ffmpeg源代码简单分析 ：avcodec_decode_video2()

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

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- 

 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 its set correctly */

 picture- extended_data = picture- data;

 return ret;

}

该函数的输入和输出参数分别为const AVPacket *avpkt和AVFrame *picture，分别表示待解码的码流和解码完成后的音视频信号，函数的核心是ret = avctx- codec- decode(avctx, picture, got_picture_ptr,  tmp)，调用预定义好的解码器进行解码。

jerry.yin 毕业于上海大学通信与信息工程学院，从事流媒体和视频编解码的研究与开发工作； 研究领域包括视频编解码标准、视频处理和流媒体技术、移动互联网技术等。