mux.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

// 对单个输出AVStream的封装的结构体
typedef struct OutputStream {
    AVStream *st;
    AVCodecContext *enc;

    // 下一帧将生成的时间戳（Presentation Timestamp）
    int64_t next_pts;
    int samples_count;

    // 存储编码前的原始帧
    AVFrame *frame;
    // 存储临时帧
    AVFrame *tmp_frame;

    // 存储临时数据包
    AVPacket *tmp_pkt;

    // 时间相关的变量，用于计算帧的时间戳
    float t, tincr, tincr2;

    // 用于图像转换
    struct SwsContext *sws_ctx;
    // 用于音频转换
    struct SwrContext *swr_ctx;
} OutputStream;


// 打印 AVPacket 的日志信息，包括时间戳和流索引等。
static void log_packet(const AVFormatContext *fmt_ctx, const AVPacket *pkt)
{   
    // 获取当前数据包对应的流的时间基准
    AVRational *time_base = &fmt_ctx->streams[pkt->stream_index]->time_base;

    // 打印时间戳和时间
    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);
}

// 将帧写入编码器并将压缩后的数据包写入媒体文件。
static int write_frame(AVFormatContext *fmt_ctx, AVCodecContext *c,
                       AVStream *st, AVFrame *frame, AVPacket *pkt)
{
    int ret;

    // 将帧发送给编码器
    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);
        }

        // 从编解码器的时间基准转换输出数据包的时间戳
        av_packet_rescale_ts(pkt, c->time_base, st->time_base);
        pkt->stream_index = st->index;

        // 将压缩后的数据包写入媒体文件
        log_packet(fmt_ctx, pkt);
        ret = av_interleaved_write_frame(fmt_ctx, pkt);
        /* pkt 现在为空（av_interleaved_write_frame() 接管了其内容并重置 pkt），
         * 因此不需要取消引用。如果使用了 av_write_frame()，情况将有所不同。 */
        if (ret < 0) {
            fprintf(stderr, "Error while writing output packet: %s\n", av_err2str(ret));
            exit(1);
        }
    }

    return ret == AVERROR_EOF ? 1 : 0;
}

// 添加一个输出流
static void add_stream(OutputStream *ost, AVFormatContext *oc,
                       const AVCodec **codec,
                       enum AVCodecID codec_id)
{
    AVCodecContext *c;
    int i;

    // 查找编码器
    *codec = avcodec_find_encoder(codec_id);
    if (!(*codec)) {
        fprintf(stderr, "Could not find encoder for '%s'\n",
                avcodec_get_name(codec_id));
        exit(1);
    }

    // 分配 AVPacket 用于临时存储数据包
    ost->tmp_pkt = av_packet_alloc();
    if (!ost->tmp_pkt) {
        fprintf(stderr, "Could not allocate AVPacket\n");
        exit(1);
    }

    // 创建一个新的输出流
    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;

    // 分配一个编码上下文
    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:
        // 设置音频流参数
        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;
            }
        }
        av_channel_layout_copy(&c->ch_layout, &(AVChannelLayout)AV_CHANNEL_LAYOUT_STEREO);
        ost->st->time_base = (AVRational){ 1, c->sample_rate };
        break;

    case AVMEDIA_TYPE_VIDEO:
        // 设置视频流参数
        c->codec_id = codec_id;

        c->bit_rate = 400000;
        // 分辨率必须是2的倍数
        c->width    = 352;
        c->height   = 288;
        /* 时间基准：这是时间的基本单位（以秒为单位），用于表示帧的时间戳。
         * 对于固定帧率的内容，时间基准应该是 1/帧率，并且时间戳增量应该是相同的。 */
        ost->st->time_base = (AVRational){ 1, STREAM_FRAME_RATE };
        c->time_base       = ost->st->time_base;

        c->gop_size      = 12; //最多每12帧发射一个关键帧（I帧）
        c->pix_fmt       = STREAM_PIX_FMT;
        if (c->codec_id == AV_CODEC_ID_MPEG2VIDEO) {
            // 仅用于测试，添加两个B帧
            c->max_b_frames = 2;
        }
        if (c->codec_id == AV_CODEC_ID_MPEG1VIDEO) {
            /* 避免使用宏块中的某些系数溢出。
             * 正常视频不会出现这种情况，只是在这里发生，因为色度平面的运动与亮度平面不匹配。 */
            c->mb_decision = 2;
        }
        break;

    default:
        break;
    }

    // 某些格式要求流头部分开
    if (oc->oformat->flags & AVFMT_GLOBALHEADER)
        c->flags |= AV_CODEC_FLAG_GLOBAL_HEADER;
}

/**************************************************************/
// 音频输出

static AVFrame *alloc_audio_frame(enum AVSampleFormat sample_fmt,
                                  const AVChannelLayout *channel_layout,
                                  int sample_rate, int nb_samples)
{
    // 分配一个音频帧
    AVFrame *frame = av_frame_alloc();
    if (!frame) {
        fprintf(stderr, "Error allocating an audio frame\n");
        exit(1);
    }

    frame->format = sample_fmt;
    av_channel_layout_copy(&frame->ch_layout, channel_layout);
    frame->sample_rate = sample_rate;
    frame->nb_samples = nb_samples;

    if (nb_samples) {
        if (av_frame_get_buffer(frame, 0) < 0) {
            fprintf(stderr, "Error allocating an audio buffer\n");
            exit(1);
        }
    }

    return frame;
}

// 打开音频输出流
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;

    // 打开音频编码器
    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);
    }

    // 初始化信号发生器
    ost->t     = 0;
    ost->tincr = 2 * M_PI * 110.0 / c->sample_rate;
    // 每秒增加110 Hz的频率
    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;

    // 分配音频帧
    ost->frame     = alloc_audio_frame(c->sample_fmt, &c->ch_layout,
                                       c->sample_rate, nb_samples);
    ost->tmp_frame = alloc_audio_frame(AV_SAMPLE_FMT_S16, &c->ch_layout,
                                       c->sample_rate, nb_samples);

    // 将流参数复制到复用器
    ret = avcodec_parameters_from_context(ost->st->codecpar, c);
    if (ret < 0) {
        fprintf(stderr, "Could not copy the stream parameters\n");
        exit(1);
    }

    // 创建重采样上下文
    ost->swr_ctx = swr_alloc();
    if (!ost->swr_ctx) {
        fprintf(stderr, "Could not allocate resampler context\n");
        exit(1);
    }

    // 设置选项
    av_opt_set_chlayout  (ost->swr_ctx, "in_chlayout",       &c->ch_layout,      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_chlayout  (ost->swr_ctx, "out_chlayout",      &c->ch_layout,      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);

    // 初始化重采样上下文
    if ((ret = swr_init(ost->swr_ctx)) < 0) {
        fprintf(stderr, "Failed to initialize the resampling context\n");
        exit(1);
    }
}

// 准备一个帧大小为'frame_size'样本，通道数为'nb_channels'的16位虚拟音频帧
static AVFrame *get_audio_frame(OutputStream *ost)
{
    AVFrame *frame = ost->tmp_frame;
    int j, i, v;
    int16_t *q = (int16_t*)frame->data[0];

    // 检查是否需要生成更多帧
    if (av_compare_ts(ost->next_pts, ost->enc->time_base,
                      STREAM_DURATION, (AVRational){ 1, 1 }) > 0)
        return NULL;

    // 遍历帧中的每个样本
    for (j = 0; j <frame->nb_samples; j++) {
        v = (int)(sin(ost->t) * 10000);
        for (i = 0; i < ost->enc->ch_layout.nb_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;
}

// 对于一个音频帧进行编码并发送给复用器
// 在编码完成时返回1，否则返回0
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) {
        // 将样本从本地格式转换为目标编解码器格式，实用重采样器
        // 计算目标样本数
        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);

        // 当我们将帧传递给编码器时，它可能在内部保留对该帧的引用；
        // 确保我们不会在此处覆盖它
        ret = av_frame_make_writable(ost->frame);
        if (ret < 0)
            exit(1);

        // 转换为目标格式
        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);
}

/**************************************************************/
// 视频输出

static AVFrame *alloc_frame(enum AVPixelFormat pix_fmt, int width, int height)
{
    AVFrame *frame;
    int ret;

    // 分配帧对象
    frame = av_frame_alloc();
    if (!frame)
        return NULL;

    // 设置帧的像素格式
    frame->format = pix_fmt;
    // 设置帧的宽度
    frame->width  = width;
    // 设置帧的高度
    frame->height = height;

    // 分配帧数据的缓冲区
    ret = av_frame_get_buffer(frame, 0);
    if (ret < 0) {
        fprintf(stderr, "Could not allocate frame data.\n");
        exit(1);
    }

    return frame;
}

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);

    // 打开编码器
    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);
    }

    // 分配和初始化可重用的帧
    ost->frame = alloc_frame(c->pix_fmt, c->width, c->height);
    if (!ost->frame) {
        fprintf(stderr, "Could not allocate video frame\n");
        exit(1);
    }

    // 如果输出格式不是YUV420P，则还需要一个临时的YUV420P图片
    // 它将被转换为所需的输出格式
    ost->tmp_frame = NULL;
    if (c->pix_fmt != AV_PIX_FMT_YUV420P) {
        ost->tmp_frame = alloc_frame(AV_PIX_FMT_YUV420P, c->width, c->height);
        if (!ost->tmp_frame) {
            fprintf(stderr, "Could not allocate temporary video frame\n");
            exit(1);
        }
    }

    // 将流参数复制到复用器
    ret = avcodec_parameters_from_context(ost->st->codecpar, c);
    if (ret < 0) {
        fprintf(stderr, "Could not copy the stream parameters\n");
        exit(1);
    }
}

// 准备一个虚拟图像
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 和 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;
        }
    }
}

// 获取视频帧
static AVFrame *get_video_frame(OutputStream *ost)
{
    AVCodecContext *c = ost->enc;

    // 检查是否需要生成更多帧
    if (av_compare_ts(ost->next_pts, c->time_base,
                      STREAM_DURATION, (AVRational){ 1, 1 }) > 0)
        return NULL;

    // 当向编码器传递帧时，它可能会在内部保留对该帧的引用
    // 确保我们不会在这里覆盖它
    if (av_frame_make_writable(ost->frame) < 0)
        exit(1);

    if (c->pix_fmt != AV_PIX_FMT_YUV420P) {
        // 由于我们只生成YUV420P图片，如果需要，我们必须将其转换为编码器的像素格式
        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;
}

// 编码一帧视频并将其发送到复用器
// 返回1表示编码完成，返回0表示未完成
static int write_video_frame(AVFormatContext *oc, OutputStream *ost)
{
    return write_frame(oc, ost->enc, ost->st, get_video_frame(ost), ost->tmp_pkt);
}

// 关闭流
static void close_stream(AVFormatContext *oc, OutputStream *ost)
{
    avcodec_free_context(&ost->enc);        // 释放编码器上下文
    av_frame_free(&ost->frame);             // 释放帧对象
    av_frame_free(&ost->tmp_frame);         // 释放临时帧对象
    av_packet_free(&ost->tmp_pkt);          // 释放临时数据包对象
    sws_freeContext(ost->sws_ctx);          // 释放图像转换上下文
    swr_free(&ost->swr_ctx);                // 释放音频重采样上下文
}

/**************************************************************/
// 媒体文件输出

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);
    }

    // 分配输出媒体上下午
    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;

    // 使用默认格式编解码器添加音频和视频流，并初始化编解码器
    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;
    }

    // 现在参数都设置好了，我们可以打开音频和视频编解码器，并分配必要的编码缓冲区
    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);

    // 如果需要，打开输出文件
    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;
        }
    }

    // 写入流头信息，（如果有的话）
    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) {
        // 选择要编码的流
        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);
        }
    }

    av_write_trailer(oc);

    // 关闭每个编解码器
    if (have_video)
        close_stream(oc, &video_st);
    if (have_audio)
        close_stream(oc, &audio_st);

    if (!(fmt->flags & AVFMT_NOFILE))
        // 关闭输出文件
        avio_closep(&oc->pb);

    // 释放流
    avformat_free_context(oc);

    return 0;
}