一：介绍

1.1 ffmpeg 的 Mux 主要分为三步操作：

avformat_write_header ：写文件头

av_write_frame/av_interleaved_write_frame：写packet

av_write_trailer ：写文件尾

本文主要分析 av_write_frame/av_interleaved_write_frame 。

可额外参考： http://blog.csdn.net/leixiaohua1020/article/details/44199673

1.2 av_write_frame

参考官网说明：http://ffmpeg.org/doxygen/3.1/group__lavf__encoding.html#gaa85cc1774f18f306cd20a40fc50d0b36

av_write_frame 直接将包写进Mux，没有缓存和重新排序，一切都需要用户自己设置。

Packet's stream_index field must be set to the index of the corresponding stream ins->streams.

The timestamps (pts, dts) must be set to correct values in the stream's timebase (unless the output format is flagged with the AVFMT_NOTIMESTAMPS flag, then they can be set to AV_NOPTS_VALUE). The dts for subsequent packets passed to this function must be strictly increasing when compared in their respective timebases (unless the output format is flagged with the AVFMT_TS_NONSTRICT, then they merely have to be nondecreasing).duration) should also be set if known.

1.3 av_interleaved_write_frame

参考官网说明： http://ffmpeg.org/doxygen/3.1/group__lavf__encoding.html#ga37352ed2c63493c38219d935e71db6c1

av_interleaved_write_frame 将对 packet 进行缓存和 pts 检查，这是区别于 av_write_frame 的地方。

If the packet is reference-counted, this function will take ownership of this reference and unreference it later when it sees fit. The caller must not access the data through this reference after this function returns. If the packet is not reference-counted, libavformat will make a copy.

This parameter can be NULL (at any time, not just at the end), to flush the interleaving queues.

Packet's stream_index field must be set to the index of the corresponding stream ins->streams.

The timestamps (pts, dts) must be set to correct values in the stream's timebase (unless the output format is flagged with the AVFMT_NOTIMESTAMPS flag, then they can be set to AV_NOPTS_VALUE). The dts for subsequent packets in one stream must be strictly increasing (unless the output format is flagged with the AVFMT_TS_NONSTRICT, then they merely have to be nondecreasing).duration) should also be set if known.

二：分析

2.1 av_write_frame

int av_write_frame(AVFormatContext *s, AVPacket *pkt)
{int ret;//对包预处理ret = prepare_input_packet(s, pkt);if (ret < 0)return ret;//空包,flushif (!pkt) {if (s->oformat->flags & AVFMT_ALLOW_FLUSH) {if (!s->internal->header_written) {ret = s->internal->write_header_ret ? s->internal->write_header_ret : write_header_internal(s);if (ret < 0)return ret;}ret = s->oformat->write_packet(s, NULL);if (s->flush_packets && s->pb && s->pb->error >= 0 && s->flags & AVFMT_FLAG_FLUSH_PACKETS)avio_flush(s->pb);if (ret >= 0 && s->pb && s->pb->error < 0)ret = s->pb->error;return ret;}return 1;}//计算包信息
#if FF_API_COMPUTE_PKT_FIELDS2 && FF_API_LAVF_AVCTXret = compute_muxer_pkt_fields(s, s->streams[pkt->stream_index], pkt);if (ret < 0 && !(s->oformat->flags & AVFMT_NOTIMESTAMPS))return ret;
#endif//写包ret = write_packet(s, pkt);if (ret >= 0 && s->pb && s->pb->error < 0)ret = s->pb->error;if (ret >= 0)s->streams[pkt->stream_index]->nb_frames++;return ret;
}

下面来看看具体做了什么。

prepare_input_packet（检查流通道和包的pts，dts是否符合）：

static int prepare_input_packet(AVFormatContext *s, AVPacket *pkt)
{int ret;//检查流通道ret = check_packet(s, pkt);if (ret < 0)return ret;//检查PTS , DTS
#if !FF_API_COMPUTE_PKT_FIELDS2 && FF_API_LAVF_AVCTX/* sanitize the timestamps */if (!(s->oformat->flags & AVFMT_NOTIMESTAMPS)) {AVStream *st = s->streams[pkt->stream_index];/* when there is no reordering (so dts is equal to pts), but* only one of them is set, set the other as well */if (!st->internal->reorder) {if (pkt->pts == AV_NOPTS_VALUE && pkt->dts != AV_NOPTS_VALUE)pkt->pts = pkt->dts;if (pkt->dts == AV_NOPTS_VALUE && pkt->pts != AV_NOPTS_VALUE)pkt->dts = pkt->pts;}/* check that the timestamps are set */if (pkt->pts == AV_NOPTS_VALUE || pkt->dts == AV_NOPTS_VALUE) {av_log(s, AV_LOG_ERROR,"Timestamps are unset in a packet for stream %d\n", st->index);return AVERROR(EINVAL);}/* check that the dts are increasing (or at least non-decreasing,* if the format allows it */if (st->cur_dts != AV_NOPTS_VALUE &&((!(s->oformat->flags & AVFMT_TS_NONSTRICT) && st->cur_dts >= pkt->dts) ||st->cur_dts > pkt->dts)) {av_log(s, AV_LOG_ERROR,"Application provided invalid, non monotonically increasing ""dts to muxer in stream %d: %" PRId64 " >= %" PRId64 "\n",st->index, st->cur_dts, pkt->dts);return AVERROR(EINVAL);}if (pkt->pts < pkt->dts) {av_log(s, AV_LOG_ERROR, "pts %" PRId64 " < dts %" PRId64 " in stream %d\n",pkt->pts, pkt->dts, st->index);return AVERROR(EINVAL);}}
#endifreturn 0;
}

compute_muxer_pkt_fields （计算duration ，pts，dts等）：

static int compute_muxer_pkt_fields(AVFormatContext *s, AVStream *st, AVPacket *pkt)
{int delay = FFMAX(st->codecpar->video_delay, st->internal->avctx->max_b_frames > 0);int num, den, i;int frame_size;if (!s->internal->missing_ts_warning &&!(s->oformat->flags & AVFMT_NOTIMESTAMPS) &&(pkt->pts == AV_NOPTS_VALUE || pkt->dts == AV_NOPTS_VALUE)) {av_log(s, AV_LOG_WARNING,"Timestamps are unset in a packet for stream %d. ""This is deprecated and will stop working in the future. ""Fix your code to set the timestamps properly\n", st->index);s->internal->missing_ts_warning = 1;}if (s->debug & FF_FDEBUG_TS)av_log(s, AV_LOG_TRACE, "compute_muxer_pkt_fields: pts:%s dts:%s cur_dts:%s b:%d size:%d st:%d\n",av_ts2str(pkt->pts), av_ts2str(pkt->dts), av_ts2str(st->cur_dts), delay, pkt->size, pkt->stream_index);if (pkt->duration < 0 && st->codec->codec_type != AVMEDIA_TYPE_SUBTITLE) {av_log(s, AV_LOG_WARNING, "Packet with invalid duration %"PRId64" in stream %d\n",pkt->duration, pkt->stream_index);pkt->duration = 0;}/* duration field */if (pkt->duration == 0) {ff_compute_frame_duration(s, &num, &den, st, NULL, pkt);if (den && num) {pkt->duration = av_rescale(1, num * (int64_t)st->time_base.den * st->codec->ticks_per_frame, den * (int64_t)st->time_base.num);}}if (pkt->pts == AV_NOPTS_VALUE && pkt->dts != AV_NOPTS_VALUE && delay == 0)pkt->pts = pkt->dts;//XXX/FIXME this is a temporary hack until all encoders output ptsif ((pkt->pts == 0 || pkt->pts == AV_NOPTS_VALUE) && pkt->dts == AV_NOPTS_VALUE && !delay) {static int warned;if (!warned) {av_log(s, AV_LOG_WARNING, "Encoder did not produce proper pts, making some up.\n");warned = 1;}pkt->dts =
//        pkt->pts= st->cur_dts;pkt->pts = st->priv_pts->val;}//calculate dts from ptsif (pkt->pts != AV_NOPTS_VALUE && pkt->dts == AV_NOPTS_VALUE && delay <= MAX_REORDER_DELAY) {st->pts_buffer[0] = pkt->pts;for (i = 1; i < delay + 1 && st->pts_buffer[i] == AV_NOPTS_VALUE; i++)st->pts_buffer[i] = pkt->pts + (i - delay - 1) * pkt->duration;for (i = 0; i<delay && st->pts_buffer[i] > st->pts_buffer[i + 1]; i++)FFSWAP(int64_t, st->pts_buffer[i], st->pts_buffer[i + 1]);pkt->dts = st->pts_buffer[0];}if (st->cur_dts && st->cur_dts != AV_NOPTS_VALUE &&((!(s->oformat->flags & AVFMT_TS_NONSTRICT) &&st->codecpar->codec_type != AVMEDIA_TYPE_SUBTITLE &&st->codecpar->codec_type != AVMEDIA_TYPE_DATA &&st->cur_dts >= pkt->dts) || st->cur_dts > pkt->dts)) {av_log(s, AV_LOG_ERROR,"Application provided invalid, non monotonically increasing dts to muxer in stream %d: %s >= %s\n",st->index, av_ts2str(st->cur_dts), av_ts2str(pkt->dts));return AVERROR(EINVAL);}if (pkt->dts != AV_NOPTS_VALUE && pkt->pts != AV_NOPTS_VALUE && pkt->pts < pkt->dts) {av_log(s, AV_LOG_ERROR,"pts (%s) < dts (%s) in stream %d\n",av_ts2str(pkt->pts), av_ts2str(pkt->dts),st->index);return AVERROR(EINVAL);}if (s->debug & FF_FDEBUG_TS)av_log(s, AV_LOG_TRACE, "av_write_frame: pts2:%s dts2:%s\n",av_ts2str(pkt->pts), av_ts2str(pkt->dts));st->cur_dts = pkt->dts;st->priv_pts->val = pkt->dts;/* update pts */switch (st->codec->codec_type) {case AVMEDIA_TYPE_AUDIO:frame_size = (pkt->flags & AV_PKT_FLAG_UNCODED_FRAME) ?((AVFrame *)pkt->data)->nb_samples :av_get_audio_frame_duration(st->codec, pkt->size);/* HACK/FIXME, we skip the initial 0 size packets as they are most* likely equal to the encoder delay, but it would be better if we* had the real timestamps from the encoder */if (frame_size >= 0 && (pkt->size || st->priv_pts->num != st->priv_pts->den >> 1 || st->priv_pts->val)) {frac_add(st->priv_pts, (int64_t)st->time_base.den * frame_size);}break;case AVMEDIA_TYPE_VIDEO:frac_add(st->priv_pts, (int64_t)st->time_base.den * st->time_base.num);break;}return 0;
｝

write_packet（写包）：

static int write_packet(AVFormatContext *s, AVPacket *pkt)
{int ret, did_split;if (s->output_ts_offset) {AVStream *st = s->streams[pkt->stream_index];int64_t offset = av_rescale_q(s->output_ts_offset, AV_TIME_BASE_Q, st->time_base);if (pkt->dts != AV_NOPTS_VALUE)pkt->dts += offset;if (pkt->pts != AV_NOPTS_VALUE)pkt->pts += offset;}if (s->avoid_negative_ts > 0) {AVStream *st = s->streams[pkt->stream_index];int64_t offset = st->mux_ts_offset;int64_t ts = s->internal->avoid_negative_ts_use_pts ? pkt->pts : pkt->dts;if (s->internal->offset == AV_NOPTS_VALUE && ts != AV_NOPTS_VALUE &&(ts < 0 || s->avoid_negative_ts == AVFMT_AVOID_NEG_TS_MAKE_ZERO)) {s->internal->offset = -ts;s->internal->offset_timebase = st->time_base;}if (s->internal->offset != AV_NOPTS_VALUE && !offset) {offset = st->mux_ts_offset =av_rescale_q_rnd(s->internal->offset,s->internal->offset_timebase,st->time_base,AV_ROUND_UP);}if (pkt->dts != AV_NOPTS_VALUE)pkt->dts += offset;if (pkt->pts != AV_NOPTS_VALUE)pkt->pts += offset;if (s->internal->avoid_negative_ts_use_pts) {if (pkt->pts != AV_NOPTS_VALUE && pkt->pts < 0) {av_log(s, AV_LOG_WARNING, "failed to avoid negative ""pts %s in stream %d.\n""Try -avoid_negative_ts 1 as a possible workaround.\n",av_ts2str(pkt->dts),pkt->stream_index);}} else {av_assert2(pkt->dts == AV_NOPTS_VALUE || pkt->dts >= 0 || s->max_interleave_delta > 0);if (pkt->dts != AV_NOPTS_VALUE && pkt->dts < 0) {av_log(s, AV_LOG_WARNING,"Packets poorly interleaved, failed to avoid negative ""timestamp %s in stream %d.\n""Try -max_interleave_delta 0 as a possible workaround.\n",av_ts2str(pkt->dts),pkt->stream_index);}}}did_split = av_packet_split_side_data(pkt);if (!s->internal->header_written) {ret = s->internal->write_header_ret ? s->internal->write_header_ret : write_header_internal(s);if (ret < 0)goto fail;}if ((pkt->flags & AV_PKT_FLAG_UNCODED_FRAME)) {AVFrame *frame = (AVFrame *)pkt->data;av_assert0(pkt->size == UNCODED_FRAME_PACKET_SIZE);ret = s->oformat->write_uncoded_frame(s, pkt->stream_index, &frame, 0);av_frame_free(&frame);} else {ret = s->oformat->write_packet(s, pkt);}if (s->pb && ret >= 0) {if (s->flush_packets && s->flags & AVFMT_FLAG_FLUSH_PACKETS)avio_flush(s->pb);if (s->pb->error < 0)ret = s->pb->error;}fail:if (did_split)av_packet_merge_side_data(pkt);return ret;
}

写包调用了AVOutputFormat 的 write_packet 。与write_header类似，具体的写包实现在各个封装的 AVOutputFormat 中。对此处写包未做详细分析...(-.-)...(/飙泪)。

2.2 av_interleaved_write_frame

int av_interleaved_write_frame(AVFormatContext *s, AVPacket *pkt)
{int ret, flush = 0, i;//对包预处理ret = prepare_input_packet(s, pkt);if (ret < 0)goto fail;//bitstream filters (过滤器,也是区别于av_write_frame 的地方)if (pkt) {AVStream *st = s->streams[pkt->stream_index];if (s->oformat->check_bitstream) {if (!st->internal->bitstream_checked) {if ((ret = s->oformat->check_bitstream(s, pkt)) < 0)goto fail;else if (ret == 1)st->internal->bitstream_checked = 1;}}for (i = 0; i < st->internal->nb_bsfcs; i++) {AVBSFContext *ctx = st->internal->bsfcs[i];if (i > 0) {AVBSFContext* prev_ctx = st->internal->bsfcs[i - 1];if (prev_ctx->par_out->extradata_size != ctx->par_in->extradata_size) {if ((ret = avcodec_parameters_copy(ctx->par_in, prev_ctx->par_out)) < 0)goto fail;}}// TODO: when any bitstream filter requires flushing at EOF, we'll need to// flush each stream's BSF chain on write_trailer.if ((ret = av_bsf_send_packet(ctx, pkt)) < 0) {av_log(ctx, AV_LOG_ERROR,"Failed to send packet to filter %s for stream %d",ctx->filter->name, pkt->stream_index);goto fail;}// TODO: when any automatically-added bitstream filter is generating multiple// output packets for a single input one, we'll need to call this in a loop// and write each output packet.if ((ret = av_bsf_receive_packet(ctx, pkt)) < 0) {if (ret == AVERROR(EAGAIN) || ret == AVERROR_EOF)return 0;av_log(ctx, AV_LOG_ERROR,"Failed to send packet to filter %s for stream %d",ctx->filter->name, pkt->stream_index);goto fail;}if (i == st->internal->nb_bsfcs - 1) {if (ctx->par_out->extradata_size != st->codecpar->extradata_size) {if ((ret = avcodec_parameters_copy(st->codecpar, ctx->par_out)) < 0)goto fail;}}}if (s->debug & FF_FDEBUG_TS)av_log(s, AV_LOG_TRACE, "av_interleaved_write_frame size:%d dts:%s pts:%s\n",pkt->size, av_ts2str(pkt->dts), av_ts2str(pkt->pts));#if FF_API_COMPUTE_PKT_FIELDS2 && FF_API_LAVF_AVCTXif ((ret = compute_muxer_pkt_fields(s, st, pkt)) < 0 && !(s->oformat->flags & AVFMT_NOTIMESTAMPS))goto fail;
#endifif (pkt->dts == AV_NOPTS_VALUE && !(s->oformat->flags & AVFMT_NOTIMESTAMPS)) {ret = AVERROR(EINVAL);goto fail;}} else {av_log(s, AV_LOG_TRACE, "av_interleaved_write_frame FLUSH\n");flush = 1;}//写包for (;; ) {AVPacket opkt;//此处也是与av_write_frame的区别int ret = interleave_packet(s, &opkt, pkt, flush);if (pkt) {memset(pkt, 0, sizeof(*pkt));av_init_packet(pkt);pkt = NULL;}if (ret <= 0) //FIXME cleanup needed for ret<0 ?return ret;//写入ret = write_packet(s, &opkt);if (ret >= 0)s->streams[opkt.stream_index]->nb_frames++;av_packet_unref(&opkt);if (ret < 0)return ret;if(s->pb && s->pb->error)return s->pb->error;}
fail:av_packet_unref(pkt);return ret;
}

prepare_input_packet 函数与 av_write_frame 的一致，主要是 bitstream filters 和 interleave_packet 的区别：

static int interleave_packet(AVFormatContext *s, AVPacket *out, AVPacket *in, int flush)
{if (s->oformat->interleave_packet) {int ret = s->oformat->interleave_packet(s, out, in, flush);if (in)av_packet_unref(in);return ret;} elsereturn ff_interleave_packet_per_dts(s, out, in, flush);
}

后续可参考： http://blog.csdn.net/dancing_night/article/details/46469865

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