20 Getfreeauto User Profile Ganthony Get Free Auto 如何获取的nb_streams数目?(页 1) - FFMPEG工程组 - ffmpeg应用 - 视频技术论坛中华视频网|ffmpeg工程组

popy 发表于 2008-8-22 03:12 PM

如何获取的nb_streams数目?

分析FFmpeg的代码，发现获取nb_streams值的过程有些隐晦，追踪了一下，还是没有发现怎么获取的该值。

考虑一个最简单的流程：

[list=1][*][b]av_register_all() 这就不用说什么了[/b][*][b]av_open_input_file() 主要是调用demuxer的xxx_probe以探测文件的格式
[/b][*][b]av_find_stream_info() 说的是从文件中获取流的相关信息，但没有找到怎么样获取nb_streams的过程，敬请指教
[/b][/list]

Fastreaming 发表于 2008-8-22 03:19 PM

>># av_open_input_file() 主要是调用demuxer的xxx_probe以探测文件的格式
No,
av_open_input_file() will calculte nb_streams by calling the demuxer's read_header

popy 发表于 2008-8-22 03:27 PM

回复 #2 Fastreaming 的帖子

哦，难道我看漏地方了，再看看

popy 发表于 2008-8-22 03:49 PM

回复 #2 Fastreaming 的帖子

给出一个简单的流程，关于av_open_input_file() 的执行过程：

/** size of probe buffer, for guessing file type from file contents */
#define PROBE_BUF_MIN 2048
#define PROBE_BUF_MAX (1<<20)

int av_open_input_file(AVFormatContext **ic_ptr, const char *filename,
                     AVInputFormat *fmt,
                     int buf_size,
                     AVFormatParameters *ap)
{}

/* Do not open file if the format does not need it. XXX: specific
   hack needed to handle RTSP/TCP */
if (!fmt || !(fmt->flags & AVFMT_NOFILE)) {}
      if (buf_size > 0) {}

      for(probe_size= PROBE_BUF_MIN; probe_size<=PROBE_BUF_MAX && !fmt; probe_size<<=1){}
         }
         /* guess file format */
         fmt = av_probe_input_format2(pd, 1, &score);
      }
      av_freep(&pd->buf);
}

/* if still no format found, error */
if (!fmt) {}

/* check filename in case an image number is expected */
if (fmt->flags & AVFMT_NEEDNUMBER) {}
}
#########################################
获取流的信息 ???????
#########################################
err = av_open_input_stream(ic_ptr, pb, filename, fmt, ap);
if (err)
      goto fail;
return 0;
fail:
av_freep(&pd->buf);
if (pb)
      url_fclose(pb);
*ic_ptr = NULL;
return err;

}

假设现在的demuxer是mpegts_demuxer:

int av_open_input_stream(AVFormatContext **ic_ptr,
                     ByteIOContext *pb, const char *filename,
                     AVInputFormat *fmt, AVFormatParameters *ap)
{}

if(!ap->prealloced_context)
      ic = av_alloc_format_context();
else
      ic = *ic_ptr;
if (!ic) {}
ic->iformat = fmt;
ic->pb = pb;
ic->duration = AV_NOPTS_VALUE;
ic->start_time = AV_NOPTS_VALUE;
av_strlcpy(ic->filename, filename, sizeof(ic->filename));

/* allocate private data */
if (fmt->priv_data_size > 0) {}
} else {}

if (ic->iformat->read_header) {}

if (pb && !ic->data_offset)
      ic->data_offset = url_ftell(ic->pb);

*ic_ptr = ic;
return 0;
fail:
if (ic) {}
         av_free(st);
      }
}
av_free(ic);
*ic_ptr = NULL;
return err;
}

接下来，可以看看mpegts_read_header()[libavformat/mpegts.c]

static int mpegts_read_header(AVFormatContext *s,
                           AVFormatParameters *ap)
{}
}

/* read the first 1024 bytes to get packet size */
pos = url_ftell(pb);
len = get_buffer(pb, buf, sizeof(buf));
if (len != sizeof(buf))
      goto fail;
ts->raw_packet_size = get_packet_size(buf, sizeof(buf));
if (ts->raw_packet_size <= 0)
      goto fail;
ts->stream = s;
ts->auto_guess = 0;

if (s->iformat == &mpegts_demuxer) {} else {}
         nb_packets++;
      }

      /* NOTE1: the bitrate is computed without the FEC */
      /* NOTE2: it is only the bitrate of the start of the stream */
      ts->pcr_incr = (pcrs[1] - pcrs[0]) / (packet_count[1] - packet_count[0]);
      ts->cur_pcr = pcrs[0] - ts->pcr_incr * packet_count[0];
      s->bit_rate = (TS_PACKET_SIZE * 8) * 27e6 / ts->pcr_incr;
      st->codec->bit_rate = s->bit_rate;
      st->start_time = ts->cur_pcr;
#if 0
      av_log(ts->stream, AV_LOG_DEBUG, "start=%0.3f pcr=%0.3f incr=%d\n",
            st->start_time / 1000000.0, pcrs[0] / 27e6, ts->pcr_incr);
#endif
}

url_fseek(pb, pos, SEEK_SET);
return 0;
fail:
return -1;
}

考虑一个简单一点的情况，假设在probe阶段已经找到输入的格式为mpegts_demuxer，因此上面就只用关心：
/* normal demux */

      /* first do a scaning to get all the services */
      url_fseek(pb, pos, SEEK_SET);
      mpegts_scan_sdt(ts);

      mpegts_set_service(ts);

      handle_packets(ts, s->probesize);
      /* if could not find service, enable auto_guess */

      ts->auto_guess = 1;
这几行代码而已，这几个函数我都简单看过，并没发现怎么设置的nb_streams

popy 发表于 2008-8-22 04:38 PM

回复 #2 Fastreaming 的帖子

找到地方了，多谢指点：

其实所有所有stream的查找过程，最终都会调用到AVStream *av_new_stream(AVFormatContext *s, int id)；这个函数，只是MPEGTS比较特殊一些，它有可能不止有stream，还可能有program，而此时调用的就是AVProgram *av_new_program(AVFormatContext *ac, int id)；我分析的时候分析到了这个函数，但其它的分支是有可能到AVStream *av_new_stream(AVFormatContext *s, int id)；函数的。

其实之前就看到过AVStream *av_new_stream(AVFormatContext *s, int id)里的s->streams[s->nb_streams++] = st;
推测最可能最终调用的是它，但分析分支情况的时候遗漏了

页: [1]

Get Free Auto Fast

20 Getfreeauto User Profile Ganthony Get Free Auto 如何获取的nb_streams数目?(页 1) - FFMPEG工程组 - ffmpeg应用 - 视频技术论坛中华视频网|ffmpeg工程组 - Powered by Discuz! Archiver