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• JavaCV FFmpegFrameRecorder properties explanation needed

  29 décembre 2014, par Leron

  I’m using FFmpegFrameRecorder to get the video input from my webcam and record it into a video file. The problem is that I’m building my application using a few different demo source codes that I found and I use properties some of which are not completely clear to me.

  First, here is my code snippet :

  FFmpegFrameRecorder recorder = new FFmpegFrameRecorder(FILENAME,  grabber.getImageWidth(),grabber.getImageHeight());
  
  
  
          recorder.setVideoCodec(13);
  
          recorder.setFormat("mp4");
  
          recorder.setPixelFormat(avutil.PIX_FMT_YUV420P);
  
          recorder.setFrameRate(30);
  
          recorder.setVideoBitrate(10 * 1024 * 1024);
  
  
  
          recorder.start();

  • setVideoCodec(13) - What is the meaning of this (13) how can I understand what actual codec stands behind any number ?
  • setPixelFormat - Just get this, don’t know what it’s doing in general
  • setFrameRate(30) - I think this should be pretty clear but still what is the logic behind what frame rate we choose (isn’t the high the better ?)
  • setVideoBitrate(10*1024*1024) - again almost no idea what this does and what’s the logic behind the numbers ?

  At the end I just want to mention one last problem that I get recording video like this. If the actual length of the video is let’s say 20secs. When I play the video file created from the program it runs significantly faster. Can’t tell if it’s exactly 2 times faster than it should be but in general if I record a 20sec video then it’s played for about 10secs. What may cause this and how can I fix it ?

• avcodec/v4l2_m2m_dec : remove redundant packet and fix double free

  16 juillet 2020, par Andriy Gelman

  avcodec/v4l2_m2m_dec : remove redundant packet and fix double free
  v4l2_receive_frame() uses two packets s->buf_pkt and avpkt. If avpkt
  
  cannot be enqueued, the packet is buffered in s->buf_pkt and enqueued in
  
  the next call. Currently the ownership transfer between the two packets
  
  is not properly handled. A double free occurs if
  
  ff_v4l2_context_enqueue_packet() returns EAGAIN and v4l2_try_start
  
  returns EINVAL.
  In fact, having two AVPackets is not needed and everything can be
  
  handled by s->buf_pkt.
  This commit removes the local avpkt from v4l2_receive_frame(), meaning
  
  that the ownership transfer doesn't need to be handled and the double
  
  free is fixed.
  Signed-off-by : Andriy Gelman <andriy.gelman@gmail.com>
  

  • [DH] libavcodec/v4l2_m2m_dec.c

• Convert encoded audio file to text with signal values

  15 août 2017, par Niccolò Cirone

  I’ve been programming in c for the first time with audio files. I found this code which supposedly should read an audio file and then write a csv file containing several info in order to analyse the audio waves,that in case will be a simple voice : i’m interested in amplitude of the waves, in the timbre of the voice and its hight and extension.

             main () {   
  
             // Create a 20 ms audio buffer (assuming Fs = 44.1 kHz)
  
             int16_t buf[N] = {0}; // buffer
  
             int n;                // buffer index
  
  
  
            // Open WAV file with FFmpeg and read raw samples via the pipe.
  
            FILE *pipein;
  
            pipein = popen("ffmpeg -i whistle.wav -f s16le -ac 1 -", "r");
  
            fread(buf, 2, N, pipein);
  
            pclose(pipein);
  
  
  
            // Print the sample values in the buffer to a CSV file
  
            FILE *csvfile;
  
            csvfile = fopen("samples.csv", "w");
  
            for (n=0 ; n<n></n>code>

  Could someone explain me in detail how can I read an audio file so that I could extract from it the info I need ? Referring to this code, could someone explain me the meaning of the pipe at line 8

  pipein = popen("ffmpeg -i whistle.wav -f s16le -ac 1 -", "r");

  p.s. I already know how to read the header of the audio file, which contains a lot of useful info, but I also want to analyse the entire audio file, sample by sample.