百倍变速--解码到底能不能丢 非参考帧 ？FFmpeg 有话说！！！

标签: FFmpeg  

forbidden_zero_bit(1bit) + nal_ref_idc(2bit) + nal_unit_type(5bit)

/* this thread gets the stream from the disk or the network */
static int read_thread(void *arg)
{
    // 省略部分代码
    for (i = 0; i < ic->nb_streams; i++) {
        AVStream *st = ic->streams[i];
        enum AVMediaType type = st->codecpar->codec_type;
        // AVDISCARD_ALL  抛弃所有的帧
        st->discard = AVDISCARD_ALL;
        if (type >= 0 && wanted_stream_spec[type] && st_index[type] == -1)
            if (avformat_match_stream_specifier(ic, st, wanted_stream_spec[type]) > 0)
                st_index[type] = i;
    }
    
    // 省略部分代码
    if (!video_disable)
        st_index[AVMEDIA_TYPE_VIDEO] =
            av_find_best_stream(ic, AVMEDIA_TYPE_VIDEO,
                                st_index[AVMEDIA_TYPE_VIDEO], -1, NULL, 0);
    if (!audio_disable)
        st_index[AVMEDIA_TYPE_AUDIO] =
            av_find_best_stream(ic, AVMEDIA_TYPE_AUDIO,
                                st_index[AVMEDIA_TYPE_AUDIO],
                                st_index[AVMEDIA_TYPE_VIDEO],
                                NULL, 0);

    /* open the streams */
    if (st_index[AVMEDIA_TYPE_AUDIO] >= 0) {
        // 开启解码线程
        stream_component_open(is, st_index[AVMEDIA_TYPE_AUDIO]);
    }
    ret = -1;
    if (st_index[AVMEDIA_TYPE_VIDEO] >= 0) {
       // 开启解码线程
        ret = stream_component_open(is, st_index[AVMEDIA_TYPE_VIDEO]);
    }
    // 省略代码
}

/* open a given stream. Return 0 if OK */
static int stream_component_open(VideoState *is, int stream_index)
{
    // 省略部分代码
    // AVDISCARD_DEFAULT 默认模式，过滤无效数据
    ic->streams[stream_index]->discard = AVDISCARD_DEFAULT;
    switch (avctx->codec_type) {
    case AVMEDIA_TYPE_AUDIO:
     // 省略部分代码
        if ((ret = decoder_start(&is->auddec, audio_thread, "audio_decoder", is)) < 0)
            goto out;
    // 省略部分代码
    case AVMEDIA_TYPE_VIDEO:
     // 省略部分代码
        if ((ret = decoder_start(&is->viddec, video_thread, "video_decoder", is)) < 0)
            goto out;
    // 省略部分代码
}

/**
 * @ingroup lavc_decoding
 */
enum AVDiscard{
    /* We leave some space between them for extensions (drop some
     * keyframes for intra-only or drop just some bidir frames). */
     // 不抛弃，不放弃任何数据
    AVDISCARD_NONE    =-16, ///< discard nothing
    //  丢掉无用的数据，比如 size 为 0 这种
    AVDISCARD_DEFAULT =  0, ///< discard useless packets like 0 size packets in avi
    // 丢掉所有的非参考帧
    AVDISCARD_NONREF  =  8, ///< discard all non reference
    // 丢掉所有的双向帧
    AVDISCARD_BIDIR   = 16, ///< discard all bidirectional frames
    // 丢掉所有的非内帧
    AVDISCARD_NONINTRA= 24, ///< discard all non intra frames
    // 丢掉所有的非关键帧
    AVDISCARD_NONKEY  = 32, ///< discard all frames except keyframes
    // 丢掉所有帧
    AVDISCARD_ALL     = 48, ///< discard all
};

/**
 * Internally, this call will copy relevant AVCodecContext fields, which can
 * influence decoding per-packet, and apply them when the packet is actually
 * decoded. (For example AVCodecContext.skip_frame, which might direct the
 * decoder to drop the frame contained by the packet sent with this function.)
 *  省略部分注释
 */
int avcodec_send_packet(AVCodecContext *avctx, const AVPacket *avpkt);