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• How to prevent the video generated by ffmpeg from flickering when converting from RGB to YUV using sws_scale() ?

  30 mai 2019, par LeFrosch

  I am generating images and loading them as a RGBA Bitmaps into my c++ program. Then I want to generate a video with ffmpeg using those images. As far as I found out I need to convert them to YUV using sws_scale(). Everything works fine except the generated video is flickering.

  I already tried to change the sws_getContext() flag. SWS_BILINEAR | SWS_ACCURATE_RND are preventing some of the flickering but not all. I also tried to change my EncodeVideo method but this also did effect the result.

  // creating the SwsContext
  
  this->data->convertContext = sws_getContext(
  
              data->outputCodecContext->width,
  
              data->outputCodecContext->height,
  
              IN_PXEL_FORMAT, // libffmpeg::AV_PIX_FMT_RGB24
  
              data->outputCodecContext->width,
  
              data->outputCodecContext->height,
  
              PIXEL_FORMAT, // libffmpeg::AV_PIX_FMT_YUV420P
  
              SWS_BILINEAR | SWS_ACCURATE_RND,
  
              NULL, NULL, NULL);
  
  
  
  // the bitmap which contains the rgb image
  
  Bitmap^ frame = generateImage();
  
  
  
  // copying the bitmap to the libffmpeg::AVFrame 
  
  for (int x = 0; x < width; x++)
  
  {
  
      for (int y = 0; y < height; y++)
  
      {
  
          Color c = frame->GetPixel(x, y);
  
          data->outputFrameRGB->data[0][y * data->outputFrameRGB->linesize[0] + 3 * x] = c.R;
  
          data->outputFrameRGB->data[0][y * data->outputFrameRGB->linesize[0] + 3 * x + 1] = c.G;
  
          data->outputFrameRGB->data[0][y * data->outputFrameRGB->linesize[0] + 3 * x + 2] = c.B;
  
      }
  
  }
  
  
  
  // converting the rgb frame to the yuv frame
  
  libffmpeg::sws_scale(data->convertContext, data->outputFrameRGB->data, data->outputFrameRGB->linesize, 0, height, data->outputFrameYUV->data, data->outputFrameYUV->linesize);
  
  
  
  for (int i = 0; i < time.TotalSeconds * frameRate; i++)
  
  {
  
      data->outputFrameYUV->pts = data->frameCount++;
  
  
  
      Helper::EncodeVideo(data->outputCodecContext, data->outputFrameYUV, data->packet, data->outputFile);
  
  }

  The Methode which encodes the video :

  void EncodeVideo(libffmpeg::AVCodecContext *context, libffmpeg::AVFrame *frame, libffmpeg::AVPacket *packet, libffmpeg::FILE *file)
  
  {
  
      int ret = 0;
  
  
  
      /* Send the frame to the encoder */
  
      ret = libffmpeg::avcodec_send_frame(context, frame);
  
      if (ret < 0)
  
      {
  
          ThrowError("Could not send frame to the encoder.", ret);
  
      }
  
  
  
      while (ret >= 0)
  
      {
  
          ret = libffmpeg::avcodec_receive_packet(context, packet);
  
          if (ret == AVERROR(EAGAIN) || ret == AVERROR_EOF)
  
          {
  
              return;
  
          }
  
          else if (ret < 0)
  
          {
  
              ThrowError("Error during encoding.", ret);
  
          }
  
  
  
          libffmpeg::fwrite(packet->data, 1, packet->size, file);
  
          libffmpeg::av_packet_unref(packet);
  
      }
  
  }

  The original image and this is a screenshot of the video. In the screenshot the area around the text is blurry which causes a flickering effect when played. How can I prevent this from happening ?

• Insert Random silence multiple times between Audios in ffmpeg

  23 septembre 2021, par Gracie williams

  suppose I have MP3 of around 1 minute duration and increase its duration longer.Duration can be any.

  


  I want to randomly add N seconds of silence to that audio.

  


  I can generate the silence like below

  


  ffmpeg -f lavfi -i anullsrc=r=44100:cl=mono -t <seconds> -q:a 9 -acodec libmp3lame out.mp3&#xA;</seconds>

  


  And I can insert silence to audio like below

  


  ffmpeg -i in.wav -filter_complex "anullsrc,atrim=0:4[s];[0]atrim=0:14[a];[0]atrim=14,asetpts=N/SR/TB[b];[a][s][b]concat=3:v=0:a=1" out.wav


  


  Is there anyway to insert silence at multiple areas.With php I can generate random time to insert based on 60 seconds audio like below.

  


  $arr = [];
$arr[3] = 2; //here 3 is original audio insert time and 2 is the duration of silence
$arr[6] = 3;  //here 6 is original audio insert time and 3 is the duration of silence
$arr[10] = 1;
$arr[12] = 3;


  


  Now I want to insert silence between seconds based on above array like

  


  2 seconds silence at 3rd second
3 seconds silence at 6th second
1 second silence at 10th second


  


  and son on..

  


• Fast seeking ffmpeg multiple times for screenshots

  7 mars 2017, par user3786834

  I have come across http://askubuntu.com/questions/377579/ffmpeg-output-screenshot-gallery/377630#377630, it’s perfect. That has done exactly what I wanted.

  However, I’m using remote URLs to generate the screenshot timeline. I do know it’s possible to fast seek with remote files using https://trac.ffmpeg.org/wiki/Seeking%20with%20FFmpeg (using -ss before the -i) but this only runs the once.

  I’m looking for a way to use the

  ./ffmpeg -i input -vf "select=gt(scene\,0.4),scale=160:-1,tile,scale=600:-1" \
  
  -frames:v 1 -qscale:v 3 preview.jpg

  command but using the fast seek method as it’s currently very slow when used with a remote file. I use PHP but I am aware that a C method exists by using av_seek_frame, I barely know C so I’m unable to implement this into a PHP script I’m writing. So hopefully, it is possible to do this directly with ffmpeg in the PHP system() function.

  Currently, I run seperate ffmpeg commands (with the -ss method) and then combine the screenshots together in PHP. However, with this method it will be refetching the metadata each time and a more optimized method would be to have it all happen in the same command line because I want to reduce the amount of requests made to the remote url so I can run more scripts in sequence with each other.

  Thank you for your help.