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• FFMPEG documentation example for chromakey doesn't work for green background png file

  15 septembre 2021, par JPM

  Im new in FFMPEG and I try to make just simple green background chroma keying of png images.
In FFMPEG documentation in 38.16 Chromakey - there is an example script :

  



  ffmpeg -i input.png -vf chromakey=green out.png

  



  for which is said that "Make every green pixel in the input image transparent"
That exactly what i wanted.
I tried with many different png images but the result is always a copy of input image.
Also i replaced the word "green" to 0x00FF00 or 0x008000 and still no successes. Only when replacing green to black and then the result is as expected - transparent.
Im working on Windows with pre-build binaries of FFMpeg.
What am I missing ?
Thanks.

  


• Audiopulse record multiple xfbv screens with python

  7 octobre 2020, par Eric Lagarda

  I’m trying to do something complicated and I need some help.

  


  I’m able to record a session (video and audio) with python using xvfb and ffmpeg.

  


  I just want to visit a page using selenium and record the video and audio. It’s working but when I try to run multiple instances, the audio is mixing between records.

  


  How can achieve that ? I’m using pulseaudio. I know that with also you can set like a interface per instance, but I don’t know how to do it.

  


  Thanks for you support and comments.

  


• When decoding video with ffmpeg and playing the decoded audio data with SDL2, it's all noise [closed]

  18 février, par jodan feng

  it is the code in the main thread

  


  int main(int argc, char *argv[])
{
    QApplication a(argc, argv);
    Ffplay f;
    f.start();
    return a.exec();
}


  


  Ffplay is the thread that decode the video and play it

  


  class Ffplay : public QThread
{
   Q_OBJECT
public:
    Ffplay();
    protected:
      void run() override;
public:
    AVFormatContext* format2;
    Thread2 t2;
};
Ffplay::Ffplay() {}

void Ffplay::run(){
    format2=avformat_alloc_context();
    avformat_open_input(&format2,"c:/Qt/project/player/movie.mp4",NULL,NULL);
    avformat_find_stream_info(format2,NULL);
    t2.format=format2;
    t2.start();
}


  


  Thread2 is the thread that decoed the audio part and play it

  


  class Thread2 : public QThread
{
    Q_OBJECT
public:
    Thread2();
protected:
    void run() override;
public:
    AVFormatContext* format;
    AVCodecContext* codec;
    const AVCodec* c;
    AVPacket* packet;
    AVFrame* frame;
    SwrContext* s;
    FILE* f;
    FILE* f2;

};
Thread2::Thread2() {}

struct Sound
{
    Uint8* data;
    Uint32 position=0;
    Uint32 len;
};

Sound* d=new Sound;

void write(void* userdata,Uint8* stream,int len){
    SDL_memset(stream,0,len);
    int size2=std::min((Uint32)len,d->len-d->position);
    SDL_memcpy(stream,d->data+d->position,size2);
    d->position=d->position+size2;
}


void Thread2::run(){
    int index=-1;
    for(unsigned int i=0;inb_streams;i++){
        if(format->streams[i]->codecpar->codec_type==AVMEDIA_TYPE_AUDIO){
            index=i;
            break;
        }
    }
    c=avcodec_find_decoder(format->streams[index]->codecpar->codec_id);
    codec=avcodec_alloc_context3(c);
    avcodec_parameters_to_context(codec,format->streams[index]->codecpar);
    avcodec_open2(codec,c,NULL);
    packet=av_packet_alloc();
    frame=av_frame_alloc();
    s=swr_alloc();
    swr_alloc_set_opts2(&s,&codec->ch_layout,AV_SAMPLE_FMT_S16,codec->sample_rate,&codec->ch_layout,codec->sample_fmt,codec->sample_rate,0,NULL);
    swr_init(s);
    f=fopen("yinpin","wb");
    uint8_t* buff=(uint8_t*)av_malloc(2*1024*4);  
    while(av_read_frame(format,packet)==0){
        if(packet->stream_index==1){
            avcodec_send_packet(codec,packet);
            while(avcodec_receive_frame(codec,frame)>=0){
                swr_convert(s,&buff,2*1024*2*2,(const uint8_t **)frame->data,frame->nb_samples);
                int size=av_samples_get_buffer_size(NULL,2,frame->nb_samples,AV_SAMPLE_FMT_S16,1);
                fwrite(buff,1,size,f);
            }
        }
    }


    SDL_Init(SDL_INIT_AUDIO);
    SDL_AudioSpec spec;
    spec.freq=48000;
    spec.format=AUDIO_S16;
    spec.channels=2;
    spec.samples=1024;
    spec.callback=write;
    SDL_OpenAudio(&spec,NULL);
    SDL_PauseAudio(0);
    f2=fopen("yinpin","rb");
    uint8_t* buff2=(uint8_t*)av_malloc(2*2*1024*2);
    size_t size=10;
    while(size>0){
        size=fread(buff2,1,2*2*1024*2,f2);
        d->data=buff2;
        d->len=(Uint32)size;
    }
}


  


  After the program is run, the sound played is all noise, which makes me a little confused because I don't know whether it is a decoding problem or a problem with SDL2 audio playback. Is there any troubleshooting method ? Has anyone figured out the source of the problem ?