Recherche avancée

Sur d’autres sites (4632)

• How to capture camera devices on Windows using Libav ?

  22 septembre 2011, par Occulta

  Is there any way to capture frames from as many camera types as DirectShow do on Windows platform using Libav ? I need to capture a camera output without using DirectShow filters and I want my application to work with many camera devices types.

  I have searched the Internet about this capability of libav and found that it can be done via libav using special input format "vfwcap". Something like that (don't sure about code correctness - I wrote it by myself) :

  AVFormatParameters formatParams = NULL;
  
  AVInputFormat* pInfmt = NULL;
  
  pInFormatCtx*  pInFormatCtx = NULL;
  
  
  
  av_register_all();
  
  
  
  //formatParams.device = NULL; //this was probably deprecated and then removed
  
  formatParams.channel = 0;
  
  formatParams.standard = "ntsc"; //deprecated too but still available
  
  formatParams.width = 640;
  
  formatParams.height = 480;
  
  formatParams.time_base.num = 1000;
  
  formatParams.time_base.den = 30000; //so we want 30000/1000 = 30 frames per second
  
  formatParams.prealloced_context = 0;
  
  
  
  
  
  pInfmt = av_find_input_format("vfwcap");
  
  if( !pInfmt )
  
  {
  
    fprintf(stderr,"Unknown input format\n");
  
    return -1;
  
  }
  
  
  
  // Open video file (formatParams can be NULL for autodetecting probably)
  
  if (av_open_input_file(&pInFormatCtx, 0, pInfmt, 0, formatParams) < 0)
  
     return -1; // Couldn't open device
  
  
  
  /* Same as video4linux code*/

  So another question is : how many devices are supported by Libav ? All I have found about capture cameras output with libav on windows is advice to use DirectShow for this purpose because libav supports too few devices. Maybe situation has already changed now and it does support enough devices to use it in production applications ?

  If this isn't possible.. Well I hope my question won't be useless and this composed from different sources piece of code will help someone interested in this theme 'coz there are really too few information about it in the whole internet.

• Windows media player showing wrong bit rate

  30 octobre 2011, par Akash

  I used ffmpeg to change the bit rate of a .mp3 file from 128kbps to 64kbps

  here are the final lines of output of ffmpeg :

  size=    2261kB time=00:04:49.38 bitrate=  64.0kbits/s
  
  video:0kB audio:2261kB global headers:0kB muxing overhead 0.011274%

  Tough the file size is reduced by half, windows media player while playing the file shows the bit-rate is 159 K bits/second

  Any ideas on why is this so ?

• how to create or extract a silent mp3 from a video file using ffmpeg or mencoder ? [migrated]

  25 octobre 2011, par mirswith

  I am looking to create an mp3 of silence that is the same duration as the input video file using either ffmpeg or mencoder.

  Using ffmpeg I have tried this :

  ffmpeg -y -i in.avi -ab 8k -ac 1 -vol 0 -acodec libmp3lame out.mp3

  Which works great if the video has an audio track but fails miserably if it does not.

  Any ideas ?

  Thanks !