FFmpeg - avcodec_receive_frame returns AVERROR(EAG

问题:

I'm using an QOpenGL widget to draw frames. However, I'm struggling to get frames by using avcodec_receive_frame. It ended within the block else if (ret == AVERROR(EAGAIN)) and returned -11. I have no idea what made this happen. Also I checked that codecs were fine, so I guess the problem wasn't caused by codecs.

MyDemux.cpp

AVPacket* MyDemux::allocatePacket()
{
    AVPacket* packet = av_packet_alloc();
    return packet;
}

AVFrame* MyDemux::allocateFrame()
{
    AVFrame* frame = av_frame_alloc();
    return frame;
}

int MyDemux::readFrame(AVPacket* packet)
{
    mux.lock();
    if (!formatCtx) {
        std::cout << "formaetCtx is null" << std::endl;
        mux.unlock();
        return -1;
    }
    int ret = av_read_frame(formatCtx, packet);
    if (ret == AVERROR_EOF) {
        return -2;
    }
    if (ret != 0) {
        mux.unlock();
        av_packet_free(&packet);
        return -1;
    }
    media_type = packet->stream_index;
    mux.unlock();
    return 0;
}

MyDecode.cpp

void MyDecode::decode(AVFrame* frame, AVPacket* packet)
{
    int ret;
    ret = avcodec_send_packet(codecCtx, packet); // this returned 0
    while (ret == 0) {
        ret = avcodec_receive_frame(codecCtx, frame); // this returned -11
        if (ret == AVERROR(EINVAL)) {
            std::cout << "codec issue" << std::endl;
            av_frame_free(&frame);
            return;
        }
        else if (ret == AVERROR(EAGAIN)) { // program ends here
            std::cout << "output is not available this state" << std::endl;
            av_frame_free(&frame);
            return;
        }
        else if (ret == AVERROR(EINVAL)) {
            std::cout << "no more frames" << std::endl;
            av_frame_free(&frame);
            return;
        }
    }
}

main.cpp

class MyThread : public QThread
{
public:

    MyDemux demux;
    MyDecode video_decode;
    myDecode audio_decode;
    MyVideoWidget* videoWidget;
    AVPacket* packet;
    AVFrame* frame;

    void initThread()
    {
        char* url = "demo.mp4";
        demux.openFile(url);
        video_decode.openCodec(demux.copy_video_codec_par());
        audio_decode.openCodec(demux.copy_audio_codec_par());
        packet = demux.allocatePacket();
        frame = demux.allocateFrame();
    }
    void run()
    {
        while (demux.readFrame(packet) != -2) {
            if (demux.get_media_type() == 0) {
                video_decode.decode(frame, packet);
                videoWidget->paintFrame(frame);
            }
            else if (demux.get_media_type() == 1) {
            }
        }
        video_decode.decode(frame, nullptr);
        demux.clear();
        demux.close();
    }
};

回答1:

There is a delay between sending and receiving a frame. When receive returns EAGAIN, you should call send with the next encoded frame, then call receive again.

回答2:

When you call avcodec_receive_frame and get EAGAIN error that means your decoder does not get enough data to decode (e.x you send a B-frame in video). So each time you get that error you should ignore it and go to next avcodec_send_packet