//解码函数

int attribute_align_arg avcodec_decode_video2(AVCodecContext *avctx, AVFrame *picture,

                                              int *got_picture_ptr,
                                              const AVPacket *avpkt)
{
    AVCodecInternal *avci = avctx->internal;
    int ret;
    // copy to ensure we do not change avpkt
    AVPacket tmp = *avpkt;

    if (avctx->codec->type != AVMEDIA_TYPE_VIDEO) {    //如果不是视频类型，则退出
        av_log(avctx, AV_LOG_ERROR, "Invalid media type for video\n");
        return AVERROR(EINVAL);
    }

    *got_picture_ptr = 0;  /
    if ((avctx->coded_width || avctx->coded_height) && av_image_check_size(avctx->coded_width, avctx->coded_height, 0, avctx))
        return AVERROR(EINVAL);

    avcodec_get_frame_defaults(picture);

    if (!avctx->refcounted_frames)
        av_frame_unref(&avci->to_free);

//如果是帧并行解码的话

//需要在codec初始化的时候设置codec->capabilities |= CODEC_CAP_DELAY，查看注释，设置avctx->active_thread_type |= FF_THREAD_FRAME

    if ((avctx->codec->capabilities & CODEC_CAP_DELAY) || avpkt->size || (avctx->active_thread_type & FF_THREAD_FRAME)) {
        int did_split = av_packet_split_side_data(&tmp);
        apply_param_change(avctx, &tmp);

        avctx->pkt = &tmp;

//需要配置configure  --enable-pthreads(我现在最新的默认生成的config.h就是#define  HAVE_THREADS 1)  ,设置avctx->active_thread_type |= FF_THREAD_FRAME

        if (HAVE_THREADS && avctx->active_thread_type & FF_THREAD_FRAME)
            ret = ff_thread_decode_frame(avctx, picture, got_picture_ptr,&tmp);/ /进入帧级并行解码
        else {//进入单线程解码
            ret = avctx->codec->decode(avctx, picture, got_picture_ptr, &tmp);
            picture->pkt_dts = avpkt->dts;

            if(!avctx->has_b_frames){
                av_frame_set_pkt_pos(picture, avpkt->pos);
            }
            //FIXME these should be under if(!avctx->has_b_frames)
            /* get_buffer is supposed to set frame parameters */
            if (!(avctx->codec->capabilities & CODEC_CAP_DR1)) {
                if (!picture->sample_aspect_ratio.num)    picture->sample_aspect_ratio = avctx->sample_aspect_ratio;
                if (!picture->width)                      picture->width               = avctx->width;
                if (!picture->height)                     picture->height              = avctx->height;
                if (picture->format == AV_PIX_FMT_NONE)   picture->format              = avctx->pix_fmt;
            }
        }
        add_metadata_from_side_data(avctx, picture);

        emms_c(); //needed to avoid an emms_c() call before every return;

        avctx->pkt = NULL;
        if (did_split) {
            ff_packet_free_side_data(&tmp);
            if(ret == tmp.size)
                ret = avpkt->size;
        }

        if (ret < 0 && picture->data[0])
            av_frame_unref(picture);

        if (*got_picture_ptr) {
            if (!avctx->refcounted_frames) {
                avci->to_free = *picture;
                avci->to_free.extended_data = avci->to_free.data;
            }

            avctx->frame_number++;
            av_frame_set_best_effort_timestamp(picture,
                                               guess_correct_pts(avctx,
                                                                 picture->pkt_pts,
                                                                 picture->pkt_dts));
        }
    } else
        ret = 0;

    /* many decoders assign whole AVFrames, thus overwriting extended_data;

     * make sure it's set correctly */

//许多解码器分配整个的AVFrames ，从而覆盖extended_data相关

    picture->extended_data = picture->data;

    return ret;
}

 

注释：

/**
 * Encoder or decoder requires flushing with NULL input at the end in order to
 * give the complete and correct output.
 *
 * NOTE: If this flag is not set, the codec is guaranteed to never be fed with
 *       with NULL data. The user can still send NULL data to the public encode
 *       or decode function, but libavcodec will not pass it along to the codec
 *       unless this flag is set.
 *
 * Decoders:
 * The decoder has a non-zero delay and needs to be fed with avpkt->data=NULL,
 * avpkt->size=0 at the end to get the delayed data until the decoder no longer
 * returns frames.
 *
 * Encoders:
 * The encoder needs to be fed with NULL data at the end of encoding until the
 * encoder no longer returns data.
 *
 * NOTE: For encoders implementing the AVCodec.encode2() function, setting this
 *       flag also means that the encoder must set the pts and duration for
 *       each output packet. If this flag is not set, the pts and duration will
 *       be determined by libavcodec from the input frame.
 */

#define CODEC_CAP_DELAY           0x0020