目录

1. 参考
2. 示例说明
3. 示例代码

1. 参考

• [1] FFmpeg/doc/examples/muxing.c

2. 示例说明

示例来源于[1]，提供了一个音视频封装的处理流程。

示例本程序会生成一个合成的音频和视频流，并将它们编码和封装输出到输出文件，输出格式是根据文件扩展名自动猜测的。

示例的流程图如下所示。

 

FFmpeg_muxing.png

流程中的各部分在其他示例中都有介绍过，这里不展开介绍。

• 编解码API的详细介绍参见 FFmpeg音频解码。
• 其中音频重采样流程的介绍见 FFmpeg音频重采样API(libswresample)。
• 视频像素格式转换参见 FFmpeg libswscale使用示例。

3. 示例代码

代码来源于[1]。

/*
 * Copyright (c) 2003 Fabrice Bellard
 *
 * Permission is hereby granted, free of charge, to any person obtaining a copy
 * of this software and associated documentation files (the "Software"), to deal
 * in the Software without restriction, including without limitation the rights
 * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
 * copies of the Software, and to permit persons to whom the Software is
 * furnished to do so, subject to the following conditions:
 *
 * The above copyright notice and this permission notice shall be included in
 * all copies or substantial portions of the Software.
 *
 * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
 * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
 * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL
 * THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
 * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
 * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
 * THE SOFTWARE.
 */

/**
 * @file
 * libavformat API example.
 *
 * Output a media file in any supported libavformat format. The default
 * codecs are used.
 * @example 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 <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;

  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, const AVRational *time_base, AVStream *st, AVPacket *pkt)
{
  /* rescale output packet timestamp values from codec to stream timebase */
  av_packet_rescale_ts(pkt, *time_base, st->time_base);
  pkt->stream_index = st->index;

  /* Write the compressed frame to the media file. */
  log_packet(fmt_ctx, pkt);
  return av_interleaved_write_frame(fmt_ctx, pkt);
}

/* Add an output stream. */
static void add_stream(OutputStream *ost, AVFormatContext *oc,
            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->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, 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;
  AVPacket pkt = { 0 }; // data and size must be 0;
  AVFrame *frame;
  int ret;
  int got_packet;
  int dst_nb_samples;

  av_init_packet(&pkt);
  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;
  }

  ret = avcodec_encode_audio2(c, &pkt, frame, &got_packet);
  if (ret < 0) {
    fprintf(stderr, "Error encoding audio frame: %s\n", av_err2str(ret));
    exit(1);
  }

  if (got_packet) {
    ret = write_frame(oc, &c->time_base, ost->st, &pkt);
    if (ret < 0) {
      fprintf(stderr, "Error while writing audio frame: %s\n",
          av_err2str(ret));
      exit(1);
    }
  }

  return (frame || got_packet) ? 0 : 1;
}

/**************************************************************/
/* 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, 32);
  if (ret < 0) {
    fprintf(stderr, "Could not allocate frame data.\n");
    exit(1);
  }

  return picture;
}

static void open_video(AVFormatContext *oc, 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)
{
  int ret;
  AVCodecContext *c;
  AVFrame *frame;
  int got_packet = 0;
  AVPacket pkt = { 0 };

  c = ost->enc;

  frame = get_video_frame(ost);

  av_init_packet(&pkt);

  /* encode the image */
  ret = avcodec_encode_video2(c, &pkt, frame, &got_packet);
  if (ret < 0) {
    fprintf(stderr, "Error encoding video frame: %s\n", av_err2str(ret));
    exit(1);
  }

  if (got_packet) {
    ret = write_frame(oc, &c->time_base, ost->st, &pkt);
  } else {
    ret = 0;
  }

  if (ret < 0) {
    fprintf(stderr, "Error while writing video frame: %s\n", av_err2str(ret));
    exit(1);
  }

  return (frame || got_packet) ? 0 : 1;
}

static void close_stream(AVFormatContext *oc, OutputStream *ost)
{
  avcodec_free_context(&ost->enc);
  av_frame_free(&ost->frame);
  av_frame_free(&ost->tmp_frame);
  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 char *filename;
  AVOutputFormat *fmt;
  AVFormatContext *oc;
  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;
}

说明：

1. 其中编解码的API是旧的。