ffmpeg 源代码简单分析 ： avcodec_decode_video2()

int avcodec_decode_video2(AVCodecContext *avctx, AVFrame *picture,  
                         int *got_picture_ptr,  
                         const AVPacket *avpkt)  
{  
    int ret;  
    // copy to ensure we do not change avpkt  
    AVPacket tmp = *avpkt;  
  
    *got_picture_ptr= 0;  
    if((avctx->coded_width||avctx->coded_height) && av_image_check_size(avctx->coded_width, avctx->coded_height, 0, avctx))  
        return -1;  
  
    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){  
            picture->pkt_pos= avpkt->pos;  
            }  
            //FIXME these should be under if(!avctx->





has_b_frames)  
            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 == PIX_FMT_NONE)  
                picture->format = avctx->pix_fmt;  
        }  
  
        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 (*got_picture_ptr){  
            avctx->frame_number++;  
            picture->best_effort_timestamp = guess_correct_pts(avctx,  
                                                            picture->pkt_pts,  
                                                            picture->pkt_dts);  
        }  
    }else  
        ret= 0;  
  
    return ret;  
}