 00:00:03.60, start: 0.017000, bitrate: 513 kb/s
    Stream #0:0: Video: h264 (High), yuv420p(progressive), 784x480, 25 fps, 25 tbr, 1k tbn, 50 tbc
    Stream #0:1: Audio: aac (LC), 44100 Hz, stereo, fltp, 128 kb/s

// Internal time base represented as integer
#define AV_TIME_BASE            1000000
// Internal time base represented as fractional value
#define AV_TIME_BASE_Q          (AVRational){1, AV_TIME_BASE}

/**
 * Convert an AVRational to a `double`.
 * @param a AVRational to convert
 * @return `a` in floating-point form
 * @see av_d2q()
static inline double av_q2d(AVRational a){
    return a.num / (double) a.den;
}

时刻值：timestamp(单位秒) = pts × av_q2d(stream->time_base);
时长值：duration(单位秒) = stream->duration × av_q2d(stream->time_base);

/**
 * Rescale a 64-bit integer by 2 rational numbers.
 * The operation is mathematically equivalent to `a × bq / cq`.
 * This function is equivalent to av_rescale_q_rnd() with #AV_ROUND_NEAR_INF.
 * @see av_rescale(), av_rescale_rnd(), av_rescale_q_rnd()
int64_t av_rescale_q(int64_t a, AVRational bq, AVRational cq) av_const;

/**
 * Convert valid timing fields (timestamps / durations) in a packet from one
 * timebase to another. Timestamps with unknown values (AV_NOPTS_VALUE) will be
 * ignored.
 * @param pkt packet on which the conversion will be performed
 * @param tb_src source timebase, in which the timing fields in pkt are
 *               expressed
 * @param tb_dst destination timebase, to which the timing fields will be
 *               converted
void av_packet_rescale_ts(AVPacket *pkt, AVRational tb_src, AVRational tb_dst);

typedef struct AVStream {
    ......
     * This is the fundamental unit of time (in seconds) in terms
     * of which frame timestamps are represented.
     * decoding: set by libavformat
     * encoding: May be set by the caller before avformat_write_header() to
     *           provide a hint to the muxer about the desired timebase. In
     *           avformat_write_header(), the muxer will overwrite this field
     *           with the timebase that will actually be used for the timestamps
     *           written into the file (which may or may not be related to the
     *           user-provided one, depending on the format).
    AVRational time_base;
    ......
}

av_read_frame(ifmt_ctx, &pkt);
pkt.pts = av_rescale_q_rnd(pkt.pts, in_stream->time_base, out_stream->time_base, AV_ROUND_NEAR_INF|AV_ROUND_PASS_MINMAX);
pkt.dts = av_rescale_q_rnd(pkt.dts, in_stream->time_base, out_stream->time_base, AV_ROUND_NEAR_INF|AV_ROUND_PASS_MINMAX);
pkt.duration = av_rescale_q(pkt.duration, in_stream->time_base, out_stream->time_base);

// 从输入文件中读取packet
av_read_frame(ifmt_ctx, &pkt);
// 将packet中的各时间值从输入流封装格式时间基转换到输出流封装格式时间基
av_packet_rescale_ts(&pkt, in_stream->time_base, out_stream->time_base);

think@opensuse> ffprobe -show_frames -select_streams v tnmil3.flv | grep pkt_pts  
Input #0, flv, from 'tnmil3.flv':
  Metadata:
    encoder         : Lavf58.20.100
  Duration: 00:00:03.60, start: 0.017000, bitrate: 513 kb/s
    Stream #0:0: Video: h264 (High), yuv420p(progressive), 784x480, 25 fps, 25 tbr, 1k tbn, 50 tbc
    Stream #0:1: Audio: aac (LC), 44100 Hz, stereo, fltp, 128 kb/s
pkt_pts=80
pkt_pts_time=0.080000
pkt_pts=120
pkt_pts_time=0.120000
pkt_pts=160
pkt_pts_time=0.160000
pkt_pts=200
pkt_pts_time=0.200000

think@opensuse> ffprobe -show_frames -select_streams v tnmil3.ts | grep pkt_pts  
Input #0, mpegts, from 'tnmil3.ts':
  Duration: 00:00:03.58, start: 0.017000, bitrate: 619 kb/s
  Program 1 
    Metadata:
      service_name    : Service01
      service_provider: FFmpeg
    Stream #0:0[0x100]: Video: h264 (High) ([27][0][0][0] / 0x001B), yuv420p(progressive), 784x480, 25 fps, 25 tbr, 90k tbn, 50 tbc
    Stream #0:1[0x101]: Audio: aac (LC) ([15][0][0][0] / 0x000F), 44100 Hz, stereo, fltp, 127 kb/s
pkt_pts=7200
pkt_pts_time=0.080000
pkt_pts=10800
pkt_pts_time=0.120000
pkt_pts=14400
pkt_pts_time=0.160000
pkt_pts=18000
pkt_pts_time=0.200000

typedef struct AVCodecContext {
    ......
     * This is the fundamental unit of time (in seconds) in terms
     * of which frame timestamps are represented. For fixed-fps content,
     * timebase should be 1/framerate and timestamp increments should be
     * identically 1.
     * This often, but not always is the inverse of the frame rate or field rate
     * for video. 1/time_base is not the average frame rate if the frame rate is not
     * constant.
     * Like containers, elementary streams also can store timestamps, 1/time_base
     * is the unit in which these timestamps are specified.
     * As example of such codec time base see ISO/IEC 14496-2:2001(E)
     * vop_time_increment_resolution and fixed_vop_rate
     * (fixed_vop_rate == 0 implies that it is different from the framerate)
     * - encoding: MUST be set by user.
     * - decoding: the use of this field for decoding is deprecated.
     *             Use framerate instead.
    AVRational time_base;
    ......
}

AVFormatContext *ifmt_ctx;
AVStream *in_stream;
AVCodecContext *dec_ctx;
AVPacket packet;
AVFrame *frame;
// 从输入文件中读取编码帧
av_read_frame(ifmt_ctx, &packet);
// 时间基转换
int raw_video_time_base = av_inv_q(dec_ctx->framerate);
av_packet_rescale_ts(packet, in_stream->time_base, raw_video_time_base);
// 解码
avcodec_send_packet(dec_ctx, packet)
avcodec_receive_frame(dec_ctx, frame);

AVFormatContext *ofmt_ctx;
AVStream *out_stream;
AVCodecContext *dec_ctx;
AVCodecContext *enc_ctx;
AVPacket packet;
AVFrame *frame;
// 编码
avcodec_send_frame(enc_ctx, frame);
avcodec_receive_packet(enc_ctx, packet);
// 时间基转换
packet.stream_index = out_stream_idx;
enc_ctx->time_base = av_inv_q(dec_ctx->framerate);
av_packet_rescale_ts(&opacket, enc_ctx->time_base, out_stream->time_base);
// 将编码帧写入输出媒体文件
av_interleaved_write_frame(o_fmt_ctx, &packet);

AVFormatContext *ifmt_ctx;
AVStream *in_stream;
AVCodecContext *dec_ctx;
AVPacket packet;
AVFrame *frame;
// 从输入文件中读取编码帧
av_read_frame(ifmt_ctx, &packet);
// 时间基转换
int raw_audio_time_base = av_inv_q(dec_ctx->sample_rate);
av_packet_rescale_ts(packet, in_stream->time_base, raw_audio_time_base);
// 解码
avcodec_send_packet(dec_ctx, packet)
avcodec_receive_frame(dec_ctx, frame);

AVFormatContext *ofmt_ctx;
AVStream *out_stream;
AVCodecContext *dec_ctx;
AVCodecContext *enc_ctx;
AVPacket packet;
AVFrame *frame;
// 编码
avcodec_send_frame(enc_ctx, frame);
avcodec_receive_packet(enc_ctx, packet);
// 时间基转换