使用FFmpeg库进行音视频处理时，经常需要处理视频时间、编解码的dts/pts时间戳，时间的准确处理对音视频处理至关重要。

time_base

时间基(time_base)是FFmpeg中作为时间单位的概念。不同的音视频封装格式，可以使用不同的time_base。

FFmpeg内部的时间都会基于AV_TIME_BASE作为时间单位，比如AVStream中的duration表示这个流的长度为duration个AV_TIME_BASE。

/**
 * Internal time base represented as integer
 */

#define AV_TIME_BASE            1000000

AVStream中的time_base的定义：

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;

	/**
     * Decoding: duration of the stream, in stream time base.
     * If a source file does not specify a duration, but does specify
     * a bitrate, this value will be estimated from bitrate and file size.
     *
     * Encoding: May be set by the caller before avformat_write_header() to
     * provide a hint to the muxer about the estimated duration.
     */
  int64_t duration;

	// ...
};

为了时间计算的精度，FFmpeg引入有理数的时间基形式 AV_TIME_BASE_Q

/**
 * Rational number (pair of numerator and denominator).
 */
typedef struct AVRational{
    int num; ///< Numerator
    int den; ///< Denominator
} AVRational;

/**
 * Internal time base represented as fractional value
 */

#define AV_TIME_BASE_Q          (AVRational){1, AV_TIME_BASE}

ffmpeg提供了一个把AVRatioal有理数结构转换成double的函数：

/**
 * 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;
}

时间戳换算

比如计算视频的总时长：

AVFormatContext *ifmt_ctx = NULL;
avformat_open_input(&ifmt_ctx, filename, NULL, NULL);
double totle_seconds = ifmt_ctx->duration * av_q2d(AV_TIME_BASE_Q);

根据pts计算一帧在视频中对应的秒数位置：

double sec = enc_pkt.pts * av_q2d(ofmt_ctx->streams[stream_index]->time_base);

ffmpeg内部的时间戳与标准的时间转换方法

timestamp(ffmpeg内部时间戳) = AV_TIME_BASE * time(秒)
time(秒) = AV_TIME_BASE_Q * timestamp(ffmpeg内部时间戳)

当需要把视频跳转到N秒的时候：

int64_t timestamp = N * AV_TIME_BASE; 
av_seek_frame(fmtctx, index_of_video, timestamp, AVSEEK_FLAG_BACKWARD);

时间基转换函数

另外，FFmpeg为我们提供了不同时间基之间的转换函数，把时间戳从一个时基调整到另外一个时基：

/**
 * 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;

dts/pts计算

转码流程中的dts/pts转换：

输入

AVPacket packet;
av_read_frame(ifmt_ctx, &packet)

解码

AVFrame *frame = NULL;
// 转换为解码器的时间戳
av_packet_rescale_ts(&packet,
                             ifmt_ctx->streams[stream_index]->time_base,
                             ifmt_ctx->streams[stream_index]->codec->time_base);
dec_func = (type == AVMEDIA_TYPE_VIDEO) ? avcodec_decode_video2 :
        avcodec_decode_audio4;
ret = dec_func(ifmt_ctx->streams[stream_index]->codec, frame,
                       &got_frame, &packet);

编码

AVPacket enc_pkt;
int (*enc_func)(AVCodecContext *, AVPacket *, const AVFrame *, int *) =
  (ifmt_ctx->streams[stream_index]->codec->codec_type ==
   AVMEDIA_TYPE_VIDEO) ? avcodec_encode_video2 : avcodec_encode_audio2;
  
/* encode filtered frame */
enc_pkt.data = NULL;
enc_pkt.size = 0;
av_init_packet(&enc_pkt);
ret = enc_func(ofmt_ctx->streams[stream_index]->codec, &enc_pkt,
                filt_frame, got_frame);

输出

// 编码后的enc_pkt的时间戳以AVCodecContext->time_base为单位
// 编码器时间戳转为输出流AVStream时间戳
av_packet_rescale_ts(&enc_pkt,
                       ofmt_ctx->streams[stream_index]->codec->time_base,
                       ofmt_ctx->streams[stream_index]->time_base);
av_interleaved_write_frame(ofmt_ctx, &enc_pkt);

参考