avcodec_decode_video2()

ffmpeg中的avcodec_decode_video2()的作用：是解码一帧视频数据。输入一个压缩编码的结构体AVPacket，输出一个解码后的结构体AVFrame。

该函数的声明位于libavcodec\avcodec.h

avcodec_decode_video2()主要做了以下几个方面的工作：

• 对输入的字段进行了一系列的检查工作：例如宽高是否正确，输入是否为视频等等。
• 通过ret = avctx->codec->decode(avctx, picture, got_picture_ptr,&tmp)这句代码，调用了相应AVCodec的decode()函数，完成了解码操作。
• 对得到的AVFrame的一些字段进行了赋值，例如宽高、像素格式等等。

其中第二部是关键的一步，它调用了AVCodec的decode()方法完成了解码。

AVCodec的decode()方法是一个函数指针，指向了具体解码器的解码函数。

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)
        return AVERROR(EINVAL);
    if (avctx->codec->type != AVMEDIA_TYPE_VIDEO) {
        av_log(avctx, AV_LOG_ERROR, "Invalid media type for video\n");
        return AVERROR(EINVAL);
    }

    if (!avctx->codec->decode) {
        av_log(avctx, AV_LOG_ERROR, "This decoder requires using the avcodec_send_packet() API.\n");
        return AVERROR(ENOSYS);
    }

    *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);

    avctx->internal->pkt = avpkt;
    ret = apply_param_change(avctx, avpkt);
    if (ret < 0)
        return ret;

    av_frame_unref(picture);

    if ((avctx->codec->capabilities & AV_CODEC_CAP_DELAY) || avpkt->size ||
        (avctx->active_thread_type & FF_THREAD_FRAME)) {
        int did_split = av_packet_split_side_data(&tmp);
        ret = apply_param_change(avctx, &tmp);
        if (ret < 0)
            goto fail;

        avctx->internal->pkt = &tmp;
        if (HAVE_THREADS && avctx->active_thread_type & FF_THREAD_FRAME)
            ret = ff_thread_decode_frame(avctx, picture, got_picture_ptr,
                                         &tmp);
        else {
            ret = avctx->codec->decode(avctx, picture, got_picture_ptr,
                                       &tmp);
            if (!(avctx->codec->caps_internal & FF_CODEC_CAP_SETS_PKT_DTS))
                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 & AV_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;
            }
        }

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

        avctx->internal->pkt = NULL;
        if (did_split) {
            av_packet_free_side_data(&tmp);
            if(ret == tmp.size)
                ret = avpkt->size;
        }
        if (picture->flags & AV_FRAME_FLAG_DISCARD) {
            *got_picture_ptr = 0;
        }
        if (*got_picture_ptr) {
            if (!avctx->refcounted_frames) {
                int err = unrefcount_frame(avci, picture);
                if (err < 0)
                    return err;
            }

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

    /* many decoders assign whole AVFrames, thus overwriting extended_data;
     * make sure it's set correctly */
    av_assert0(!picture->extended_data || picture->extended_data == picture->data);

#if FF_API_AVCTX_TIMEBASE
    if (avctx->framerate.num > 0 && avctx->framerate.den > 0)
        avctx->time_base = av_inv_q(av_mul_q(avctx->framerate, (AVRational){avctx->ticks_per_frame, 1}));
#endif

    return ret;
}