FFmpeg的API使用篇（三）：提取纯音频/纯画面

本文将在上一篇文章FFmpeg的API使用篇（二）：转封装（不涉及转码）的基础上，拓展从视频中提取纯音频或者纯视频的功能。依旧不涉及转码操作，仅仅是将音频流提取出来，然后封装成对应的格式。比如从mp4格式的视频中提取音频，并保存为aac格式。或者提取纯视频转为mp4格式。

2、提取流程

提取音频流的过程和视频转封装的过程大部分都类似。只不过，我们需要的只是音频或者只是视频而已。因此，与转封装不同的是：首先，在复制编码参数（AVCodecParameters）时，我们只需要复制视频或者音频。而在使用AVPacket读取帧并写入容器时，我们也只需要音频流或视频流。

其中代码的关键部分在这里指出，看的时候可以多多关注。

//获取音频索引
    this->audio_stream_index = av_find_best_stream(ifmt_ctx_, AVMEDIA_TYPE_AUDIO, -1, -1, NULL, 0);

//逐帧读取音频
    while(av_read_frame(ifmt_ctx_, &packet) >=0 ){
        AVStream *in_stream, *out_stream;
        in_stream = ifmt_ctx_->streams[pkt->stream_index];
        out_stream = ofmt_ctx_->streams[pkt->stream_index];

         if(packet.stream_index == this->audio_stream_index){
             //时间基计算，音频pts和dts一致
             packet.pts = av_rescale_q_rnd(packet.pts, in_stream->time_base, out_stream->time_base,
                                           (AVRounding)(AV_ROUND_NEAR_INF|AV_ROUND_PASS_MINMAX));
             packet.dts = packet.pts;
             packet.duration = av_rescale_q(packet.duration, in_stream->time_base, out_stream->time_base);
             packet.pos = -1;
             packet.stream_index = 0;
             //将包写到输出媒体文件
             av_interleaved_write_frame(ofmt_ctx_, &packet);
             //减少引用计数，避免内存泄漏
             av_packet_unref(&packet);
         }
     }

提取纯音频/纯视频头文件（extractaudioorvideo.h）

// Copyright (c) 2021 LucasNan <[email protected]> <www.kevinnan.org.cn>

// 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.

#ifndef EXTRACTAUDIOORVIDEO_H
#define EXTRACTAUDIOORVIDEO_H

#include <iostream>
#include <string>

extern "C"{

    #include "libavformat/avformat.h"
    #include "libavcodec/avcodec.h"
}


class ExtractAudioORVideo
{
public:
    ExtractAudioORVideo();

    ~ExtractAudioORVideo();


    //@brief: 得到输入文件
    //@param: input_file: 输入文件
    //@ret  : void
    //@birth: created by LucasNan on 20210221
    void getInputFile(std::string input_file){
        this->input_file_ = input_file;
    }

    //@brief: 得到输出文件
    //@param: output_file: 输出文件
    //@ret  : void
    //@birth: created by LucasNan on 20210221
    void getOutputFile(std::string output_file){
        this->output_file_ = output_file;
    }

    //@brief: 提取纯音频流
    //@param: void
    //@ret  : void
    //@birth: created by LucasNan on 20210221
    void extractAudio();

    //@brief: 提取纯画面
    //@param: void
    //@ret  : void
    //@birth: created by LucasNan on 20210221
    void extractVideo();


private:
    //输出文件AVFormatContext
    AVFormatContext* ifmt_ctx_;
    //输出文件AVFormatContext
    AVFormatContext* ofmt_ctx_;

    std::string input_file_;
    std::string output_file_;

    int audio_stream_index;
    int video_stream_index;
};

#endif // EXTRACTAUDIOORVIDEO_H

类实现（extractaudioorvideo.cpp）

// Copyright (c) 2021 LucasNan <[email protected]> <www.kevinnan.org.cn>

// 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.

#include "extractaudioorvideo.h"

ExtractAudioORVideo::ExtractAudioORVideo(){

    ifmt_ctx_ = avformat_alloc_context();
    ofmt_ctx_ = avformat_alloc_context();
    audio_stream_index = -1;
    video_stream_index = -1;
}


ExtractAudioORVideo::~ExtractAudioORVideo(){

    avformat_free_context(ifmt_ctx_);
    avformat_free_context(ofmt_ctx_);
}


void ExtractAudioORVideo::extractAudio(){

    AVOutputFormat* ofmt;
    AVPacket packet;
    av_init_packet(&packet);
    packet.data = NULL;
    packet.size = 0;
    AVPacket *pkt = av_packet_alloc();

    int ret;
    //打开媒体文件
    if(avformat_open_input(&ifmt_ctx_, this->input_file_.c_str(), NULL, NULL) < 0){
        printf("file open error!");
        return;
    }
    //读取音视频流信息
    if(avformat_find_stream_info(ifmt_ctx_, 0) < 0){
        printf("stream info error!");
        return;
    }
    //打印视频信息
    av_dump_format(ifmt_ctx_, 0, input_file_.c_str(), 0);

    //给AVFormatContext分配动态内存
    avformat_alloc_output_context2(&ofmt_ctx_, NULL, NULL, output_file_.c_str());
    //ofmt = av_guess_format(NULL, output_file_.c_str(), NULL);
    if(!ofmt_ctx_){
        printf("can not create output");
        return;
    }

    //得到AVOutputFormat对象
    ofmt = ofmt_ctx_->oformat;
    //ofmt_ctx_->oformat = ofmt;

    //读取音频流
    for(unsigned int i = 0; i < ifmt_ctx_->nb_streams; i++){
        AVStream* out_stream = NULL;
        AVStream* in_stream = ifmt_ctx_->streams[i];
        AVCodecParameters* in_codepar = in_stream->codecpar;

        if(in_codepar->codec_type == AVMEDIA_TYPE_VIDEO ||
           in_codepar->codec_type == AVMEDIA_TYPE_SUBTITLE){
            continue;
        }

        //新建输出流
        out_stream = avformat_new_stream(ofmt_ctx_, NULL);
        if(!out_stream){
            printf("Failed allocat output stream");
            return;
        }

        //copy编码参数，上下文信息
        ret = avcodec_parameters_copy(out_stream->codecpar, in_codepar);
        if(ret < 0){
            printf("codec copy error!");
            return;
        }

        out_stream->codecpar->codec_tag = 0;
        break;
    }

    av_dump_format(ofmt_ctx_, 0, output_file_.c_str(), 1);

    //如果flags没有设置为AVFMT_NOFILE,则使用pb
    //处初始化AVIOContext，文件操作由他完成
    if(!(ofmt->flags & AVFMT_NOFILE)){
        printf("avio_open\n");
        ret = avio_open(&ofmt_ctx_->pb, output_file_.c_str(), AVIO_FLAG_WRITE);
        if(ret < 0){
            printf("can not open file");
            return;
        }
    }

    //获取音频索引
    this->audio_stream_index = av_find_best_stream(ifmt_ctx_, AVMEDIA_TYPE_AUDIO, -1, -1, NULL, 0);
    if(this->audio_stream_index < 0){
        printf("can not find audio stream!");
        return;
    }

    //写入媒体头文件
    ret = avformat_write_header(ofmt_ctx_, NULL);
    if(ret < 0){
        printf("can not write");
        return;
    }

    //逐帧读取音频
    while(av_read_frame(ifmt_ctx_, &packet) >=0 ){
        AVStream *in_stream, *out_stream;
        in_stream = ifmt_ctx_->streams[pkt->stream_index];
        out_stream = ofmt_ctx_->streams[pkt->stream_index];

         if(packet.stream_index == this->audio_stream_index){
             //时间基计算，音频pts和dts一致
             packet.pts = av_rescale_q_rnd(packet.pts, in_stream->time_base, out_stream->time_base,
                                           (AVRounding)(AV_ROUND_NEAR_INF|AV_ROUND_PASS_MINMAX));
             packet.dts = packet.pts;
             packet.duration = av_rescale_q(packet.duration, in_stream->time_base, out_stream->time_base);
             packet.pos = -1;
             packet.stream_index = 0;
             //将包写到输出媒体文件
             av_interleaved_write_frame(ofmt_ctx_, &packet);
             //减少引用计数，避免内存泄漏
             av_packet_unref(&packet);
         }
     }

    //写入尾部信息
    av_write_trailer(ofmt_ctx_);

    //释放AVPacket
    av_packet_free(&pkt);

    //打印输出文件信息
    av_dump_format(ofmt_ctx_, 0, output_file_.c_str(), 1);

    //关闭内存
    if (ofmt_ctx_ && !(ofmt->flags & AVFMT_NOFILE))
            avio_close(ofmt_ctx_->pb);
    //在析构函数中释放内存
    printf("over");
}


void ExtractAudioORVideo::extractVideo(){
    AVOutputFormat* ofmt;
    AVPacket packet;
    av_init_packet(&packet);
    packet.data = NULL;
    packet.size = 0;
    AVPacket *pkt = av_packet_alloc();

    int ret;
    //打开媒体文件
    if(avformat_open_input(&ifmt_ctx_, this->input_file_.c_str(), NULL, NULL) < 0){
        printf("file open error!");
        return;
    }
    //读取音视频流信息
    if(avformat_find_stream_info(ifmt_ctx_, 0) < 0){
        printf("stream info error!");
        return;
    }
    //打印视频信息
    av_dump_format(ifmt_ctx_, 0, input_file_.c_str(), 0);

    //给AVFormatContext分配动态内存
    avformat_alloc_output_context2(&ofmt_ctx_, NULL, NULL, output_file_.c_str());
    //ofmt = av_guess_format(NULL, output_file_.c_str(), NULL);
    if(!ofmt_ctx_){
        printf("can not create output");
        return;
    }

    //得到AVOutputFormat对象
    ofmt = ofmt_ctx_->oformat;
    //ofmt_ctx_->oformat = ofmt;

    //读取视频流
    for(unsigned int i = 0; i < ifmt_ctx_->nb_streams; i++){
        AVStream* out_stream = NULL;
        AVStream* in_stream = ifmt_ctx_->streams[i];
        AVCodecParameters* in_codepar = in_stream->codecpar;

        if(in_codepar->codec_type == AVMEDIA_TYPE_AUDIO ||
           in_codepar->codec_type == AVMEDIA_TYPE_SUBTITLE){
            continue;
        }

        //新建输出流        
        out_stream = avformat_new_stream(ofmt_ctx_, NULL);
        if(!out_stream){
            printf("Failed allocat output stream");
            return;
        }

        //copy编码参数，上下文信息
        ret = avcodec_parameters_copy(out_stream->codecpar, in_codepar);
        if(ret < 0){
            printf("codec copy error!");
            return;
        }

        out_stream->codecpar->codec_tag = 0;
        break;
    }

    av_dump_format(ofmt_ctx_, 0, output_file_.c_str(), 1);

    //如果flags没有设置为AVFMT_NOFILE,则使用pb
    //处初始化AVIOContext，文件操作由他完成
    if(!(ofmt->flags & AVFMT_NOFILE)){
        ret = avio_open(&ofmt_ctx_->pb, output_file_.c_str(), AVIO_FLAG_WRITE);
        if(ret < 0){
            printf("can not open file");
            return;
        }
    }

    video_stream_index = av_find_best_stream(ifmt_ctx_, AVMEDIA_TYPE_VIDEO, -1, -1, NULL, 0);
    if(video_stream_index < 0){
        printf("can not find audio stream!");
        return;
    }

    //写入媒体头文件
    ret = avformat_write_header(ofmt_ctx_, NULL);
    if(ret < 0){
        printf("can not write");
        return;
    }


    while(av_read_frame(ifmt_ctx_, &packet) >=0 ){
        AVStream *in_stream, *out_stream;
        in_stream = ifmt_ctx_->streams[pkt->stream_index];
        out_stream = ofmt_ctx_->streams[pkt->stream_index];

         if(packet.stream_index == this->video_stream_index){
             //时间基计算，音频pts和dts一致
             packet.pts = av_rescale_q_rnd(packet.pts, in_stream->time_base, out_stream->time_base,
                                           (AVRounding)(AV_ROUND_NEAR_INF|AV_ROUND_PASS_MINMAX));
             packet.dts = packet.pts;
             packet.duration = av_rescale_q(packet.duration, in_stream->time_base, out_stream->time_base);
             packet.pos = -1;
             packet.stream_index = 0;
             //将包写到输出媒体文件
             av_interleaved_write_frame(ofmt_ctx_, &packet);
             //减少引用计数，避免内存泄漏
             av_packet_unref(&packet);
         }
     }

    //写入尾部信息
    av_write_trailer(ofmt_ctx_);

    //释放AVPacket
    av_packet_free(&pkt);

    //打印输出文件信息
    av_dump_format(ofmt_ctx_, 0, output_file_.c_str(), 1);

    //关闭内存
    if (ofmt_ctx_ && !(ofmt->flags & AVFMT_NOFILE))
            avio_close(ofmt_ctx_->pb);
    //在析构函数中释放内存
    printf("over");
}

main.cpp

// Copyright (c) 2021 LucasNan <[email protected]> <www.kevinnan.org.cn>

// 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.


#include <QCoreApplication>

#include <iostream>

#include "extractaudioorvideo.h"


int main(int argc, char *argv[])
{
    QCoreApplication a(argc, argv);  

    const char* file = "F:/google/durant.mp4";

    ExtractAudioORVideo *ext = new ExtractAudioORVideo();
    ext->getInputFile(file);
    ext->getOutputFile("./output_audio.aac");
    ext->extractAudio();
    //ext->extractVideo();
    delete ext;
    
    return a.exec();
}

2、提取流程

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