AVFormatContext编解码层：理论与实战

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前言

AVFormatContext 是一个贯穿始终的数据结构，很多函数都用到它作为参数，是输入输出相关信息的一个容器，本文讲解 AVFormatContext 的编解码层，主要包括三大数据结构：AVStream，AVCodecContex，AVCodec。

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一、FFmpeg 解码流程

得到输入文件 -> 解封格式 -> 得到编码的数据包 -> 解码数据包 -> 得到解码后的数据帧 -> 处理数据帧 -> 编码 -> 得到编码后的数据包 -> 封装格式 -> 输出文件

涉及到下面的 API 函数：

• 注册所有容器格式和 CODEC：av_register_all()；
• 打开文件：av_open_input_file()；
• 从文件中提取流信息：av_find_stream_info()；
• 穷举所有的流，查找其中种类为 CODEC_TYPE_VIDEO；
• 查找对应的解码器：avcodec_find_decoder()；
• 打开编解码器：avcodec_open()；
• 为解码帧分配内存：avcodec_alloc_frame()；
• 不停地从码流中提取出帧数据：av_read_frame()；
• 判断帧的类型，对于视频帧调用：avcodec_decode_video()；
• 解码完后，释放解码器：avcodec_close()；
• 关闭输入文件：av_close_input_file()；

程序流程图如下图所示：

二、FFmpeg 转码流程

• 大流程可以划分为输入、输出、转码、播放四大块；
• 其中转码涉及比较多的处理环节，从图中可以看出，转码功能在整个功能图中占比很大。转码的核心功能在解码和编码两个部分，但在一个可用的示例程序中，编码解码与输入输出是难以分割的。
• 解复用器为解码器提供输入，解码器会输出原始帧，对原始帧可进行各种复杂的滤镜处理，滤镜处理后的帧经编码器生成编码帧，多路流的编码帧经复用器输出到输出文件。

三、编解码 API 详解

• 解码使用 avcodec_send_packet() 和 avcodec_receive_frame() 两个函数。
• 编码使用 avcodec_send_frame() 和 avcodec_receive_packet() 两个函数。

1、解码 API 使用详解

关于 avcodec_send_packet() 与 avcodec_receive_frame() 的使用说明：

• ①、按 dts 递增的顺序向解码器送入编码帧 packet，解码器按 pts 递增的顺序输出原始帧 frame，实际上解码器不关注输入 packet 的 dts（错值都没关系），它只管依次处理收到的 packet，按需缓冲和解码；
• ②、avcodec_receive_frame() 输出 frame 时，会根据各种因素设置好 frame->best_effort_timestamp（文档明确说明），实测 frame->pts 也会被设置（通常直接拷贝自对应的 packet.pts，文档未明确说明）用户应确保 avcodec_send_packet() 发送的 packet 具有正确的 pts，编码帧 packet 与原始帧 frame 间的对应关系通过 pts 确定；
• ③、avcodec_receive_frame() 输 出 frame 时 ，frame->pkt_dts 拷贝自当前 avcodec_send_packet() 发送的 packet 中的 dts， 如果当前 packet 为 NULL（flush packet），解码器进入 flush 模式，当前及剩余的 frame->pkt_dts 值总为 AV_NOPTS_VALUE。因为解码器中有缓存帧，当前输出的 frame 并不是由当前输入的 packet 解码得到的，所以这个 frame->pkt_dts 没什么实际意义，可以不必关注；
• ④、avcodec_send_packet() 发送第一个 NULL 会返回成功，后续的 NULL 会返回 AVERROR_EOF；
• ⑤、avcodec_send_packet() 多次发送 NULL 并不会导致解码器中缓存的帧丢失，使用 avcodec_flush_buffers() 可以立即丢掉解码器中缓存帧。因此播放完毕时应 avcodec_send_packet(NULL) 来取完缓存的帧，而 SEEK 操作或切换流时应调用 avcodec_flush_buffers() 来直接丢弃缓存帧；
• ⑥、解码器通常的冲洗方法：调用一次 avcodec_send_packet(NULL)(返回成功)，然后不停调用 avcodec_receive_frame() 直到其返回 AVERROR_EOF，取出所有缓存帧，avcodec_receive_frame() 返回 AVERROR_EOF 这一次是没有有效数据的，仅仅获取到一个结束标志；

2、编码 API 使用详解

关于 avcodec_send_frame() 与 avcodec_receive_packet() 的使用说明：

• ①、按 pts 递增的顺序向编码器送入原始帧 frame， 编码器按 dts 递增的顺序输出编码帧 packet，实际上编码器关注输入 frame 的 pts 不关注其 dts，它只管依次处理收到的 frame，按需缓冲和编码；
• ②、avcodec_receive_packet() 输出 packet 时，会设置 packet.dts，从 0 开始，每次输出的 packet 的 dts 加 1，这是视频层的 dts，用户写输出前应将其转换为容器层的 dts；
• ③、avcodec_receive_packet() 输出 packet 时，packet.pts 拷贝自对应的 frame.pts，这是视频层的 pts，用户写输出前应将其转换为容器层的 pts；
• ④、avcodec_send_frame() 发送 NULL frame 时， 编码器进入 flush 模式；
• ⑤、avcodec_send_frame() 发送第一个 NULL 会返回成功 ，后续的 NULL 会返回 AVERROR_EOF；
• ⑥、avcodec_send_frame() 多次发送 NULL 并不会导致编码器中缓存的帧丢失，使用 avcodec_flush_buffers() 可以立即丢掉编码器中缓存帧。因此编码完毕时应使用 avcodec_send_frame(NULL) 来取完缓存的帧，而 SEEK 操作或切换流时应调用 avcodec_flush_buffers() 来直接丢弃缓存帧；
• ⑦、编码器通常的冲洗方法：调用一次 avcodec_send_frame(NULL)(返回成功)，然后不停调用avcodec_receive_packet() 直到其返回 AVERROR_EOF，取出所有缓存帧，avcodec_receive_packet() 返回AVERROR_EOF 这一次是没有有效数据的，仅仅获取到一个结束标志；
• ⑧、对音频来说，如果 AV_CODEC_CAP_VARIABLE_FRAME_SIZE（在 AVCodecContext.codec.capabilities 变量中，只读）标志有效，表示编码器支持可变尺寸音频帧，送入编码器的音频帧可以包含任意数量的采样点。如果此标志无效，则每一个音频帧的采样点数目（frame->nb_samples）必须等于编码器设定的音频帧尺寸（avctx->frame_size），最后一帧除外，最后一帧音频帧采样点数可以小于 avctx->frame_size；

四、编码案例实战

下面代码使用实现了一个简单的视频编码器，将指定的图像序列编码为 MPEG 视频文件

1、示例源码

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

#include <libavcodec/avcodec.h>

#include <libavutil/opt.h>
#include <libavutil/imgutils.h>

static void encode(AVCodecContext *enc_ctx, AVFrame *frame, AVPacket *pkt,
                   FILE *outfile)
{
    
    
    int ret;

    /* send the frame to the encoder */
    if (frame)
        printf("Send frame %3"PRId64"\n", frame->pts);

    ret = avcodec_send_frame(enc_ctx, frame);
    if (ret < 0) {
    
    
        fprintf(stderr, "Error sending a frame for encoding\n");
        exit(1);
    }

    while (ret >= 0) {
    
    
        ret = avcodec_receive_packet(enc_ctx, pkt);
        if (ret == AVERROR(EAGAIN) || ret == AVERROR_EOF)
            return;
        else if (ret < 0) {
    
    
            fprintf(stderr, "Error during encoding\n");
            exit(1);
        }

        printf("Write packet %3"PRId64" (size=%5d)\n", pkt->pts, pkt->size);
        fwrite(pkt->data, 1, pkt->size, outfile);
        av_packet_unref(pkt);
    }
}

int main(int argc, char **argv)
{
    
    
    const char *filename, *codec_name;
    const AVCodec *codec;
    AVCodecContext *c= NULL;
    int i, ret, x, y;
    FILE *f;
    AVFrame *frame;
    AVPacket *pkt;
    uint8_t endcode[] = {
    
     0, 0, 1, 0xb7 };

    filename = "./debug/test666.mp4";
    codec_name = "mpeg1video";

    /* find the mpeg1video encoder */
    codec = avcodec_find_encoder_by_name(codec_name);
    if (!codec) {
    
    
        fprintf(stderr, "Codec '%s' not found\n", codec_name);
        exit(1);
    }

    c = avcodec_alloc_context3(codec);
    if (!c) {
    
    
        fprintf(stderr, "Could not allocate video codec context\n");
        exit(1);
    }

    pkt = av_packet_alloc();
    if (!pkt)
        exit(1);

    /* put sample parameters */
    c->bit_rate = 400000;
    /* resolution must be a multiple of two */
    c->width = 352;
    c->height = 288;
    /* frames per second */
    c->time_base = (AVRational){
    
    1, 25};
    c->framerate = (AVRational){
    
    25, 1};

    /* emit one intra frame every ten frames
     * check frame pict_type before passing frame
     * to encoder, if frame->pict_type is AV_PICTURE_TYPE_I
     * then gop_size is ignored and the output of encoder
     * will always be I frame irrespective to gop_size
     */
    c->gop_size = 10;
    c->max_b_frames = 1;
    c->pix_fmt = AV_PIX_FMT_YUV420P;

    if (codec->id == AV_CODEC_ID_H264)
        av_opt_set(c->priv_data, "preset", "slow", 0);

    /* open it */
    ret = avcodec_open2(c, codec, NULL);
    if (ret < 0) {
    
    
        fprintf(stderr, "Could not open codec: %s\n", av_err2str(ret));
        exit(1);
    }

    f = fopen(filename, "wb");
    if (!f) {
    
    
        fprintf(stderr, "Could not open %s\n", filename);
        exit(1);
    }

    frame = av_frame_alloc();
    if (!frame) {
    
    
        fprintf(stderr, "Could not allocate video frame\n");
        exit(1);
    }
    frame->format = c->pix_fmt;
    frame->width  = c->width;
    frame->height = c->height;

    ret = av_frame_get_buffer(frame, 0);
    if (ret < 0) {
    
    
        fprintf(stderr, "Could not allocate the video frame data\n");
        exit(1);
    }

    /* encode 1 second of video */
    for (i = 0; i < 25; i++) {
    
    
        fflush(stdout);

        /* make sure the frame data is writable */
        ret = av_frame_make_writable(frame);
        if (ret < 0)
            exit(1);

        /* prepare a dummy image */
        /* Y */
        for (y = 0; y < c->height; y++) {
    
    
            for (x = 0; x < c->width; x++) {
    
    
                frame->data[0][y * frame->linesize[0] + x] = x + y + i * 3;
            }
        }

        /* Cb and Cr */
        for (y = 0; y < c->height/2; y++) {
    
    
            for (x = 0; x < c->width/2; x++) {
    
    
                frame->data[1][y * frame->linesize[1] + x] = 128 + y + i * 2;
                frame->data[2][y * frame->linesize[2] + x] = 64 + x + i * 5;
            }
        }

        frame->pts = i;

        /* encode the image */
        encode(c, frame, pkt, f);
    }

    /* flush the encoder */
    encode(c, NULL, pkt, f);

    /* add sequence end code to have a real MPEG file */
    if (codec->id == AV_CODEC_ID_MPEG1VIDEO || codec->id == AV_CODEC_ID_MPEG2VIDEO)
        fwrite(endcode, 1, sizeof(endcode), f);
    fclose(f);

    avcodec_free_context(&c);
    av_frame_free(&frame);
    av_packet_free(&pkt);

    return 0;
}

2、运行结果

Send frame   0
Send frame   1
Write packet   0 (size= 6731)
Send frame   2
Write packet   2 (size= 3727)
Send frame   3
Write packet   1 (size= 1680)
Send frame   4
Write packet   4 (size= 2744)
Send frame   5
Write packet   3 (size= 1678)
Send frame   6
Write packet   6 (size= 2963)
Send frame   7
Write packet   5 (size= 1819)
Send frame   8
Write packet   8 (size= 3194)
Send frame   9
Write packet   7 (size= 1977)
Send frame  10
Write packet  10 (size=12306)
Send frame  11
Write packet   9 (size= 2231)
Send frame  12
Write packet  12 (size= 3762)
Send frame  13
Write packet  11 (size= 2039)
[mpeg1video @ 02562100] warning, clipping 1 dct coefficients to -255..255
....
[mpeg1video @ 02562100] warning, clipping 1 dct coefficients to -255..255
[mpeg1video @ 02562100] waSend frame  14
Write packet  14 (size= 3278)
Send frame  15
Write packet  13 (size= 1939)
Send frame  16
Write packet  16 (size= 3150)
Send frame  17
Write packet  15 (size= 1929)
Send frame  18
Write packet  18 (size= 3422)
Send frame  19
Write packet  17 (size= 2116)
Send frame  20
Write packet  20 (size=12236)
Send frame  21
Write packet  19 (size= 2055)
Send frame  22
Write packet  22 (size= 4054)
Send frame  23
Write packet  21 (size= 2048)
Send frame  24
Write packet  24 (size= 3191)
Write packet  23 (size= 1955)
rning, clipping 1 dct coefficients to -255..255
[mpeg1video @ 02562100] warning, clipping 1 dct coefficients to -255..255
....
[mpeg1video @ 02562100] warning, clipping 1 dct coefficients to -255..255

在 debug 目录下生成一个 test666.mp4，使用 MediaInfo 查看相关信息如下：

可以看到只有一路视频流。使用 VLC 播放可以看到 1s 的视频如下：

五、解码案例实战

下面代码使用实现了一个视频解码，将指定的视频 test.flv 解码为原始视频数据 yuv 和原始音频数据 pcm

1、示例源码

/**
 * @file
 * Demuxing and decoding example.
 *
 * Show how to use the libavformat and libavcodec API to demux and
 * decode audio and video data.
 * @example demuxing_decoding.c
 */

#include <libavutil/imgutils.h>
#include <libavutil/samplefmt.h>
#include <libavutil/timestamp.h>
#include <libavformat/avformat.h>

static AVFormatContext *fmt_ctx = NULL;
static AVCodecContext *video_dec_ctx = NULL, *audio_dec_ctx;
static int width, height;
static enum AVPixelFormat pix_fmt;
static AVStream *video_stream = NULL, *audio_stream = NULL;
static const char *src_filename = NULL;
static const char *video_dst_filename = NULL;
static const char *audio_dst_filename = NULL;
static FILE *video_dst_file = NULL;
static FILE *audio_dst_file = NULL;

static uint8_t *video_dst_data[4] = {
    
    NULL};
static int      video_dst_linesize[4];
static int video_dst_bufsize;

static int video_stream_idx = -1, audio_stream_idx = -1;
static AVFrame *frame = NULL;
static AVPacket pkt;
static int video_frame_count = 0;
static int audio_frame_count = 0;

static int output_video_frame(AVFrame *frame)
{
    
    
    if (frame->width != width || frame->height != height ||
        frame->format != pix_fmt) {
    
    
        /* To handle this change, one could call av_image_alloc again and
         * decode the following frames into another rawvideo file. */
        fprintf(stderr, "Error: Width, height and pixel format have to be "
                "constant in a rawvideo file, but the width, height or "
                "pixel format of the input video changed:\n"
                "old: width = %d, height = %d, format = %s\n"
                "new: width = %d, height = %d, format = %s\n",
                width, height, av_get_pix_fmt_name(pix_fmt),
                frame->width, frame->height,
                av_get_pix_fmt_name(frame->format));
        return -1;
    }

    printf("video_frame n:%d coded_n:%d\n",
           video_frame_count++, frame->coded_picture_number);

    /* copy decoded frame to destination buffer:
     * this is required since rawvideo expects non aligned data */
    av_image_copy(video_dst_data, video_dst_linesize,
                  (const uint8_t **)(frame->data), frame->linesize,
                  pix_fmt, width, height);

    /* write to rawvideo file */
    fwrite(video_dst_data[0], 1, video_dst_bufsize, video_dst_file);
    return 0;
}

static int output_audio_frame(AVFrame *frame)
{
    
    
    size_t unpadded_linesize = frame->nb_samples * av_get_bytes_per_sample(frame->format);
    printf("audio_frame n:%d nb_samples:%d pts:%s\n",
           audio_frame_count++, frame->nb_samples,
           av_ts2timestr(frame->pts, &audio_dec_ctx->time_base));

    /* Write the raw audio data samples of the first plane. This works
     * fine for packed formats (e.g. AV_SAMPLE_FMT_S16). However,
     * most audio decoders output planar audio, which uses a separate
     * plane of audio samples for each channel (e.g. AV_SAMPLE_FMT_S16P).
     * In other words, this code will write only the first audio channel
     * in these cases.
     * You should use libswresample or libavfilter to convert the frame
     * to packed data. */
    fwrite(frame->extended_data[0], 1, unpadded_linesize, audio_dst_file);

    return 0;
}

static int decode_packet(AVCodecContext *dec, const AVPacket *pkt)
{
    
    
    int ret = 0;

    // submit the packet to the decoder
    ret = avcodec_send_packet(dec, pkt);
    if (ret < 0) {
    
    
        fprintf(stderr, "Error submitting a packet for decoding (%s)\n", av_err2str(ret));
        return ret;
    }

    // get all the available frames from the decoder
    while (ret >= 0) {
    
    
        ret = avcodec_receive_frame(dec, frame);
        if (ret < 0) {
    
    
            // those two return values are special and mean there is no output
            // frame available, but there were no errors during decoding
            if (ret == AVERROR_EOF || ret == AVERROR(EAGAIN))
                return 0;

            fprintf(stderr, "Error during decoding (%s)\n", av_err2str(ret));
            return ret;
        }

        // write the frame data to output file
        if (dec->codec->type == AVMEDIA_TYPE_VIDEO)
            ret = output_video_frame(frame);
        else
            ret = output_audio_frame(frame);

        av_frame_unref(frame);
        if (ret < 0)
            return ret;
    }

    return 0;
}

static int open_codec_context(int *stream_idx,
                              AVCodecContext **dec_ctx, AVFormatContext *fmt_ctx, enum AVMediaType type)
{
    
    
    int ret, stream_index;
    AVStream *st;
    AVCodec *dec = NULL;
    AVDictionary *opts = NULL;

    ret = av_find_best_stream(fmt_ctx, type, -1, -1, NULL, 0);
    if (ret < 0) {
    
    
        fprintf(stderr, "Could not find %s stream in input file '%s'\n",
                av_get_media_type_string(type), src_filename);
        return ret;
    } else {
    
    
        stream_index = ret;
        st = fmt_ctx->streams[stream_index];

        /* find decoder for the stream */
        dec = avcodec_find_decoder(st->codecpar->codec_id);
        if (!dec) {
    
    
            fprintf(stderr, "Failed to find %s codec\n",
                    av_get_media_type_string(type));
            return AVERROR(EINVAL);
        }

        /* Allocate a codec context for the decoder */
        *dec_ctx = avcodec_alloc_context3(dec);
        if (!*dec_ctx) {
    
    
            fprintf(stderr, "Failed to allocate the %s codec context\n",
                    av_get_media_type_string(type));
            return AVERROR(ENOMEM);
        }

        /* Copy codec parameters from input stream to output codec context */
        if ((ret = avcodec_parameters_to_context(*dec_ctx, st->codecpar)) < 0) {
    
    
            fprintf(stderr, "Failed to copy %s codec parameters to decoder context\n",
                    av_get_media_type_string(type));
            return ret;
        }

        /* Init the decoders */
        if ((ret = avcodec_open2(*dec_ctx, dec, &opts)) < 0) {
    
    
            fprintf(stderr, "Failed to open %s codec\n",
                    av_get_media_type_string(type));
            return ret;
        }
        *stream_idx = stream_index;
    }

    return 0;
}

static int get_format_from_sample_fmt(const char **fmt,
                                      enum AVSampleFormat sample_fmt)
{
    
    
    int i;
    struct sample_fmt_entry {
    
    
        enum AVSampleFormat sample_fmt; const char *fmt_be, *fmt_le;
    } sample_fmt_entries[] = {
    
    
        {
    
     AV_SAMPLE_FMT_U8,  "u8",    "u8"    },
        {
    
     AV_SAMPLE_FMT_S16, "s16be", "s16le" },
        {
    
     AV_SAMPLE_FMT_S32, "s32be", "s32le" },
        {
    
     AV_SAMPLE_FMT_FLT, "f32be", "f32le" },
        {
    
     AV_SAMPLE_FMT_DBL, "f64be", "f64le" },
    };
    *fmt = NULL;

    for (i = 0; i < FF_ARRAY_ELEMS(sample_fmt_entries); i++) {
    
    
        struct sample_fmt_entry *entry = &sample_fmt_entries[i];
        if (sample_fmt == entry->sample_fmt) {
    
    
            *fmt = AV_NE(entry->fmt_be, entry->fmt_le);
            return 0;
        }
    }

    fprintf(stderr,
            "sample format %s is not supported as output format\n",
            av_get_sample_fmt_name(sample_fmt));
    return -1;
}

int main(int argc, char **argv)
{
    
    
    int ret = 0;

    src_filename = "./debug/test.flv";
    video_dst_filename = "./debug/test.yuv";
    audio_dst_filename = "./debug/test.pcm";

    /* open input file, and allocate format context */
    if (avformat_open_input(&fmt_ctx, src_filename, NULL, NULL) < 0) {
    
    
        fprintf(stderr, "Could not open source file %s\n", src_filename);
        exit(1);
    }

    /* retrieve stream information */
    if (avformat_find_stream_info(fmt_ctx, NULL) < 0) {
    
    
        fprintf(stderr, "Could not find stream information\n");
        exit(1);
    }

    if (open_codec_context(&video_stream_idx, &video_dec_ctx, fmt_ctx, AVMEDIA_TYPE_VIDEO) >= 0) {
    
    
        video_stream = fmt_ctx->streams[video_stream_idx];

        video_dst_file = fopen(video_dst_filename, "wb");
        if (!video_dst_file) {
    
    
            fprintf(stderr, "Could not open destination file %s\n", video_dst_filename);
            ret = 1;
            goto end;
        }

        /* allocate image where the decoded image will be put */
        width = video_dec_ctx->width;
        height = video_dec_ctx->height;
        pix_fmt = video_dec_ctx->pix_fmt;
        ret = av_image_alloc(video_dst_data, video_dst_linesize,
                             width, height, pix_fmt, 1);
        if (ret < 0) {
    
    
            fprintf(stderr, "Could not allocate raw video buffer\n");
            goto end;
        }
        video_dst_bufsize = ret;
    }

    if (open_codec_context(&audio_stream_idx, &audio_dec_ctx, fmt_ctx, AVMEDIA_TYPE_AUDIO) >= 0) {
    
    
        audio_stream = fmt_ctx->streams[audio_stream_idx];
        audio_dst_file = fopen(audio_dst_filename, "wb");
        if (!audio_dst_file) {
    
    
            fprintf(stderr, "Could not open destination file %s\n", audio_dst_filename);
            ret = 1;
            goto end;
        }
    }

    /* dump input information to stderr */
    av_dump_format(fmt_ctx, 0, src_filename, 0);

    if (!audio_stream && !video_stream) {
    
    
        fprintf(stderr, "Could not find audio or video stream in the input, aborting\n");
        ret = 1;
        goto end;
    }

    frame = av_frame_alloc();
    if (!frame) {
    
    
        fprintf(stderr, "Could not allocate frame\n");
        ret = AVERROR(ENOMEM);
        goto end;
    }

    /* initialize packet, set data to NULL, let the demuxer fill it */
    av_init_packet(&pkt);
    pkt.data = NULL;
    pkt.size = 0;

    if (video_stream)
        printf("Demuxing video from file '%s' into '%s'\n", src_filename, video_dst_filename);
    if (audio_stream)
        printf("Demuxing audio from file '%s' into '%s'\n", src_filename, audio_dst_filename);

    /* read frames from the file */
    while (av_read_frame(fmt_ctx, &pkt) >= 0) {
    
    
        // check if the packet belongs to a stream we are interested in, otherwise
        // skip it
        if (pkt.stream_index == video_stream_idx)
            ret = decode_packet(video_dec_ctx, &pkt);
        else if (pkt.stream_index == audio_stream_idx)
            ret = decode_packet(audio_dec_ctx, &pkt);
        av_packet_unref(&pkt);
        if (ret < 0)
            break;
    }

    /* flush the decoders */
    if (video_dec_ctx)
        decode_packet(video_dec_ctx, NULL);
    if (audio_dec_ctx)
        decode_packet(audio_dec_ctx, NULL);

    printf("Demuxing succeeded.\n");

    if (video_stream) {
    
    
        printf("Play the output video file with the command:\n"
               "ffplay -f rawvideo -pix_fmt %s -video_size %dx%d %s\n",
               av_get_pix_fmt_name(pix_fmt), width, height,
               video_dst_filename);
    }

    if (audio_stream) {
    
    
        enum AVSampleFormat sfmt = audio_dec_ctx->sample_fmt;
        int n_channels = audio_dec_ctx->channels;
        const char *fmt;

        if (av_sample_fmt_is_planar(sfmt)) {
    
    
            const char *packed = av_get_sample_fmt_name(sfmt);
            printf("Warning: the sample format the decoder produced is planar "
                   "(%s). This example will output the first channel only.\n",
                   packed ? packed : "?");
            sfmt = av_get_packed_sample_fmt(sfmt);
            n_channels = 1;
        }

        if ((ret = get_format_from_sample_fmt(&fmt, sfmt)) < 0)
            goto end;

        printf("Play the output audio file with the command:\n"
               "ffplay -f %s -ac %d -ar %d %s\n",
               fmt, n_channels, audio_dec_ctx->sample_rate,
               audio_dst_filename);
    }

end:
    avcodec_free_context(&video_dec_ctx);
    avcodec_free_context(&audio_dec_ctx);
    avformat_close_input(&fmt_ctx);
    if (video_dst_file)
        fclose(video_dst_file);
    if (audio_dst_file)
        fclose(audio_dst_file);
    av_frame_free(&frame);
    av_free(video_dst_data[0]);

    return ret < 0;
}

2、运行结果

Demuxing video from file './debug/test.flv' into './debug/test.yuv'
Demuxing audio from file './debug/test.flv' into './debug/test.pcm'
video_frame n:0 coded_n:0
audio_frame n:0 nb_samples:1024 pts:0
audio_frame n:1 nb_samples:1024 pts:0.0004375
video_frame n:1 coded_n:1
audio_frame n:2 nb_samples:1024 pts:0.000895833
audio_frame n:3 nb_samples:1024 pts:0.00133333
video_frame n:2 coded_n:2
audio_frame n:4 nb_samples:1024 pts:0.00177083
audio_frame n:5 nb_samples:1024 pts:0.00222917
....
video_frame n:2931 coded_n:2931
audio_frame n:5496 nb_samples:1024 pts:2.44267
audio_frame n:5497 nb_samples:1024 pts:2.4431
audio_frame n:5498 nb_samples:1024 pts:2.44356
Demuxing succeeded.
Play the output video file with the command:
ffplay -f rawvideo -pix_fmt yuv420p -video_size 1280x720 ./debug/test.yuv
Warning: the sample format the decoder produced is planar (fltp). This example will output the first channel only.
Play the output audio file with the command:
ffplay -f f32le -ac 1 -ar 48000 ./debug/test.pcm
Input #0, flv, from './debug/test.flv':
  Metadata:
    encoder         : Lavf58.45.100
  Duration: 00:01:57.31, start: 0.000000, bitrate: 1442 kb/s
    Stream #0:0: Video: h264 (Main), yuv420p(progressive), 1280x720 [SAR 1:1 DAR 16:9], 1048 kb/s, 25 fps, 25 tbr, 1k tbn, 50 tbc
    Stream #0:1: Audio: aac (LC), 48000 Hz, 5.1, fltp, 383 kb/s

在 debug 目录下生成了原始视频数据 test.yuv 以及原始音频数据 test.pcm

在 debug 目录下打开终端使用 ffplay 播放 test.yuv：

ffplay -f rawvideo -video_size 1280x720 test.yuv

此时可以看到原视频中的视频（无音频）。

在 debug 目录下打开终端使用 ffplay 播放 test.pcm：

ffplay -f f32le -ac 1 -ar 48000 test.pcm

此时可以听到原视频中的音频（无视频）。

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