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libavformat（lavf）是一个用于处理各种媒体容器格式的库。它的主要两个目的是去复用（即将媒体文件拆分为组件流）和复用的反向过程（以指定的容器格式写入提供的数据）。它还有一个I/O模块，支持多种访问数据的协议（如文件、tcp、http等）。在使用lavf之前，您需要调用av_register_all（）来注册所有已编译的复用器、解复用器和协议。除非您绝对确定不会使用libavformat的网络功能，否则还应该调用avformat_network_init（）；
一、媒体流封装（Muxing）
媒体流封装主要是指以AVPackets的形式获取编码后的数据后，以指定的容器格式将其写入到文件或者其他方式输出到字节流中。
Muxing实际执行的主要API有：
初始化：avformat_alloc_output_context2();
创建媒体流（如果有的话）：avformat_new_stream()；
写文件头：avformat_write_header()；
写数据包：av_write_frame()/av_interleaved_write_frame();
写文件尾：av_write_trailer();
流程图：
代码示例：
在官方源码：/doc/examples/muxing.c
#include <stdlib.h> #include <stdio.h> #include <string.h> #include <math.h> #include <libavutil/avassert.h> #include <libavutil/channel_layout.h> #include <libavutil/opt.h> #include <libavutil/mathematics.h> #include <libavutil/timestamp.h> #include <libavcodec/avcodec.h> #include <libavformat/avformat.h> #include <libswscale/swscale.h> #include <libswresample/swresample.h> #define STREAM_DURATION 10.0 #define STREAM_FRAME_RATE 25 /* 25 images/s */ #define STREAM_PIX_FMT AV_PIX_FMT_YUV420P /* default pix_fmt */ #define SCALE_FLAGS SWS_BICUBIC // a wrapper around a single output AVStream typedef struct OutputStream { AVStream *st; AVCodecContext *enc; /* pts of the next frame that will be generated */ int64_t next_pts; int samples_count; AVFrame *frame; AVFrame *tmp_frame; AVPacket *tmp_pkt; float t, tincr, tincr2; struct SwsContext *sws_ctx; struct SwrContext *swr_ctx; } OutputStream; static void log_packet(const AVFormatContext *fmt_ctx, const AVPacket *pkt) { AVRational *time_base = &fmt_ctx->streams[pkt->stream_index]->time_base; printf("pts:%s pts_time:%s dts:%s dts_time:%s duration:%s duration_time:%s stream_index:%d\n", av_ts2str(pkt->pts), av_ts2timestr(pkt->pts, time_base), av_ts2str(pkt->dts), av_ts2timestr(pkt->dts, time_base), av_ts2str(pkt->duration), av_ts2timestr(pkt->duration, time_base), pkt->stream_index); } static int write_frame(AVFormatContext *fmt_ctx, AVCodecContext *c, AVStream *st, AVFrame *frame, AVPacket *pkt) { int ret; // send the frame to the encoder ret = avcodec_send_frame(c, frame); if (ret < 0) { fprintf(stderr, "Error sending a frame to the encoder: %s\n", av_err2str(ret)); exit(1); } while (ret >= 0) { ret = avcodec_receive_packet(c, pkt); if (ret == AVERROR(EAGAIN) || ret == AVERROR_EOF) break; else if (ret < 0) { fprintf(stderr, "Error encoding a frame: %s\n", av_err2str(ret)); exit(1); } /* rescale output packet timestamp values from codec to stream timebase */ av_packet_rescale_ts(pkt, c->time_base, st->time_base); pkt->stream_index = st->index; /* Write the compressed frame to the media file. */ log_packet(fmt_ctx, pkt); ret = av_interleaved_write_frame(fmt_ctx, pkt); /* pkt is now blank (av_interleaved_write_frame() takes ownership of * its contents and resets pkt), so that no unreferencing is necessary. * This would be different if one used av_write_frame(). */ if (ret < 0) { fprintf(stderr, "Error while writing output packet: %s\n", av_err2str(ret)); exit(1); } } return ret == AVERROR_EOF ? 1 : 0; } /* Add an output stream. */ static void add_stream(OutputStream *ost, AVFormatContext *oc, const AVCodec **codec, enum AVCodecID codec_id) { AVCodecContext *c; int i; /* find the encoder */ *codec = avcodec_find_encoder(codec_id); if (!(*codec)) { fprintf(stderr, "Could not find encoder for '%s'\n", avcodec_get_name(codec_id)); exit(1); } ost->tmp_pkt = av_packet_alloc(); if (!ost->tmp_pkt) { fprintf(stderr, "Could not allocate AVPacket\n"); exit(1); } ost->st = avformat_new_stream(oc, NULL); if (!ost->st) { fprintf(stderr, "Could not allocate stream\n"); exit(1); } ost->st->id = oc->nb_streams-1; c = avcodec_alloc_context3(*codec); if (!c) { fprintf(stderr, "Could not alloc an encoding context\n"); exit(1); } ost->enc = c; switch ((*codec)->type) { case AVMEDIA_TYPE_AUDIO: c->sample_fmt = (*codec)->sample_fmts ? (*codec)->sample_fmts[0] : AV_SAMPLE_FMT_FLTP; c->bit_rate = 64000; c->sample_rate = 44100; if ((*codec)->supported_samplerates) { c->sample_rate = (*codec)->supported_samplerates[0]; for (i = 0; (*codec)->supported_samplerates[i]; i++) { if ((*codec)->supported_samplerates[i] == 44100) c->sample_rate = 44100; } } c->channels = av_get_channel_layout_nb_channels(c->channel_layout); c->channel_layout = AV_CH_LAYOUT_STEREO; if ((*codec)->channel_layouts) { c->channel_layout = (*codec)->channel_layouts[0]; for (i = 0; (*codec)->channel_layouts[i]; i++) { if ((*codec)->channel_layouts[i] == AV_CH_LAYOUT_STEREO) c->channel_layout = AV_CH_LAYOUT_STEREO; } } c->channels = av_get_channel_layout_nb_channels(c->channel_layout); ost->st->time_base = (AVRational){ 1, c->sample_rate }; break; case AVMEDIA_TYPE_VIDEO: c->codec_id = codec_id; c->bit_rate = 400000; /* Resolution must be a multiple of two. */ c->width = 352; c->height = 288; /* timebase: 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 * identical to 1. */ ost->st->time_base = (AVRational){ 1, STREAM_FRAME_RATE }; c->time_base = ost->st->time_base; c->gop_size = 12; /* emit one intra frame every twelve frames at most */ c->pix_fmt = STREAM_PIX_FMT; if (c->codec_id == AV_CODEC_ID_MPEG2VIDEO) { /* just for testing, we also add B-frames */ c->max_b_frames = 2; } if (c->codec_id == AV_CODEC_ID_MPEG1VIDEO) { /* Needed to avoid using macroblocks in which some coeffs overflow. * This does not happen with normal video, it just happens here as * the motion of the chroma plane does not match the luma plane. */ c->mb_decision = 2; } break; default: break; } /* Some formats want stream headers to be separate. */ if (oc->oformat->flags & AVFMT_GLOBALHEADER) c->flags |= AV_CODEC_FLAG_GLOBAL_HEADER; } /**************************************************************/ /* audio output */ static AVFrame *alloc_audio_frame(enum AVSampleFormat sample_fmt, uint64_t channel_layout, int sample_rate, int nb_samples) { AVFrame *frame = av_frame_alloc(); int ret; if (!frame) { fprintf(stderr, "Error allocating an audio frame\n"); exit(1); } frame->format = sample_fmt; frame->channel_layout = channel_layout; frame->sample_rate = sample_rate; frame->nb_samples = nb_samples; if (nb_samples) { ret = av_frame_get_buffer(frame, 0); if (ret < 0) { fprintf(stderr, "Error allocating an audio buffer\n"); exit(1); } } return frame; } static void open_audio(AVFormatContext *oc, const AVCodec *codec, OutputStream *ost, AVDictionary *opt_arg) { AVCodecContext *c; int nb_samples; int ret; AVDictionary *opt = NULL; c = ost->enc; /* open it */ av_dict_copy(&opt, opt_arg, 0); ret = avcodec_open2(c, codec, &opt); av_dict_free(&opt); if (ret < 0) { fprintf(stderr, "Could not open audio codec: %s\n", av_err2str(ret)); exit(1); } /* init signal generator */ ost->t = 0; ost->tincr = 2 * M_PI * 110.0 / c->sample_rate; /* increment frequency by 110 Hz per second */ ost->tincr2 = 2 * M_PI * 110.0 / c->sample_rate / c->sample_rate; if (c->codec->capabilities & AV_CODEC_CAP_VARIABLE_FRAME_SIZE) nb_samples = 10000; else nb_samples = c->frame_size; ost->frame = alloc_audio_frame(c->sample_fmt, c->channel_layout, c->sample_rate, nb_samples); ost->tmp_frame = alloc_audio_frame(AV_SAMPLE_FMT_S16, c->channel_layout, c->sample_rate, nb_samples); /* copy the stream parameters to the muxer */ ret = avcodec_parameters_from_context(ost->st->codecpar, c); if (ret < 0) { fprintf(stderr, "Could not copy the stream parameters\n"); exit(1); } /* create resampler context */ ost->swr_ctx = swr_alloc(); if (!ost->swr_ctx) { fprintf(stderr, "Could not allocate resampler context\n"); exit(1); } /* set options */ av_opt_set_int (ost->swr_ctx, "in_channel_count", c->channels, 0); av_opt_set_int (ost->swr_ctx, "in_sample_rate", c->sample_rate, 0); av_opt_set_sample_fmt(ost->swr_ctx, "in_sample_fmt", AV_SAMPLE_FMT_S16, 0); av_opt_set_int (ost->swr_ctx, "out_channel_count", c->channels, 0); av_opt_set_int (ost->swr_ctx, "out_sample_rate", c->sample_rate, 0); av_opt_set_sample_fmt(ost->swr_ctx, "out_sample_fmt", c->sample_fmt, 0); /* initialize the resampling context */ if ((ret = swr_init(ost->swr_ctx)) < 0) { fprintf(stderr, "Failed to initialize the resampling context\n"); exit(1); } } /* Prepare a 16 bit dummy audio frame of 'frame_size' samples and * 'nb_channels' channels. */ static AVFrame *get_audio_frame(OutputStream *ost) { AVFrame *frame = ost->tmp_frame; int j, i, v; int16_t *q = (int16_t*)frame->data[0]; /* check if we want to generate more frames */ if (av_compare_ts(ost->next_pts, ost->enc->time_base, STREAM_DURATION, (AVRational){ 1, 1 }) > 0) return NULL; for (j = 0; j <frame->nb_samples; j++) { v = (int)(sin(ost->t) * 10000); for (i = 0; i < ost->enc->channels; i++) *q++ = v; ost->t += ost->tincr; ost->tincr += ost->tincr2; } frame->pts = ost->next_pts; ost->next_pts += frame->nb_samples; return frame; } /* * encode one audio frame and send it to the muxer * return 1 when encoding is finished, 0 otherwise */ static int write_audio_frame(AVFormatContext *oc, OutputStream *ost) { AVCodecContext *c; AVFrame *frame; int ret; int dst_nb_samples; c = ost->enc; frame = get_audio_frame(ost); if (frame) { /* convert samples from native format to destination codec format, using the resampler */ /* compute destination number of samples */ dst_nb_samples = av_rescale_rnd(swr_get_delay(ost->swr_ctx, c->sample_rate) + frame->nb_samples, c->sample_rate, c->sample_rate, AV_ROUND_UP); av_assert0(dst_nb_samples == frame->nb_samples); /* when we pass a frame to the encoder, it may keep a reference to it * internally; * make sure we do not overwrite it here */ ret = av_frame_make_writable(ost->frame); if (ret < 0) exit(1); /* convert to destination format */ ret = swr_convert(ost->swr_ctx, ost->frame->data, dst_nb_samples, (const uint8_t **)frame->data, frame->nb_samples); if (ret < 0) { fprintf(stderr, "Error while converting\n"); exit(1); } frame = ost->frame; frame->pts = av_rescale_q(ost->samples_count, (AVRational){1, c->sample_rate}, c->time_base); ost->samples_count += dst_nb_samples; } return write_frame(oc, c, ost->st, frame, ost->tmp_pkt); } /**************************************************************/ /* video output */ static AVFrame *alloc_picture(enum AVPixelFormat pix_fmt, int width, int height) { AVFrame *picture; int ret; picture = av_frame_alloc(); if (!picture) return NULL; picture->format = pix_fmt; picture->width = width; picture->height = height; /* allocate the buffers for the frame data */ ret = av_frame_get_buffer(picture, 0); if (ret < 0) { fprintf(stderr, "Could not allocate frame data.\n"); exit(1); } return picture; } static void open_video(AVFormatContext *oc, const AVCodec *codec, OutputStream *ost, AVDictionary *opt_arg) { int ret; AVCodecContext *c = ost->enc; AVDictionary *opt = NULL; av_dict_copy(&opt, opt_arg, 0); /* open the codec */ ret = avcodec_open2(c, codec, &opt); av_dict_free(&opt); if (ret < 0) { fprintf(stderr, "Could not open video codec: %s\n", av_err2str(ret)); exit(1); } /* allocate and init a re-usable frame */ ost->frame = alloc_picture(c->pix_fmt, c->width, c->height); if (!ost->frame) { fprintf(stderr, "Could not allocate video frame\n"); exit(1); } /* If the output format is not YUV420P, then a temporary YUV420P * picture is needed too. It is then converted to the required * output format. */ ost->tmp_frame = NULL; if (c->pix_fmt != AV_PIX_FMT_YUV420P) { ost->tmp_frame = alloc_picture(AV_PIX_FMT_YUV420P, c->width, c->height); if (!ost->tmp_frame) { fprintf(stderr, "Could not allocate temporary picture\n"); exit(1); } } /* copy the stream parameters to the muxer */ ret = avcodec_parameters_from_context(ost->st->codecpar, c); if (ret < 0) { fprintf(stderr, "Could not copy the stream parameters\n"); exit(1); } } /* Prepare a dummy image. */ static void fill_yuv_image(AVFrame *pict, int frame_index, int width, int height) { int x, y, i; i = frame_index; /* Y */ for (y = 0; y < height; y++) for (x = 0; x < width; x++) pict->data[0][y * pict->linesize[0] + x] = x + y + i * 3; /* Cb and Cr */ for (y = 0; y < height / 2; y++) { for (x = 0; x < width / 2; x++) { pict->data[1][y * pict->linesize[1] + x] = 128 + y + i * 2; pict->data[2][y * pict->linesize[2] + x] = 64 + x + i * 5; } } } static AVFrame *get_video_frame(OutputStream *ost) { AVCodecContext *c = ost->enc; /* check if we want to generate more frames */ if (av_compare_ts(ost->next_pts, c->time_base, STREAM_DURATION, (AVRational){ 1, 1 }) > 0) return NULL; /* when we pass a frame to the encoder, it may keep a reference to it * internally; make sure we do not overwrite it here */ if (av_frame_make_writable(ost->frame) < 0) exit(1); if (c->pix_fmt != AV_PIX_FMT_YUV420P) { /* as we only generate a YUV420P picture, we must convert it * to the codec pixel format if needed */ if (!ost->sws_ctx) { ost->sws_ctx = sws_getContext(c->width, c->height, AV_PIX_FMT_YUV420P, c->width, c->height, c->pix_fmt, SCALE_FLAGS, NULL, NULL, NULL); if (!ost->sws_ctx) { fprintf(stderr, "Could not initialize the conversion context\n"); exit(1); } } fill_yuv_image(ost->tmp_frame, ost->next_pts, c->width, c->height); sws_scale(ost->sws_ctx, (const uint8_t * const *) ost->tmp_frame->data, ost->tmp_frame->linesize, 0, c->height, ost->frame->data, ost->frame->linesize); } else { fill_yuv_image(ost->frame, ost->next_pts, c->width, c->height); } ost->frame->pts = ost->next_pts++; return ost->frame; } /* * encode one video frame and send it to the muxer * return 1 when encoding is finished, 0 otherwise */ static int write_video_frame(AVFormatContext *oc, OutputStream *ost) { return write_frame(oc, ost->enc, ost->st, get_video_frame(ost), ost->tmp_pkt); } static void close_stream(AVFormatContext *oc, OutputStream *ost) { avcodec_free_context(&ost->enc); av_frame_free(&ost->frame); av_frame_free(&ost->tmp_frame); av_packet_free(&ost->tmp_pkt); sws_freeContext(ost->sws_ctx); swr_free(&ost->swr_ctx); } /**************************************************************/ /* media file output */ int main(int argc, char **argv) { OutputStream video_st = { 0 }, audio_st = { 0 }; const AVOutputFormat *fmt; const char *filename; AVFormatContext *oc; const AVCodec *audio_codec, *video_codec; int ret; int have_video = 0, have_audio = 0; int encode_video = 0, encode_audio = 0; AVDictionary *opt = NULL; int i; if (argc < 2) { printf("usage: %s output_file\n" "API example program to output a media file with libavformat.\n" "This program generates a synthetic audio and video stream, encodes and\n" "muxes them into a file named output_file.\n" "The output format is automatically guessed according to the file extension.\n" "Raw images can also be output by using '%%d' in the filename.\n" "\n", argv[0]); return 1; } filename = argv[1]; for (i = 2; i+1 < argc; i+=2) { if (!strcmp(argv[i], "-flags") || !strcmp(argv[i], "-fflags")) av_dict_set(&opt, argv[i]+1, argv[i+1], 0); } /* allocate the output media context */ avformat_alloc_output_context2(&oc, NULL, NULL, filename); if (!oc) { printf("Could not deduce output format from file extension: using MPEG.\n"); avformat_alloc_output_context2(&oc, NULL, "mpeg", filename); } if (!oc) return 1; fmt = oc->oformat; /* Add the audio and video streams using the default format codecs * and initialize the codecs. */ if (fmt->video_codec != AV_CODEC_ID_NONE) { add_stream(&video_st, oc, &video_codec, fmt->video_codec); have_video = 1; encode_video = 1; } if (fmt->audio_codec != AV_CODEC_ID_NONE) { add_stream(&audio_st, oc, &audio_codec, fmt->audio_codec); have_audio = 1; encode_audio = 1; } /* Now that all the parameters are set, we can open the audio and * video codecs and allocate the necessary encode buffers. */ if (have_video) open_video(oc, video_codec, &video_st, opt); if (have_audio) open_audio(oc, audio_codec, &audio_st, opt); av_dump_format(oc, 0, filename, 1); /* open the output file, if needed */ if (!(fmt->flags & AVFMT_NOFILE)) { ret = avio_open(&oc->pb, filename, AVIO_FLAG_WRITE); if (ret < 0) { fprintf(stderr, "Could not open '%s': %s\n", filename, av_err2str(ret)); return 1; } } /* Write the stream header, if any. */ ret = avformat_write_header(oc, &opt); if (ret < 0) { fprintf(stderr, "Error occurred when opening output file: %s\n", av_err2str(ret)); return 1; } while (encode_video || encode_audio) { /* select the stream to encode */ if (encode_video && (!encode_audio || av_compare_ts(video_st.next_pts, video_st.enc->time_base, audio_st.next_pts, audio_st.enc->time_base) <= 0)) { encode_video = !write_video_frame(oc, &video_st); } else { encode_audio = !write_audio_frame(oc, &audio_st); } } /* Write the trailer, if any. The trailer must be written before you * close the CodecContexts open when you wrote the header; otherwise * av_write_trailer() may try to use memory that was freed on * av_codec_close(). */ av_write_trailer(oc); /* Close each codec. */ if (have_video) close_stream(oc, &video_st); if (have_audio) close_stream(oc, &audio_st); if (!(fmt->flags & AVFMT_NOFILE)) /* Close the output file. */ avio_closep(&oc->pb); /* free the stream */ avformat_free_context(oc); return 0; }