FFMPEG avformat_find_stream_info替换

前提

因为使用avformat_find_stream_info接口读取一部分视音频数据并且获得一些相关的信息。耗时太长，在网上查找了一些资料，看了雷神的讲解稍微了解了这个函数的功能，但是并没有得到如何降低耗时的问题，最后在网上找到了这篇文章 —— [ VLC优化（1） avformat_find_stream_info接口延迟降低 ]

这篇文章详细的说明了如何使用一些参数来降低avformat_find_stream_info接口的耗时，并提出了一种极端的解决方案，在已知发送端的流信息的情况下，跳过avformat_find_stream_info接口，自定义初始化解码环境。
我的音视频播放用的是雷神的 [ Simplest FFmpeg Player 2 ]中的SU（SDL Update）版。

过程

我稍微修改了读取数据的方式为rmpt，然后使用，但是我使用了他的方案后发现无法读取到一帧的数据，然后我反复对比avformat_find_stream_info接口和init_decode接口后的fmt_ctx参数的不同。

增加了一些codec信息，比如：

s->streams[audio_index]->codec->codec_type = AVMEDIA_TYPE_AUDIO;s->streams[audio_index]->codec->bit_rate = 16000;s->streams[audio_index]->codec->refs = 1;s->streams[audio_index]->codec->sample_fmt = AV_SAMPLE_FMT_FLTP;s->streams[audio_index]->codec->profile = 1;s->streams[audio_index]->codec->level = -99;s->streams[video_index]->codec->pix_fmt = AV_PIX_FMT_YUV420P;s->streams[video_index]->codec->sample_fmt = AV_SAMPLE_FMT_NONE;s->streams[video_index]->codec->codec_type = AVMEDIA_TYPE_VIDEO;

最后发现还是不行，在对比av_dump_format打印出来的音视频信息的不同，增加了一些配置：

char option_key[] = "encoder";char option_value[] = "Lavf57.0.100";ret = av_dict_set(&(s->metadata), option_key, option_value, 0);// AVDictionaryEntry *tag = NULL;// tag = av_dict_get((s->metadata), "", tag, AV_DICT_IGNORE_SUFFIX);s->duration = 0;s->start_time = 0;s->bit_rate = 0;s->iformat->flags = 0;s->duration_estimation_method = AVFMT_DURATION_FROM_STREAM;

发现av_dump_format打印出来的音视频消息都对了，但是还是无法读取视频数据。
继续找不同，发现s->packet_buffer里面没有数据，查找资料发现This buffer is only needed when packets were already buffered but not decoded, for example to get the codec parameters in MPEG streams. 看样子是里面要有数据啊，从av_dump_format源代码中搜索packet_buffer找到相关的代码：

ret = read_frame_internal(ic, &pkt1);          if (ret == AVERROR(EAGAIN))              continue;          if (ret < 0) {              /* EOF or error*/              break;          }          if (ic->flags & AVFMT_FLAG_NOBUFFER)              free_packet_buffer(&ic->packet_buffer, &ic->packet_buffer_end);          {              pkt = add_to_pktbuf(&ic->packet_buffer, &pkt1,                                  &ic->packet_buffer_end);              if (!pkt) {                  ret = AVERROR(ENOMEM);                  goto find_stream_info_err;              }              if ((ret = av_dup_packet(pkt)) < 0)                  goto find_stream_info_err;          }  

看样子是通过调用read_frame_internal读取一帧的数据放入packet_buffer，所以拷贝出相关代码read_frame_internal这个不能调用，但是有个av_read_frame这个函数对read_frame_internal进行了封装，可以调用这个函数

    AVPacket packet;    av_init_packet(&packet);    int ret = av_read_frame(s, &packet);    add_to_pktbuf(&(s->packet_buffer), &packet, &(s->packet_buffer_end));

还是不成功，断点查看，发现没有得到packet数据，在看看avformat_find_stream_info源代码：

ret = read_frame_internal(ic, &pkt1);  if (ret == AVERROR(EAGAIN))      continue;

再改：

AVPacket packet;av_init_packet(&packet);while (true){    int ret1 = av_read_frame(s, &packet);    if (packet.size > 0)    {        break;    }}add_to_pktbuf(&(s->packet_buffer), &packet, &(s->packet_buffer_end));

终于得到了packet_buff的数据了，可是还是不行，在对比avformat_find_stream_info接口和init_decode接口后的fmt_ctx参数的不同，发现avformat_find_stream_info执行完后，avio_tell(s->pb), s->pb->seek_count两个的数据是相同的，而init_decode执行完avio_tell(s->pb), s->pb->seek_count两个的数据不同，查看init_decode中有关的代码get_video_extradata()，只知道这里面有对s->pb的操作，看不懂就先注释了，先看看效果，如果不行在自己修改s->streams[video_index]->codec->extradata和s->streams[video_index]->codec->extradata_size的数据，结果竟然可以了！！！完全没想到，怀疑avio_*** 这类函数可能有问题。放代码吧：

enum {    FLV_TAG_TYPE_AUDIO = 0x08,    FLV_TAG_TYPE_VIDEO = 0x09,    FLV_TAG_TYPE_META  = 0x12,};static AVStream *create_stream(AVFormatContext *s, int codec_type){    AVStream *st = avformat_new_stream(s, NULL);    if (!st)        return NULL;    st->codec->codec_type = (AVMediaType)codec_type;    return st;}static AVPacket *add_to_pktbuf(AVPacketList **packet_buffer, AVPacket *pkt, AVPacketList **plast_pktl){    AVPacketList *pktl = (AVPacketList *)av_mallocz(sizeof(AVPacketList));    if (!pktl)        return NULL;    if (*packet_buffer)        (*plast_pktl)->next = pktl;    else        *packet_buffer = pktl;    /* Add the packet in the buffered packet list. */    *plast_pktl = pktl;    pktl->pkt   = *pkt;    return &pktl->pkt;}static int init_decode(AVFormatContext *s){     int video_index = -1;     int audio_index = -1;     int ret = -1;     if (!s)          return ret;     /*     Get video stream index, if no video stream then create it.     And audio so on.     */     if (0 == s->nb_streams) {        create_stream(s, AVMEDIA_TYPE_VIDEO);        create_stream(s, AVMEDIA_TYPE_AUDIO);        video_index = 0;        audio_index = 1;     } else if (1 == s->nb_streams) {        if (AVMEDIA_TYPE_VIDEO == s->streams[0]->codec->codec_type) {            create_stream(s, AVMEDIA_TYPE_AUDIO);            video_index = 0;            audio_index = 1;        } else if (AVMEDIA_TYPE_AUDIO == s->streams[0]->codec->codec_type) {           create_stream(s, AVMEDIA_TYPE_VIDEO);           video_index = 1;           audio_index = 0;        }     } else if (2 == s->nb_streams) {        if (AVMEDIA_TYPE_VIDEO == s->streams[0]->codec->codec_type) {           video_index = 0;           audio_index = 1;        } else if (AVMEDIA_TYPE_VIDEO == s->streams[1]->codec->codec_type) {           video_index = 1;           audio_index = 0;        }     }     /*Error. I can't find video stream.*/     if (video_index != 0 && video_index != 1)          return ret;     //Init the audio codec(AAC).     s->streams[audio_index]->codec->codec_id = AV_CODEC_ID_AAC;     s->streams[audio_index]->codec->sample_rate = 8000;     s->streams[audio_index]->codec->time_base.den = 44100;     s->streams[audio_index]->codec->time_base.num = 1;     s->streams[audio_index]->codec->bits_per_coded_sample = 16; //     s->streams[audio_index]->codec->channels = 1;     s->streams[audio_index]->codec->channel_layout = 4;     s->streams[audio_index]->codec->codec_type = AVMEDIA_TYPE_AUDIO;     s->streams[audio_index]->codec->bit_rate = 16000;     s->streams[audio_index]->codec->refs = 1;     s->streams[audio_index]->codec->sample_fmt = AV_SAMPLE_FMT_FLTP;     s->streams[audio_index]->codec->profile = 1;     s->streams[audio_index]->codec->level = -99;     s->streams[audio_index]->pts_wrap_bits = 32;     s->streams[audio_index]->time_base.den = 1000;     s->streams[audio_index]->time_base.num = 1;    //Init the video codec(H264).     s->streams[video_index]->codec->codec_id = AV_CODEC_ID_H264;     s->streams[video_index]->codec->width = 1280;     s->streams[video_index]->codec->height = 720;     s->streams[video_index]->codec->ticks_per_frame = 2;     s->streams[video_index]->codec->pix_fmt = AV_PIX_FMT_YUV420P;     s->streams[video_index]->codec->time_base.den = 2000;     s->streams[video_index]->codec->time_base.num = 1;     s->streams[video_index]->codec->sample_fmt = AV_SAMPLE_FMT_NONE;     s->streams[video_index]->codec->frame_size = 0;     s->streams[video_index]->codec->frame_number = 7;     s->streams[video_index]->codec->has_b_frames = 0;     s->streams[video_index]->codec->codec_type = AVMEDIA_TYPE_VIDEO;     s->streams[video_index]->codec->codec_tag = 0;     s->streams[video_index]->codec->bit_rate = 0;     s->streams[video_index]->codec->refs = 1;     s->streams[video_index]->codec->sample_rate = 0;     s->streams[video_index]->codec->channels = 0;     s->streams[video_index]->codec->profile = 66;     s->streams[video_index]->codec->level = 31;     s->streams[video_index]->pts_wrap_bits = 32;     s->streams[video_index]->time_base.den = 1000;     s->streams[video_index]->time_base.num = 1;     s->streams[video_index]->avg_frame_rate.den = 1;     s->streams[video_index]->avg_frame_rate.num = 25;        char option_key[] = "encoder";     char option_value[] = "Lavf57.0.100";     ret = av_dict_set(&(s->metadata), option_key, option_value, 0);     AVDictionaryEntry *tag = NULL;     tag = av_dict_get((s->metadata), "", tag, AV_DICT_IGNORE_SUFFIX);     s->duration = 0;     s->start_time = 0;     s->bit_rate = 0;     s->iformat->flags = 0;     s->duration_estimation_method = AVFMT_DURATION_FROM_STREAM;    AVPacket packet;    av_init_packet(&packet);    while (true)    {        int ret1 = av_read_frame(s, &packet);        if (packet.flags & AV_PKT_FLAG_KEY)        {            break;        }    }    add_to_pktbuf(&(s->packet_buffer), &packet, &(s->packet_buffer_end));   /*Need to change, different condition has different frame_rate. 'r_frame_rate' is new in ffmepg2.3.3*/     s->streams[video_index]->r_frame_rate.den = 1;     s->streams[video_index]->r_frame_rate.num = 25;     /*Update the AVFormatContext Info*/     s->nb_streams = 2;     /*     something wrong.     TODO: find out the 'pos' means what.     then set it.     */     s->pb->pos = (int64_t)s->pb->buf_end;     return ret;}if(avformat_open_input(&pFormatCtx,filepath,NULL,NULL)!=0){    printf("Couldn't open input stream.\n");    return -1;}init_decode(pFormatCtx);

相关参数按照自己的要求修改哈

参考：
[1]https://jiya.io/archives/vlc_optimize_1.html
[2]http://blog.csdn.net/leixiaohua1020/article/details/44084321
[3]http://blog.csdn.net/leixiaohua1020/article/details/12678577