FFMPeg代码分析：avcodec_decode_video2函数

该函数的作用是实现压缩视频的解码。在avcodec.h中的声明方式如下：

int avcodec_decode_video2(AVCodecContext *avctx, AVFrame *picture, int *got_picture_ptr, const AVPacket *avpkt);

待解码的数据保存在avpkt->data中，大小为avpkt->size；解码完成后，picture用于保存输出图像数据。

该方法的各个参数：

AVCodecContext *avctx：编解码上下文环境，定义了编解码操作的一些细节；

AVFrame *picture：输出参数；传递到该方法的对象本身必须在外部由av_frame_alloc()分配空间，而实际解码过后的数据储存区将由AVCodecContext.get_buffer2()分配；AVCodecContext.refcounted_frames表示该frame的引用计数，当这个值为1时，表示有另外一帧将该帧用作参考帧，而且参考帧返回给调用者；当参考完成时，调用者需要调用av_frame_unref()方法解除对该帧的参考；av_frame_is_writable()可以通过返回值是否为1来验证该帧是否可写。

int *got_picture_ptr：该值为0表明没有图像可以解码，否则表明有图像可以解码；

const AVPacket *avpkt：输入参数，包含待解码数据。

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);
    }

    *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);

    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;
        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);
            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) {
            av_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;
                memset(picture->buf, 0, sizeof(picture->buf));
            }

            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 */
    picture->extended_data = picture->data;

    return ret;
}

在该函数中，调用了ret = avctx->codec->decode(avctx, picture, got_picture_ptr, &tmp);实现解码功能。在当前demo中，codec类型为ff_hevc_decoder，decode指针指向的函数为hevc_decode_frame。ff_hevc_decoder的定义如下：

AVCodec ff_hevc_decoder = {
    .name                  = "hevc",
    .long_name             = NULL_IF_CONFIG_SMALL("HEVC (High Efficiency Video Coding)"),
    .type                  = AVMEDIA_TYPE_VIDEO,
    .id                    = AV_CODEC_ID_HEVC,
    .priv_data_size        = sizeof(HEVCContext),
    .priv_class            = &hevc_decoder_class,
    .init                  = hevc_decode_init,
    .close                 = hevc_decode_free,
    .decode                = hevc_decode_frame,
    .flush                 = hevc_decode_flush,
    .update_thread_context = hevc_update_thread_context,
    .init_thread_copy      = hevc_init_thread_copy,
    .capabilities   = CODEC_CAP_DR1 | CODEC_CAP_DELAY | CODEC_CAP_SLICE_THREADS | CODEC_CAP_FRAME_THREADS,
};

解码函数：

static int hevc_decode_frame(AVCodecContext *avctx, void *data, int *got_output,
                             AVPacket *avpkt)
{
    int ret;
    HEVCContext *s = avctx->priv_data;

    if (!avpkt->size) {
        ret = ff_hevc_output_frame(s, data, 1);
        if (ret < 0)
            return ret;

        *got_output = ret;
        return 0;
    }

    s->ref = NULL;
    ret = decode_nal_units(s, avpkt->data, avpkt->size);
    if (ret < 0)
        return ret;

    /* verify the SEI checksum */
    if (avctx->err_recognition & AV_EF_CRCCHECK && s->is_decoded &&
        avctx->err_recognition & AV_EF_EXPLODE &&
        s->is_md5) {
        ret = verify_md5(s, s->ref->frame);
        if (ret < 0) {
            ff_hevc_unref_frame(s, s->ref, ~0);
            return ret;
        }
    }
    s->is_md5 = 0;

    if (s->is_decoded) {
        av_log(avctx, AV_LOG_DEBUG, "Decoded frame with POC %d.\n", s->poc);
        s->is_decoded = 0;
    }

    if (s->output_frame->buf[0]) {
        av_frame_move_ref(data, s->output_frame);
        *got_output = 1;
    }

    return avpkt->size;
}

熟悉编解码标准的同学都知道，H.264和HEVC都定义了网络抽象层NAL来执行传输层的任务，每一个NAL单元都按照规定保存了某些语法元素。函数decode_nal_units执行了对这些NAL单元进行解析并对NAL的下一层视频编码层VCL进行解码的任务。