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• Evolution #3784 : Validation automatique des messages de forum par un admin

  31 mai 2016, par b b

  Certainement parce que tu as créé le ticket depuis le projet en question, c’est corrigé.

• FFMpeg library : how to precisely seek in an audio file

  16 août 2023, par matteoh

  Using the FFMpeg library in my Android app, I try to understand how I can seek in an audio file, at a very precise position.

  



  For example, I want to set the current position in my file to the frame #1234567 (in a file encoded at 44100 Hz), which is equivalent to seek at 27994.717 milliseconds.

  



  To achieve that, here is what I tried :

  



  // this:
av_seek_frame(formatContext, -1, 27994717, 0);

// or this:
av_seek_frame(formatContext, -1, 27994717, AVSEEK_FLAG_ANY);

// or even this:
avformat_seek_file(formatContext, -1, 27994617, 27994717, 27994817, 0);


  



  Using a position in microseconds gives me the best result so far.

  



  But for some reason, the positioning is not totally accurate : when I extract the samples from the audio file, it doesn't start exactly at the expected position. There is a slight delay of about 30-40 milliseconds (even if I seek to the position 0, surprisingly...).

  



  Do I use the function the right way, or even the right function ?

  



  EDIT

  



  Here is how I can get the position :

  



  AVPacket packet;
AVStream *stream = NULL;
AVFormatContext *formatContext = NULL;
AVCodec *dec = NULL;

// initialization:
avformat_open_input(&formatContext, filename, NULL, NULL);
avformat_find_stream_info(formatContext, NULL);
int audio_stream_index = av_find_best_stream(formatContext, AVMEDIA_TYPE_AUDIO, -1, -1, &dec, 0);
stream = formatContext->streams[audio_stream_index];

...

// later, when I extract samples, here is how I get my position, in microseconds:
av_read_frame(formatContext, &packet);
long position = (long) (1000000 * (packet.pts * ((float) stream->time_base.num / stream->time_base.den)));


  



  Thanks to that piece of code, I can get the position of the beginning of the current frame (frame = bloc of samples, the size depends on the audio format - 1152 samples for mp3, 128 to 1152 for ogg, ...)

  



  The problem is : the value I get in position is not accurate : it's actually 30 ms late, approximately. For example, when it says 1000000, the actual position is approximately 1030000...

  



  What did I do wrong ? Is it a bug in FFMpeg ?

  



  Thanks for your help.

  


• ffmpeg 180 degree panoramic fisheye image to equirectangular / flat

  7 juillet 2024, par Willy62

  I am trying to get my Hikvision Panovu image of a sportsfield to look like a standard camera image, similar to what would be seen with a Veo solution / traditional camera.

  


  This is what the image would ideally look like with a little bit of zoom. Note the players are all upright and it looks "correct" and not skewed with the far end of the field in line with the horizon.

  


  veo sample image

  


  The original image looks like this (same field but other side). This is a 180 degree panoramic image from a Hikvision camera as found here.

  


  It provides the following output natively.

  


  hikvision native view

  


  I have had some luck converting the image with ffmpeg using the v360 filter. Note there is a downward tilt meaning I have to apply some yaw to correct it.

  


  v360=input=fisheye:output=rectilinear:ih_fov=180:iv_fov=87.5:d_fov=87.5:pitch=20:yaw=5:w=3840:h=2160


  


  And this gets the following output :

  


  deskewed v360 image

  


  So the challenge here to make the original image flat/equirect but to address the skew such that :

  


  

  • the players are orientated "upright"

  


  • the far sideline of the field looks like a straight line in line with the horizon

  


  • the image quality is preserved as best as possible

  


  


  With these cameras the image is 32MP so there is the opportunity to do an ePTZ into the area of interest.

  


  I suspect v360 isnt the right choice here and it is some remap-style filter, or perhaps I am best going across to gstreamer or similar.

  


  I tried an ffmpeg v360 filter and it partially works, but the players are still skewed because the top of the image is not wide enough. The issue can possibly be solved by correctly applying a couplex perspective filter, but I think this will only mask the issue and perspective requires a complex filter that hasn't worked for me so far.