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• Record stream from camera to file / live stream

  8 juillet 2015, par sandman

  I’m trying to stream my camera feed to a webpage. For this I’m trying to use FFmpeg to encode the data from the camera preview and output to a file. (I’m new to this, so I’m only trying to write to a file now)

  The camera feed is captured and it starts writing to a flv file but, it freezes my app after a few seconds and the resulting flv is about half a second long.

  bos is a BufferedOutputStream which servers as an input stream for the Ffmpeg ProcessBuilder.

  When a button is clicked, the Ffmpeg process is executed and the writing begins.
  But as I said, it freezes the app after a while. I tried running the Ffmpeg process in an AsyncTask, but it keeps saying FileDescriptor closed, but the app does not freeze when I do it this way, and there is no output as well.

  Here is my onPreviewFrame() :

  public void onPreviewFrame(byte[] b, Camera c) {
  
      if (recording) {
  
          int previewFormat = p.getPreviewFormat();
  
          Log.v(LOGTAG, "Started Writing Frame");
  
  
  
          im = new YuvImage(b, previewFormat, p.getPreviewSize().width, p.getPreviewSize().height, null);
  
          Rect r = new Rect(0, 0, p.getPreviewSize().width, p.getPreviewSize().height);
  
          if (bos != null)
  
              im.compressToJpeg(r, 40, bos);
  
          im = null;
  
  
  
          Log.v(LOGTAG, "Finished Writing Frame");
  
      }
  
  }

  If I can get this fixed, I can probably move on to live streaming.

• rv34 : use ff_mpeg_update_thread_context only when decoder is fully initialized

  21 août 2014, par Janne Grunau

  rv34 : use ff_mpeg_update_thread_context only when decoder is fully initialized
  MpegEncContext based decoders are only fully initialized after the first
  
  ff_thread_get_buffer() call. The RV30/40 decoders may fail before a frame
  
  buffer was requested. ff_mpeg_update_thread_context() fails on half
  
  initialized MpegEncContexts. Since this can only happen before a the
  
  first frame was decoded there is no need to call
  
  ff_mpeg_update_thread_context().
  Based on patches by John Stebbins and tested by John Stebbins.
  CC : libav-stable@libav.org
  

  • [DBH] libavcodec/rv34.c

• How exactly does this Jaccard distance logic (get_jaccarddist) in FFmpeg work ?

  23 mars 2024, par stilloo

  I see this C code in FFmpeg source signature_lookup file for Jaccard distance in method get_jaccarddist.

  


  If you see in the code below that is calculating for 5 BagOfWords.

  


  Now for two BagOfWords with say nothing in common intersection would be 0, that means jaccarddist would be 0, so ideally its a bad pair but this if condition jaccarddist >= sc->thworddist, would be false isn't it but we want it to be true as its bad pair ?

  


  What are the default values of sc->thworddist ?

  


  Also surprised why jaccarddist is int and not float/double ?

  


  /**
 * calculates the jaccard distance and evaluates a pair of coarse signatures as good
 * @return 0 if pair is bad, 1 otherwise
 */
static int get_jaccarddist(SignatureContext *sc, CoarseSignature *first, CoarseSignature *second)
{
    int jaccarddist, i, composdist = 0, cwthcount = 0;
    for (i = 0; i < 5; i++) {
        if ((jaccarddist = intersection_word(first->data[i], second->data[i])) > 0) {
            jaccarddist /= union_word(first->data[i], second->data[i]);
        }
        if (jaccarddist >= sc->thworddist) {
            if (++cwthcount > 2) {
                /* more than half (5/2) of distances are too wide */
                return 0;
            }
        }
        composdist += jaccarddist;
        if (composdist > sc->thcomposdist) {
            return 0;
        }
    }
    return 1;
}