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• Orientation issue with mobile recordings

  18 mai 2012, par Kim Egede Jakobsen

  When I convert .mov files from iPhone 4 and .3gp from Android with FFMPEG.
  I need to rotate the video 90 degress, but iPhone 4s needs 180 degress...

  Is there any way to fix this ?

• ffmpeg iOS and SDL2

  24 décembre 2015, par user3487978

  I created a app for iPhone, the core source is based on the latest ffplay.c, I just changed the rendering part to support iOS, general it can play video, but I found the quality is not as good as enough, see screenshot below, the first image is the what expected, the second is image that my app on iPhone produced, and the last the produced by my app on iPad.
  on the iPhone’s image, you can found there is a dash line, and the colour is not correct, it seems that there is a shadow of the image.
  I am new to ffmpeg, can anyone help me what is the problem

• libswresample : swr_convert() not producing enough samples

  20 septembre 2016, par Tsherr

  I’m trying to use ffmpeg/libswresample to resample streaming audio in my c++ application. Changing the sample width works well and the result sounds as one would expect ; however, when changing the sample rate the result is somewhat crackly. I am unsure if it is due to incorrect usage of the libswresample library, or if I’m misunderstanding the resampling theory.

  Here is my resampling process, simplified for demonstration’s sake :

  //Externally supplied data
  
  const uint8_t* in_samples //contains the audio data to be resampled
  
  int in_num_samples = 256
  
  
  
  //Set up resampling context
  
  SwrContext *swr = swr_alloc();
  
  av_opt_set_channel_layout(swr, "in_channel_layout", AV_CH_LAYOUT_STEREO, 0);
  
  av_opt_set_channel_layout(swr, "out_channel_layout", AV_CH_LAYOUT_STEREO, 0);
  
  av_opt_set_int(swr, "in_sample_rate", 44100, 0);
  
  av_opt_set_int(swr, "out_sample_rate", 22050, 0);
  
  av_opt_set_sample_fmt(swr, "in_sample_fmt", AV_SAMPLE_FMT_FLT, 0);
  
  av_opt_set_sample_fmt(swr, "out_sample_fmt", AV_SAMPLE_FMT_FLT, 0);
  
  swr_init(swr);
  
  
  
  //Perform the resampe
  
  uint8_t* out_samples;
  
  int out_num_samples = av_rescale_rnd(swr_get_delay(swr, in_samplerate) + in_num_samples, out_samplerate, in_samplerate, AV_ROUND_UP);
  
  av_samples_alloc(&out_samples, NULL, out_num_channels, out_num_samples, AV_SAMPLE_FMT_FLT, 0);
  
  out_num_samples = swr_convert(swr, &out_samples, out_num_samples, &in_samples, in_num_samples);
  
  av_freep(&out_samples);
  
  swr_free(&swr);

  I suspect that the reason the resampled audio does not sound right is because swr_convert() returns 112, where I expect it to return 128 (the number of samples of the resampled audio) :
  Downsampling 256 samples from a samplerate of 44100 to a samplerate of 22050 should yield 128 samples, yet swr_convert() is producing 112 samples. When expressed in terms of audio duration this is also puzzling. 256 samples at 44100 = 5.8 ms, but 112 samples at 22050 = 5.07 ms. Shouldn’t the downsampling process not alter the duration of the resampled audio ?

  I have also stepped through an example provided with ffmpeg, in which swr_convert() also returns a smaller number than I would expect. So, I suspect that the problem is not due to a bug in libswresample but rather my own lack of understanding.