定制建站,站长工具精品,深圳设计网站建设,旅游公网站如何做参考链接 FFmpeg源代码简单分析#xff1a;avformat_write_header()_雷霄骅的博客-CSDN博客_avformat_write_header avformat_write_header() FFmpeg写文件用到的3个函数#xff1a;avformat_write_header()#xff0c;av_write_frame()以及av_write_trailer()其中av_writ…参考链接 FFmpeg源代码简单分析avformat_write_header()_雷霄骅的博客-CSDN博客_avformat_write_header avformat_write_header() FFmpeg写文件用到的3个函数avformat_write_header()av_write_frame()以及av_write_trailer()其中av_write_frame()用于写视频数据avformat_write_header()用于写视频文件头而av_write_trailer()用于写视频文件尾本文首先分析avformat_write_header()。PS需要注意的是尽管这3个函数功能是配套的但是它们的前缀却不一样写文件头Header的函数前缀是“avformat_”其他两个函数前缀是“av_”不太明白其中的原因。avformat_write_header()的声明位于libavformat\avformat.h如下所示 /*** 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 AVSTREAM_INIT_IN_WRITE_HEADER on success if the codec had not already been fully initialized in avformat_init,* AVSTREAM_INIT_IN_INIT_OUTPUT on success if the codec had already been fully initialized in avformat_init,* negative AVERROR on failure.** see av_opt_find, av_dict_set, avio_open, av_oformat_next, avformat_init_output.*/ av_warn_unused_result int avformat_write_header(AVFormatContext *s, AVDictionary **options); 简单解释一下它的参数的含义 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。函数正常执行后返回值等于0。avformat_write_header()的定义位于libavformat\mux.c如下所示 int avformat_write_header(AVFormatContext *s, AVDictionary **options) {FFFormatContext *const si ffformatcontext(s);int already_initialized si-initialized;int streams_already_initialized si-streams_initialized;int ret 0;if (!already_initialized)if ((ret avformat_init_output(s, options)) 0)return ret;if (!(s-oformat-flags AVFMT_NOFILE) s-pb)avio_write_marker(s-pb, AV_NOPTS_VALUE, AVIO_DATA_MARKER_HEADER);if (s-oformat-write_header) {ret s-oformat-write_header(s);if (ret 0 s-pb s-pb-error 0)ret s-pb-error;if (ret 0)goto fail;flush_if_needed(s);}if (!(s-oformat-flags AVFMT_NOFILE) s-pb)avio_write_marker(s-pb, AV_NOPTS_VALUE, AVIO_DATA_MARKER_UNKNOWN);if (!si-streams_initialized) {if ((ret init_pts(s)) 0)goto fail;}return streams_already_initialized;fail:deinit_muxer(s);return ret; } 从源代码可以看出avformat_write_header()完成了以下工作1调用avformat_init_output()初始化复用器 2调用AVOutputFormat的write_header()下面看一下这两个函数。函数调用关系图 avformat_write_header()的调用关系如下图所示。init_muxer函数被avformat_init_output函数替代avformat_init_output内部包含了init_muxeravformat_init_output int avformat_init_output(AVFormatContext *s, AVDictionary **options) {FFFormatContext *const si ffformatcontext(s);int ret 0;if ((ret init_muxer(s, options)) 0)return ret;si-initialized 1;si-streams_initialized ret;if (s-oformat-init ret) {if ((ret init_pts(s)) 0)return ret;return AVSTREAM_INIT_IN_INIT_OUTPUT;}return AVSTREAM_INIT_IN_WRITE_HEADER; } init_muxer() init_muxer()用于初始化复用器它的定义如下所示。 static int init_muxer(AVFormatContext *s, AVDictionary **options) {FFFormatContext *const si ffformatcontext(s);AVDictionary *tmp NULL;const AVOutputFormat *of s-oformat;AVDictionaryEntry *e;int ret 0;if (options)av_dict_copy(tmp, *options, 0);if ((ret av_opt_set_dict(s, tmp)) 0)goto fail;if (s-priv_data s-oformat-priv_class *(const AVClass**)s-priv_datas-oformat-priv_class (ret av_opt_set_dict2(s-priv_data, tmp, AV_OPT_SEARCH_CHILDREN)) 0)goto fail;if (!s-url !(s-url av_strdup())) {ret AVERROR(ENOMEM);goto fail;}// some sanity checksif (s-nb_streams 0 !(of-flags AVFMT_NOSTREAMS)) {av_log(s, AV_LOG_ERROR, No streams to mux were specified\n);ret AVERROR(EINVAL);goto fail;}for (unsigned i 0; i s-nb_streams; i) {AVStream *const st s-streams[i];FFStream *const sti ffstream(st);AVCodecParameters *const par st-codecpar;const AVCodecDescriptor *desc;if (!st-time_base.num) {/* fall back on the default timebase values */if (par-codec_type AVMEDIA_TYPE_AUDIO par-sample_rate)avpriv_set_pts_info(st, 64, 1, par-sample_rate);elseavpriv_set_pts_info(st, 33, 1, 90000);}switch (par-codec_type) {case AVMEDIA_TYPE_AUDIO:if (par-sample_rate 0) {av_log(s, AV_LOG_ERROR, sample rate not set\n);ret AVERROR(EINVAL);goto fail;}#if FF_API_OLD_CHANNEL_LAYOUT FF_DISABLE_DEPRECATION_WARNINGS/* if the caller is using the deprecated channel layout API,* convert it to the new style */if (!par-ch_layout.nb_channels par-channels) {if (par-channel_layout) {av_channel_layout_from_mask(par-ch_layout, par-channel_layout);} else {par-ch_layout.order AV_CHANNEL_ORDER_UNSPEC;par-ch_layout.nb_channels par-channels;}} FF_ENABLE_DEPRECATION_WARNINGS #endifif (!par-block_align)par-block_align par-ch_layout.nb_channels *av_get_bits_per_sample(par-codec_id) 3;break;case AVMEDIA_TYPE_VIDEO:if ((par-width 0 || par-height 0) !(of-flags AVFMT_NODIMENSIONS)) {av_log(s, AV_LOG_ERROR, dimensions not set\n);ret AVERROR(EINVAL);goto fail;}if (av_cmp_q(st-sample_aspect_ratio, par-sample_aspect_ratio) fabs(av_q2d(st-sample_aspect_ratio) - av_q2d(par-sample_aspect_ratio)) 0.004*av_q2d(st-sample_aspect_ratio)) {if (st-sample_aspect_ratio.num ! 0 st-sample_aspect_ratio.den ! 0 par-sample_aspect_ratio.num ! 0 par-sample_aspect_ratio.den ! 0) {av_log(s, AV_LOG_ERROR, Aspect ratio mismatch between muxer (%d/%d) and encoder layer (%d/%d)\n,st-sample_aspect_ratio.num, st-sample_aspect_ratio.den,par-sample_aspect_ratio.num,par-sample_aspect_ratio.den);ret AVERROR(EINVAL);goto fail;}}break;}desc avcodec_descriptor_get(par-codec_id);if (desc desc-props AV_CODEC_PROP_REORDER)sti-reorder 1;sti-is_intra_only ff_is_intra_only(par-codec_id);if (of-codec_tag) {if ( par-codec_tag par-codec_id AV_CODEC_ID_RAWVIDEO ( av_codec_get_tag(of-codec_tag, par-codec_id) 0|| av_codec_get_tag(of-codec_tag, par-codec_id) MKTAG(r, a, w, )) !validate_codec_tag(s, st)) {// the current rawvideo encoding system ends up setting// the wrong codec_tag for avi/mov, we override it herepar-codec_tag 0;}if (par-codec_tag) {if (!validate_codec_tag(s, st)) {const uint32_t otag av_codec_get_tag(s-oformat-codec_tag, par-codec_id);av_log(s, AV_LOG_ERROR,Tag %s incompatible with output codec id %d (%s)\n,av_fourcc2str(par-codec_tag), par-codec_id, av_fourcc2str(otag));ret AVERROR_INVALIDDATA;goto fail;}} elsepar-codec_tag av_codec_get_tag(of-codec_tag, par-codec_id);}if (par-codec_type ! AVMEDIA_TYPE_ATTACHMENT)si-nb_interleaved_streams;}si-interleave_packet of-interleave_packet;if (!si-interleave_packet)si-interleave_packet si-nb_interleaved_streams 1 ?ff_interleave_packet_per_dts :ff_interleave_packet_passthrough;if (!s-priv_data of-priv_data_size 0) {s-priv_data av_mallocz(of-priv_data_size);if (!s-priv_data) {ret AVERROR(ENOMEM);goto fail;}if (of-priv_class) {*(const AVClass **)s-priv_data of-priv_class;av_opt_set_defaults(s-priv_data);if ((ret av_opt_set_dict2(s-priv_data, tmp, AV_OPT_SEARCH_CHILDREN)) 0)goto fail;}}/* set muxer identification string */if (!(s-flags AVFMT_FLAG_BITEXACT)) {av_dict_set(s-metadata, encoder, LIBAVFORMAT_IDENT, 0);} else {av_dict_set(s-metadata, encoder, NULL, 0);}for (e NULL; e av_dict_get(s-metadata, encoder-, e, AV_DICT_IGNORE_SUFFIX); ) {av_dict_set(s-metadata, e-key, NULL, 0);}if (options) {av_dict_free(options);*options tmp;}if (s-oformat-init) {if ((ret s-oformat-init(s)) 0) {if (s-oformat-deinit)s-oformat-deinit(s);return ret;}return ret 0;}return 0;fail:av_dict_free(tmp);return ret; }init_muxer()代码很长但是它所做的工作比较简单可以概括成两个字检查。函数的流程可以概括成以下几步 1将传入的AVDictionary形式的选项设置到AVFormatContext2遍历AVFormatContext中的每个AVStream并作如下检查 a)AVStream的time_base是否正确设置。如果发现AVStream的time_base没有设置则会调用avpriv_set_pts_info()进行设置。b)对于音频检查采样率设置是否正确对于视频检查宽、高、宽高比。c)其他一些检查不再详述。 AVOutputFormat-write_header() avformat_write_header()中最关键的地方就是调用了AVOutputFormat的write_header()。write_header()是AVOutputFormat中的一个函数指针指向写文件头的函数。不同的AVOutputFormat有不同的write_header()的实现方法。在这里我们举例子看一下FLV封装格式对应的AVOutputFormat它的定义位于libavformat\flvenc.c如下所示。 const AVOutputFormat ff_flv_muxer {.name flv,.long_name NULL_IF_CONFIG_SMALL(FLV (Flash Video)),.mime_type video/x-flv,.extensions flv,.priv_data_size sizeof(FLVContext),.audio_codec CONFIG_LIBMP3LAME ? AV_CODEC_ID_MP3 : AV_CODEC_ID_ADPCM_SWF,.video_codec AV_CODEC_ID_FLV1,.init flv_init,.write_header flv_write_header,.write_packet flv_write_packet,.write_trailer flv_write_trailer,.check_bitstream flv_check_bitstream,.codec_tag (const AVCodecTag* const []) {flv_video_codec_ids, flv_audio_codec_ids, 0},.flags AVFMT_GLOBALHEADER | AVFMT_VARIABLE_FPS |AVFMT_TS_NONSTRICT,.priv_class flv_muxer_class, }; 从ff_flv_muxer的定义中可以看出write_header()指向的函数为flv_write_header()。我们继续看一下flv_write_header()函数。flv_write_header()的定义同样位于libavformat\flvenc.c如下所示。 static int flv_write_header(AVFormatContext *s) {int i;AVIOContext *pb s-pb;FLVContext *flv s-priv_data;avio_write(pb, FLV, 3);avio_w8(pb, 1);avio_w8(pb, FLV_HEADER_FLAG_HASAUDIO * !!flv-audio_par FLV_HEADER_FLAG_HASVIDEO * !!flv-video_par);avio_wb32(pb, 9);avio_wb32(pb, 0);for (i 0; i s-nb_streams; i)if (s-streams[i]-codecpar-codec_tag 5) {avio_w8(pb, 8); // message typeavio_wb24(pb, 0); // include flagsavio_wb24(pb, 0); // time stampavio_wb32(pb, 0); // reservedavio_wb32(pb, 11); // sizeflv-reserved 5;}if (flv-flags FLV_NO_METADATA) {pb-seekable 0;} else {write_metadata(s, 0);}for (i 0; i s-nb_streams; i) {flv_write_codec_header(s, s-streams[i]-codecpar, 0);}flv-datastart_offset avio_tell(pb);return 0; } 从源代码可以看出flv_write_header()完成了FLV文件头的写入工作。该函数的工作可以大体分为以下两部分1给FLVContext设置参数2写文件头以及相关的Tag写文件头的代码很短如下所示avio_write(pb, FLV, 3);avio_w8(pb, 1);avio_w8(pb, FLV_HEADER_FLAG_HASAUDIO * !!flv-audio_par FLV_HEADER_FLAG_HASVIDEO * !!flv-video_par);avio_wb32(pb, 9);avio_wb32(pb, 0); 可以参考下图中FLV文件头的定义比对一下上面的代码