ffmpeg time_basehtml
ffmpeg存在多個時間基準(time_base)，對應不一樣的階段(結構體)，每一個time_base具體的值不同，ffmpeg提供函數在各個time_base中進行切換。搞清楚各個time_base的來源，對於閱讀ffmpeg的代碼很重要。
1、time_base
一、AVStream(libavformat/avformat.h)app
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: pts of the first frame of the stream in presentation order, in stream time base.
* Only set this if you are absolutely 100% sure that the value you set
* it to really is the pts of the first frame.
* This may be undefined (AV_NOPTS_VALUE).
* @note The ASF header does NOT contain a correct start_time the ASF
* demuxer must NOT set this.
*/
int64_t start_time;
/**
* 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.
*/
int64_t duration;
從上面的信息能夠看到，AVStream->time_base單位爲秒。
那AVStream->time_base具體的值是多少呢？下面以mpegts_demuxer爲例：
static int mpegts_set_stream_info(AVStream *st, PESContext *pes,
uint32_t stream_type, uint32_t prog_reg_desc)
{
avpriv_set_pts_info(st, 33, 1, 90000);
void avpriv_set_pts_info(AVStream *s, int pts_wrap_bits,
unsigned int pts_num, unsigned int pts_den)
{
AVRational new_tb;
if (av_reduce(&new_tb.num, &new_tb.den, pts_num, pts_den, INT_MAX)) {
if (new_tb.num != pts_num)
av_log(NULL, AV_LOG_DEBUG,
"st:%d removing common factor %d from timebase\n",
s->index, pts_num / new_tb.num);
} else
av_log(NULL, AV_LOG_WARNING,
"st:%d has too large timebase, reducing\n", s->index);
if (new_tb.num <= 0 || new_tb.den <= 0) {
av_log(NULL, AV_LOG_ERROR,
"Ignoring attempt to set invalid timebase %d/%d for st:%d\n",
new_tb.num, new_tb.den,
s->index);
return;
}
s->time_base = new_tb;
av_codec_set_pkt_timebase(s->codec, new_tb);
s->pts_wrap_bits = pts_wrap_bits;
}
經過avpriv_set_pts_info(st, 33, 1, 90000)函數，設置AVStream->time_base爲1/90000。爲何是90000？由於mpeg的pts、dts都是以90kHz來採樣的，因此採樣間隔爲1/90000秒。
二、AVCodecContext
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.
* - encoding: MUST be set by user.
* - decoding: Set by libavcodec.
*/
AVRational time_base;
從上面的信息能夠看到，AVCodecContext->time_base單位一樣爲秒，不過精度沒有AVStream->time_base高，大小爲1/framerate。
下面以ffmpeg轉碼工具爲例：less
static int transcode_init(void)
{
if (enc_ctx->codec_type == AVMEDIA_TYPE_VIDEO) {
if (ost->filter && !ost->frame_rate.num)
ost->frame_rate = av_buffersink_get_frame_rate(ost->filter->filter);
if (ist && !ost->frame_rate.num)
ost->frame_rate = ist->framerate;
if (ist && !ost->frame_rate.num)
ost->frame_rate = ist->st->r_frame_rate;
if (ist && !ost->frame_rate.num) {
ost->frame_rate = (AVRational){25, 1};
av_log(NULL, AV_LOG_WARNING,
"No information "
"about the input framerate is available. Falling "
"back to a default value of 25fps for output stream #%d:%d. Use the -r option "
"if you want a different framerate.\n",
ost->file_index, ost->index);
}
// ost->frame_rate = ist->st->avg_frame_rate.num ? ist->st->avg_frame_rate : (AVRational){25, 1};
if (ost->enc && ost->enc->supported_framerates && !ost->force_fps) {
int idx = av_find_nearest_q_idx(ost->frame_rate, ost->enc->supported_framerates);
ost->frame_rate = ost->enc->supported_framerates[idx];
}
if (enc_ctx->codec_id == AV_CODEC_ID_MPEG4) {
av_reduce(&ost->frame_rate.num, &ost->frame_rate.den,
ost->frame_rate.num, ost->frame_rate.den, 65535);
}
}
switch (enc_ctx->codec_type) {
case AVMEDIA_TYPE_VIDEO:
enc_ctx->time_base = av_inv_q(ost->frame_rate);
if (ost->filter && !(enc_ctx->time_base.num && enc_ctx->time_base.den))
enc_ctx->time_base = ost->filter->filter->inputs[0]->time_base;
if ( av_q2d(enc_ctx->time_base) < 0.001 && video_sync_method != VSYNC_PASSTHROUGH
&& (video_sync_method == VSYNC_CFR || video_sync_method == VSYNC_VSCFR || (video_sync_method == VSYNC_AUTO && !(oc->oformat->flags & AVFMT_VARIABLE_FPS)))){
av_log(oc, AV_LOG_WARNING, "Frame rate very high for a muxer not efficiently supporting it.\n"
"Please consider specifying a lower framerate, a different muxer or -vsync 2\n");
}
首先獲取ost->frame_rate，而後計算enc_ctx->time_base = 1/ost->frame_rate。
總結：
AVStream->time_base比AVCodecContext->time_base精度要高(數值要小)，好比AVStream->time_base爲1/90000，而AVCodecContext->time_base爲1/30(假設frame_rate爲30)；一樣的pts和dts，以AVStream->time_base爲單位，數值要比以AVCodecContext->time_base爲單位要大。
2、pts、dts
那各個結構下，pts和dts使用哪一個time_base來表示呢？
一、AVPacketide
typedef struct AVPacket {
/**
* Presentation timestamp in AVStream->time_base units; the time at which
* the decompressed packet will be presented to the user.
* Can be AV_NOPTS_VALUE if it is not stored in the file.
* pts MUST be larger or equal to dts as presentation cannot happen before
* decompression, unless one wants to view hex dumps. Some formats misuse
* the terms dts and pts/cts to mean something different. Such timestamps
* must be converted to true pts/dts before they are stored in AVPacket.
*/
int64_t pts;
/**
* Decompression timestamp in AVStream->time_base units; the time at which
* the packet is decompressed.
* Can be AV_NOPTS_VALUE if it is not stored in the file.
*/
int64_t dts;
從上面能夠看到，AVPacket下的pts和dts以AVStream->time_base爲單位(數值比較大)。這也很容易理解，根據mpeg的協議，壓縮後或解壓前的數據，pts和dts是90kHz時鐘的採樣值，時間間隔就是AVStream->time_base。
二、AVFrame函數
typedef struct AVFrame {
/**
* Presentation timestamp in time_base units (time when frame should be shown to user).
*/
int64_t pts;
/**
* PTS copied from the AVPacket that was decoded to produce this frame.
*/
int64_t pkt_pts;
/**
* DTS copied from the AVPacket that triggered returning this frame. (if frame threading isn't used)
* This is also the Presentation time of this AVFrame calculated from
* only AVPacket.dts values without pts values.
*/
int64_t pkt_dts;
注意：
AVFrame裏面的pkt_pts和pkt_dts是拷貝自AVPacket，一樣以AVStream->time_base爲單位；而pts是爲輸出(顯示)準備的，以AVCodecContex->time_base爲單位)。//FIXME
三、InputStream工具
typedef struct InputStream {
int file_index;
AVStream *st;
AVCodecContext *dec_ctx;
int64_t start; /* time when read started */
/* predicted dts of the next packet read for this stream or (when there are
* several frames in a packet) of the next frame in current packet (in AV_TIME_BASE units) */
int64_t next_dts;
int64_t dts; ///< dts of the last packet read for this stream (in AV_TIME_BASE units)
int64_t next_pts; ///< synthetic pts for the next decode frame (in AV_TIME_BASE units)
int64_t pts; ///< current pts of the decoded frame (in AV_TIME_BASE units)
從上面能夠看到，InputStream下的pts和dts以AV_TIME_BASE爲單位(微秒)，至於爲何要轉化爲微妙，多是爲了不使用浮點數。
四、OutputStreamui
typedef struct OutputStream {
int file_index; /* file index */
int index; /* stream index in the output file */
int source_index; /* InputStream index */
AVStream *st; /* stream in the output file */
int encoding_needed; /* true if encoding needed for this stream */
int frame_number;
/* input pts and corresponding output pts
for A/V sync */
struct InputStream *sync_ist; /* input stream to sync against */
int64_t sync_opts; /* output frame counter, could be changed to some true timestamp */ // FIXME look at frame_number
/* pts of the first frame encoded for this stream, used for limiting
* recording time */
int64_t first_pts;
/* dts of the last packet sent to the muxer */
int64_t last_mux_dts;
AVBitStreamFilterContext *bitstream_filters;
AVCodecContext *enc_ctx;
AVCodec *enc;
int64_t max_frames;
AVFrame *filtered_frame;
OutputStream涉及音視頻同步，結構和InputStream不一樣，暫時只做記錄，不分析。
3、各個time_base之間轉換
ffmpeg提供av_rescale_q函數用於time_base之間轉換，av_rescale_q(a,b,c)做用至關於執行a*b/c，經過設置b,c的值，能夠很方便的實現time_base之間轉換。
例如：
一、InputStream(AV_TIME_BASE)到AVPacket(AVStream->time_base)this
static int decode_video(InputStream *ist, AVPacket *pkt, int *got_output)
{
pkt->dts = av_rescale_q(ist->dts, AV_TIME_BASE_Q, ist->st->time_base);
二、AVPacket(AVStream->time_base)到InputStream(AV_TIME_BASE)編碼
static int process_input_packet(InputStream *ist, const AVPacket *pkt)
{
if (pkt->dts != AV_NOPTS_VALUE) {
ist->next_dts = ist->dts = av_rescale_q(pkt->dts, ist->st->time_base, AV_TIME_BASE_Q);
4、後記：
AVFrame->pts和AVPacket->pts、AVPacket->dts的值，在解碼/編碼後，會經歷短暫的time_base不匹配的狀況：
一、解碼後url
static int decode_video(InputStream *ist, AVPacket *pkt, int *got_output)
{
decoded_frame = ist->decoded_frame;
pkt->dts = av_rescale_q(ist->dts, AV_TIME_BASE_Q, ist->st->time_base);
update_benchmark(NULL);
ret = avcodec_decode_video2(ist->dec_ctx,
decoded_frame, got_output, pkt);
best_effort_timestamp= av_frame_get_best_effort_timestamp(decoded_frame);
if(best_effort_timestamp != AV_NOPTS_VALUE)
ist->next_pts = ist->pts = av_rescale_q(decoded_frame->pts = best_effort_timestamp, ist->st->time_base, AV_TIME_BASE_Q);
解碼後，decoded_frame->pts的值使用AVStream->time_base爲單位，後在AVFilter裏面轉換成以AVCodecContext->time_base爲單位。 //FIXME
二、編碼後
static void do_video_out(AVFormatContext *s,
OutputStream *ost,
AVFrame *in_picture)
{
ret = avcodec_encode_video2(enc, &pkt, in_picture, &got_packet);
if (got_packet) {
if (debug_ts) {
av_log(NULL, AV_LOG_INFO, "encoder -> type:video "
"pkt_pts:%s pkt_pts_time:%s pkt_dts:%s pkt_dts_time:%s\n",
av_ts2str(pkt.pts), av_ts2timestr(pkt.pts, &enc->time_base),
av_ts2str(pkt.dts), av_ts2timestr(pkt.dts, &enc->time_base));
}
if (pkt.pts == AV_NOPTS_VALUE && !(enc->codec->capabilities & CODEC_CAP_DELAY))
pkt.pts = ost->sync_opts;
av_packet_rescale_ts(&pkt, enc->time_base, ost->st->time_base);
if (debug_ts) {
av_log(NULL, AV_LOG_INFO, "encoder -> type:video "
"pkt_pts:%s pkt_pts_time:%s pkt_dts:%s pkt_dts_time:%s\n",
av_ts2str(pkt.pts), av_ts2timestr(pkt.pts, &ost->st->time_base),
av_ts2str(pkt.dts), av_ts2timestr(pkt.dts, &ost->st->time_base));
}
frame_size = pkt.size;
write_frame(s, &pkt, ost);
/* if two pass, output log */
if (ost->logfile && enc->stats_out) {
fprintf(ost->logfile, "%s", enc->stats_out);
}
}
編碼後，pkt.pts、pkt.dts使用AVCodecContext->time_base爲單位，後經過調用"av_packet_rescale_ts"轉換爲AVStream->time_base爲單位。