如何使用libavfilter库给pcm音频采样数据添加音频滤镜？

一.初始化音频滤镜

　　初始化音频滤镜的方法基本上和初始化视频滤镜的方法相同，不懂的可以看上篇博客，这里直接给出代码：

//audio_filter_core.cpp
#define INPUT_SAMPLERATE 44100
#define INPUT_FORMAT AV_SAMPLE_FMT_FLTP
#define INPUT_CHANNEL_LAYOUT AV_CH_LAYOUT_STEREO
static AVFilterGraph *filter_graph;
static AVFilterContext *abuffersrc_ctx;
static AVFilterContext *volume_ctx;
static AVFilterContext *aformat_ctx;
static AVFilterContext *abuffersink_ctx;
static AVFrame *input_frame= nullptr,*output_frame= nullptr;
int32_t init_audio_filter(const char *volume_factor){
    int32_t result=0;
    char ch_layout[64];
    char options_str[1024];
    AVDictionary *options_dict= nullptr;
    //创建滤镜图
    filter_graph=avfilter_graph_alloc();
    if(!filter_graph){
        cerr<<"Error:Unable to create filter graph."<<endl;
        return -1;
    }
    //创建abuffer滤镜
    const AVFilter *abuffer= avfilter_get_by_name("abuffer");
    if(!abuffer){
        cerr<<"Error:Could not find abuffer filter."<<endl;
        return -1;
    }
    abuffersrc_ctx= avfilter_graph_alloc_filter(filter_graph,abuffer,"src");
    if(!abuffersrc_ctx){
        cerr<<"Error:could not allocate the abuffer instance."<<endl;
        return -1;
    }
    av_get_channel_layout_string(ch_layout,sizeof(ch_layout),0,INPUT_CHANNEL_LAYOUT);
    av_opt_set(abuffersrc_ctx,"channel_layout",ch_layout,AV_OPT_SEARCH_CHILDREN);
    av_opt_set(abuffersrc_ctx,"sample_fmt",av_get_sample_fmt_name(INPUT_FORMAT),AV_OPT_SEARCH_CHILDREN);
    av_opt_set_q(abuffersrc_ctx,"time_base",(AVRational){1,INPUT_SAMPLERATE},AV_OPT_SEARCH_CHILDREN);
    av_opt_set_int(abuffersrc_ctx,"sample_rate",INPUT_SAMPLERATE,AV_OPT_SEARCH_CHILDREN);
    result= avfilter_init_str(abuffersrc_ctx, nullptr);
    if(result<0){
        cerr<<"Error:could not initialize the abuffer filter."<<endl;
        return -1;
    }
    //创建volume滤镜
    const AVFilter *volume= avfilter_get_by_name("volume");
    if(!volume){
        cerr<<"Error:could not find volume filter."<<endl;
        return -1;
    }
    volume_ctx= avfilter_graph_alloc_filter(filter_graph,volume,"volume");
    if(!volume_ctx){
        cerr<<"Error:could not allocate volume filter instance."<<endl;
        return -1;
    }
    av_dict_set(&options_dict,"volume",volume_factor,0);
    result= avfilter_init_dict(volume_ctx,&options_dict);
    av_dict_free(&options_dict);
    if(result<0){
        cerr<<"Error:could not initialize volume filter instance."<<endl;
        return -1;
    }
    //创建aformat滤镜
    const AVFilter *aformat=avfilter_get_by_name("aformat");
    if(!aformat){
        cerr<<"Error:could not find aformat filter."<<endl;
        return -1;
    }
    aformat_ctx= avfilter_graph_alloc_filter(filter_graph,aformat,"aformat");
    if(!aformat_ctx){
        cerr<<"Error:could not allocate aformat filter instance."<<endl;
        return -1;
    }
    snprintf(options_str,sizeof(options_str),"sample_fmts=%s:sample_rates=%d:channel_layouts=stereo",av_get_sample_fmt_name(AV_SAMPLE_FMT_S16),44100);
    result= avfilter_init_str(aformat_ctx,options_str);
    if(result<0){
        cerr<<"Error:could not initialize aformat filter."<<endl;
        return -1;
    }
    //创建abuffersink滤镜
    const AVFilter *abuffersink= avfilter_get_by_name("abuffersink");
    if(!abuffersink){
        cerr<<"Error:could not find abuffersink filter."<<endl;
        return -1;
    }
    abuffersink_ctx= avfilter_graph_alloc_filter(filter_graph,abuffersink,"sink");
    if(!abuffersink_ctx){
        cerr<<"Error:could not allocate abuffersink filter instance."<<endl;
        return -1;
    }
    result= avfilter_init_str(abuffersink_ctx, nullptr);
    if(result<0){
        cerr<<"Error:could not initialize abuffersink filter."<<endl;
        return -1;
    }
    //连接创建好的滤镜
    result=avfilter_link(abuffersrc_ctx,0,volume_ctx,0);
    if(result>=0){
        result=avfilter_link(volume_ctx,0,aformat_ctx,0);
    }
    if(result>=0){
        result=avfilter_link(aformat_ctx,0,abuffersink_ctx,0);
    }
    if(result<0){
        fprintf(stderr,"Error connecting filters\n");
        return -1;
    }
    //配置滤镜图
    result=avfilter_graph_config(filter_graph, nullptr);
    if(result<0){
        cerr<<"Error:Error configuring the filter graph."<<endl;
        return -1;
    }
    //创建输入输出帧
    input_frame=av_frame_alloc();
    output_frame=av_frame_alloc();
    if(!input_frame||!output_frame){
        cerr<<"Error:frame allocation failed."<<endl;
        return -1;
    }
    return 0;
}

二.初始化输入音频帧

　　在这一步需要给输入音频帧设置一些参数，包括采样率，采样点个数，声道布局，音频帧格式等，然后就可以给音频帧分配内存空间了。代码如下：

//audio_filter_core.cpp
static int32_t init_frames(){
    int result=0;
    input_frame->sample_rate=44100;
    input_frame->format=AV_SAMPLE_FMT_FLTP;
    input_frame->channel_layout=AV_CH_LAYOUT_STEREO;
    input_frame->nb_samples=1024;
    result= av_frame_get_buffer(input_frame,0);
    if(result<0){
        cerr<<"Error:av_frame_get_buffer failed."<<endl;
        return -1;
    }
    result= av_frame_make_writable(input_frame);
    if(result<0){
        cerr<<"Error:av_frame_make_writable failed."<<endl;
        return -1;
    }
    return 0;
}

三.循环编辑音频帧

　　在这一步需要注意的是，每次将输入音频帧放入滤镜图前，都要做一次初始化音频帧操作，否则会报错：filter context - fmt: fltp r: 44100 layout: 3 ch: 2, incoming frame - fmt: (null) r: 0 layout: 3 ch: 2 pts_time: NOPTS【Changing audio frame properties on the fly is not supported.】。注意一定是每次，不要只初始化一次，这样只有第一帧初始化了，后面的帧还是会报错，因为输入帧的格式要和滤镜上下文保持一致，如果没有每次都初始化，后面的帧的格式和采样率就识别不到，为null了。下面给出代码：

//audio_filter_core.cpp
static int32_t filter_frame(){
    int32_t result= av_buffersrc_add_frame(abuffersrc_ctx,input_frame);
    if(result<0){
        cerr<<"Error:add frame to buffersrc failed."<<endl;
        return -1;
    }
    while(1){
        result=av_buffersink_get_frame(abuffersink_ctx,output_frame);
        if(result==AVERROR(EAGAIN)||result==AVERROR_EOF){
            return 1;
        }
        else if(result<0){
            cerr<<"Error:av_buffersink_get_frame failed."<<endl;
            return -1;
        }
        cout<<"Output channels:"<<output_frame->channels<<",nb_samples:"<<output_frame->nb_samples<<",sample_fmt:"<<output_frame->format<<endl;
        write_samples_to_pcm2(output_frame);
        av_frame_unref(output_frame);
    }
    return 0;
}
int32_t audio_filtering(){
    int32_t result=0;
    while(!end_of_input_file()){
        init_frames();//每次都要执行
        result=read_pcm_to_frame2(input_frame,INPUT_FORMAT,2);
        if(result<0){
            cerr<<"Error:read_pcm_to_frame2 failed."<<endl;
            return -1;
        }
        result=filter_frame();
        if(result<0){
            cerr<<"Error:filter_frame failed."<<endl;
            return -1;
        }
    }
    return 0;
}

四.将编辑后的数据写入输出文件

　　在这一步需要注意的是，由于在滤镜图中有一个滤镜实例将音频帧的采样格式设置为了AV_SAMPLE_FMT_S16，这是packed格式的帧，左右声道的数据交错存储在frame->data[0]指向的内存单元中，所以在写入的时候，需要注意这一点。下面给出代码：

//io_data.cpp
int32_t write_samples_to_pcm2(AVFrame* frame){
    int16_t* samples = reinterpret_cast<int16_t*>(frame->data[0]);
    int dataSize = frame->nb_samples * frame->channels * sizeof(int16_t);
    fwrite(samples, 1, dataSize, output_file);
    return 0;
}

　　数据读入代码：

//io_data.cpp

static FILE* input_file= nullptr;
static FILE* output_file= nullptr;

int32_t read_pcm_to_frame2(AVFrame *frame,enum AVSampleFormat sample_fmt,int channels){
    int data_size= av_get_bytes_per_sample(sample_fmt);
    if(data_size<0){
        cerr<<"Error:Failed to calculate data size."<<endl;
        return -1;
    }
    for(int i=0;i<frame->nb_samples;i++){
        for(int ch=0;ch<channels;ch++){
            fread(frame->data[ch]+i*data_size,1,data_size,input_file);
        }
    }
    return 0;
}
int32_t open_input_output_files(const char* input_name,const char* output_name){
    if(strlen(input_name)==0||strlen(output_name)==0){
        cout<<"Error:empty input or output file name."<<endl;
        return -1;
    }
    close_input_output_files();
    input_file=fopen(input_name,"rb");//rb：读取一个二进制文件，该文件必须存在
    if(input_file==nullptr){
        cerr<<"Error:failed to open input file."<<endl;
        return -1;
    }
    output_file=fopen(output_name,"wb");//wb:打开或新建一个二进制文件，只允许写
    if(output_file== nullptr){
        cout<<"Error:failed to open output file."<<endl;
        return -1;
    }
    return 0;
}
void close_input_output_files(){
    if(input_file!= nullptr){
        fclose(input_file);
        input_file= nullptr;
    }
    if(output_file!= nullptr){
        fclose(output_file);
        output_file= nullptr;
    }
}
int32_t end_of_input_file(){
    return feof(input_file);
}

五.销毁资源

audio_filter_core.cpp
static void free_frames(){
    av_frame_free(&input_frame);
    av_frame_free(&output_frame);
}
void destroy_audio_filter(){
    free_frames();
    avfilter_graph_free(&filter_graph);
}

六.main函数实现

int main(){
    const char *input_file_name="../input.pcm";
    const char *output_file_name="../output.pcm";
    const char *volume_factor="0.9";
    int32_t result=0;
    result= open_input_output_files(input_file_name,output_file_name);
    if(result<0){
        return -1;
    }
    result= init_audio_filter(volume_factor);
    if(result<0){
        return -1;
    }
    result=audio_filtering();
    if(result<0){
        return -1;
    }
    destroy_audio_filter();
    close_input_output_files();
    return 0;
}

　　最后，可以使用下面的指令测试输出的pcm文件：

　　ffplay -ac 2 -ar 44100 -f s16le -i output.pcm