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：
代码语言：javascript复制//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 filtersn");
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：
代码语言：javascript复制//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：
代码语言：javascript复制//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：
代码语言：javascript复制//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：
代码语言：javascript复制//io_data.cpp
五.销毁资源
代码语言：javascript复制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
代码语言：javascript复制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
