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• ffmpeg start_time is negative

  8 décembre 2016, par Alex Bollbach

  I’m using ffmpeg to segment a video by start/end times. For example, I have a set of time [1], and I’d call ffmpeg 3 times here to create the three 2 second videos at times 0, 2, and 4.

  My problem is that these times (which are correct) are not correctly segmenting the video. When I inspect the stats of the video with ffprobe -i test.mov -show_format, I noticed start_time=-0.381044. I also know that this seems to be caused by trimming a video in QuickTime to produce this video. In order to correct this, I must add (start + 0.381044) somewhere in my code, and the output is fine.

  So, why is the start_time set to a negative value by the quicktime trimming ? How can I set start_time=0, so my code can work without the + 0.381044 ?

• C++ FFmpeg pitch issue

  24 janvier 2016, par David Andrei Norgren

  I’m using swr_convert to lower/raise the pitch of incoming audio and store it in a .mp3. To change the pitch, I’m dividing the out sample rate by a factor. However, the resulting audio is slightly distorted when this factor is anything other than 1. Here’s my conversion code :

  ...
  
  
  
  // Set up resample context
  
  swrContext = swr_alloc();
  
  if (!swrContext)
  
      throw -15;
  
  
  
  av_opt_set_int(swrContext, "in_channel_count", codecContext->channels, 0);
  
  av_opt_set_int(swrContext, "in_channel_layout", codecContext->channel_layout, 0);
  
  av_opt_set_int(swrContext, "in_sample_rate", codecContext->sample_rate, 0);
  
  av_opt_set_sample_fmt(swrContext, "in_sample_fmt", codecContext->sample_fmt, 0);
  
  
  
  av_opt_set_int(swrContext, "out_channel_count", STREAM_AUDIO_CHANNELS, 0);
  
  av_opt_set_int(swrContext, "out_channel_layout", STREAM_AUDIO_CHANNEL_LAYOUT, 0);
  
  av_opt_set_int(swrContext, "out_sample_rate", STREAM_AUDIO_SAMPLE_RATE / pitch, 0);
  
  av_opt_set_sample_fmt(swrContext, "out_sample_fmt", STREAM_AUDIO_SAMPLE_FORMAT_GM, 0);
  
  
  
  if (swr_init(swrContext))
  
      throw -16;
  
  
  
  // Allocate re-usable frame
  
  frameDecoded = av_frame_alloc();
  
  if (!frameDecoded)
  
      throw -17;
  
  
  
  frameDecoded->format = codecContext->sample_fmt;
  
  frameDecoded->channel_layout = codecContext->channel_layout;
  
  frameDecoded->channels = codecContext->channels;
  
  frameDecoded->sample_rate = codecContext->sample_rate;
  
  
  
  // Load frames
  
  inPacket.data = NULL;
  
  inPacket.size = 0;
  
  
  
  int gotFrame, samples = 0;
  
  
  
  while (av_read_frame(formatContext, &inPacket) >= 0) {
  
  
  
      if (inPacket.stream_index != streamId) 
  
          continue;
  
  
  
      if (avcodec_decode_audio4(codecContext, frameDecoded, &gotFrame, &inPacket) < 0)
  
          throw -18;
  
  
  
      if (!gotFrame)
  
          continue;
  
  
  
      // Begin conversion
  
      if (swr_convert(swrContext, NULL, 0, (const uint8_t **)frameDecoded->data, frameDecoded->nb_samples) < 0)
  
          throw -19;
  
  
  
      while (swr_get_out_samples(swrContext, 0) >= RAW_AUDIO_FRAME_SIZE) {
  
  
  
          // Allocate data
  
          uint8_t **convertedData = NULL;
  
          if (av_samples_alloc_array_and_samples(&convertedData, NULL, STREAM_AUDIO_CHANNELS, RAW_AUDIO_FRAME_SIZE, STREAM_AUDIO_SAMPLE_FORMAT_GM, 0) < 0)
  
              throw -20;
  
  
  
          // Convert
  
          if (swr_convert(swrContext, convertedData, RAW_AUDIO_FRAME_SIZE, NULL, 0) < 0)
  
              throw -21;
  
  
  
          // Calculate buffer size
  
          size_t bufferSize = av_samples_get_buffer_size(NULL, STREAM_AUDIO_CHANNELS, RAW_AUDIO_FRAME_SIZE, STREAM_AUDIO_SAMPLE_FORMAT_GM, 0);
  
          if (bufferSize < 0)
  
              throw -22;
  
  
  
          fwrite(convertedData[0], 1, bufferSize, outStream);
  
          av_free(convertedData);
  
      }
  
  }
  
  
  
  ...

  STREAM_AUDIO_SAMPLE_RATE is defined as 44100.
  Here’s the entire program if it helps : http://pastebin.com/5akEwNg4

  The program generates a .mp3 with 25 notes that decrease in pitch.
  Here’s an example of the distortion : http://www.stuffbydavid.com/dl/30256478.mp3

  Can you spot anything incorrect about my conversion, or is my method of changing the pitch incorrect ? Is there another way ?

• Evolution #3284 (Nouveau) : changer le vert de l’espace privé par defaut

  12 octobre 2014, par Ben .

  Je propose de changer le vert par défaut et de passer au orange (ou une autre couleur) . C’est purement psychologique et c’est histoire de faire une différence pour se dire immédiatement tiens il y a une nouvelle couleur .