FFmpeg Muxing流程

介绍

FFmpeg的Muxing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

1、av_format_write_head

/**
 * Allocate the stream private data and write the stream header to
 * an output media file.
 *
 * @param s Media file handle, must be allocated with avformat_alloc_context().
 *          Its oformat field must be set to the desired output format;
 *          Its pb field must be set to an already opened AVIOContext.
 * @param options  An AVDictionary filled with AVFormatContext and muxer-private options.
 *                 On return this parameter will be destroyed and replaced with a dict containing
 *                 options that were not found. May be NULL.
 *
 * @return 0 on success, negative AVERROR on failure.
 *
 * @see av_opt_find, av_dict_set, avio_open, av_oformat_next.
 */
int avformat_write_header(AVFormatContext *s, AVDictionary **options);

简单描述其功能：

s：用于输出的AVFormatContext。
options：额外的选项，目前没有深入研究过，一般为NULL。

2、av_write_frame/av_interleaved_write_frame

2.1 av_write_frame

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。

/**
 * Write a packet to an output media file.
 *
 * This function passes the packet directly to the muxer, without any buffering
 * or reordering. The caller is responsible for correctly interleaving the
 * packets if the format requires it. Callers that want libavformat to handle
 * the interleaving should call av_interleaved_write_frame() instead of this
 * function.
 *
 * @param s media file handle
 * @param pkt The packet containing the data to be written. Note that unlike
 *            av_interleaved_write_frame(), this function does not take
 *            ownership of the packet passed to it (though some muxers may make
 *            an internal reference to the input packet).
 *            

 *            This parameter can be NULL (at any time, not just at the end), in
 *            order to immediately flush data buffered within the muxer, for
 *            muxers that buffer up data internally before writing it to the
 *            output.
 *            

 *            Packet's @ref AVPacket.stream_index "stream_index" field must be
 *            set to the index of the corresponding stream in @ref
 *            AVFormatContext.streams "s->streams". It is very strongly
 *            recommended that timing information (@ref AVPacket.pts "pts", @ref
 *            AVPacket.dts "dts", @ref AVPacket.duration "duration") is set to
 *            correct values.
 * @return < 0 on error, = 0 if OK, 1 if flushed and there is no more data to flush
 *
 * @see av_interleaved_write_frame()
 */
int av_write_frame(AVFormatContext *s, AVPacket *pkt);

2.2 av_interleaved_write_frame

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。

/**
 * Write a packet to an output media file ensuring correct interleaving.
 *
 * This function will buffer the packets internally as needed to make sure the
 * packets in the output file are properly interleaved in the order of
 * increasing dts. Callers doing their own interleaving should call
 * av_write_frame() instead of this function.
 *
 * @param s media file handle
 * @param pkt The packet containing the data to be written.
 *            

 *            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 @ref AVPacket.stream_index "stream_index" field must be
 *            set to the index of the corresponding stream in @ref
 *            AVFormatContext.streams "s->streams". It is very strongly
 *            recommended that timing information (@ref AVPacket.pts "pts", @ref
 *            AVPacket.dts "dts", @ref AVPacket.duration "duration") is set to
 *            correct values.
 *
 * @return 0 on success, a negative AVERROR on error. Libavformat will always
 *         take care of freeing the packet, even if this function fails.
 *
 * @see av_write_frame(), AVFormatContext.max_interleave_delta
 */
int av_interleaved_write_frame(AVFormatContext *s, AVPacket *pkt);

结论是：在有多个流的情况下要用av_interleaved_write_frame，只有单一流2个函数都可以用

3、av_write_trailer()

av_write_trailer()用于输出文件尾，它的声明位于libavformat\avformat.h，如下所示

/**
 * Write the stream trailer to an output media file and free the
 * file private data.
 *
 * May only be called after a successful call to avformat_write_header.
 *
 * @param s media file handle
 * @return 0 if OK, AVERROR_xxx on error
 */
int av_write_trailer(AVFormatContext *s);

av_write_trailer()主要完成了以下两步工作：

• （1）循环调用interleave_packet()以及write_packet()，将还未输出的AVPacket输出出来。
• （2）调用AVOutputFormat的write_trailer()，输出文件尾。