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• FFmpeg programming : exr image too dark [on hold]

  2 juillet 2017, par Francis Tesla

  Using the fantastic FFmpeg library to load images all exr files appears dark.
  I am almost sure that’s involve converting the resulting AVFrame or AVPicture from linear to sRGB but I don’t know how to do it.

  Here is how the EXR file is loaded into an AVPicture :

  AVFrame *frame = av_frame_alloc();
  
  avcodec_decode_video2(codecCtx, frame, &frameFinished, &packet);
  
  
  
  ... 
  
  
  
  AVPicture       picture;
  
  avpicture_alloc(&picture, dest_format, dest_width, dest_height);
  
  
  
  SwsContext *swsCtx = NULL;
  
  
  
  swsCtx = sws_getContext(source_width, source_height, source_format, dest_width, dest_height, dest_format, SWS_POINT, NULL, NULL, NULL);
  
  
  
  if (swsCtx == NULL) {
  
      qDebug() << "error calling sws_getContext";
  
  }
  
  
  
  sws_scale(swsCtx, frame->data, frame->linesize, 0, frame->height, picture.data, picture.linesize);
  
  
  
  QImage image(dest_width, dest_height, QImage::Format_ARGB32);
  
  
  
  for (int y = 0; y < dest_height; ++y) {
  
      memcpy(image.scanLine(y), picture.data[0] + y * picture.linesize[0], picture.linesize[0]);
  
  }
  
  
  
  av_frame_free(&frame);
  
  sws_freeContext(swsCtx);

  Regards,

  Francis

• Why does ffmpeg have bigger latency on dark images ?

  19 novembre 2017, par doodoroma

  I have a c# application to stream real camera images using ffmpeg. The input images are in raw, 8-bit gray-scale format. I created an ffmpeg stream using the standard input to feed the images and send the output packages to websocket clients.

  I start an external ffmpeg process using this config :

  -f rawvideo -pixel_format gray -video_size " + camera.Width.ToString() + "x" + camera.Height.ToString() + " -framerate 25 -i - -f mpeg1video -b:v 512k -s 320x240 -

  Typical image size is 1040*1392 pixels

  I display the stream on the browser, using jsmpeg library

  This works with a reasonable latency ( 500ms on localhost), but when the camera image is really dark (black image), the latency is extremely big ( 2-3 seconds on localhost). When there is something bright on the image again after a black period, it takes 2-3 seconds to "synchronize".

  I was thinking that black images are really easy to compress and will generate really small packages, jsmpeg has almost no information to display and wait until a complete data package arrives, but I couldn’t prove my theory.

  I played with ffmpeg parameters like bitrate and fps but nothing has changed.

  Is there any settings which I could try ?

• why does the order of arguments matter in here ?

  26 juin 2013, par Pooya

  as far as I can remember, the order of the command arguments did not matter but here I see something else :

  ffmpeg -f avi -i "s02e01.mkv" vcodec mpeg4 -b 4000k -acodec libmp3lame -ab 320k "converted.avi" 

  and this is the result :

  s02e01.mkv: Operation not permitted

  but after changing the place of "-f" and "-i" everything goes ok :

  ffmpeg -i "s02e01.mkv" -f avi vcodec mpeg4 -b 4000k -acodec libmp3lame -ab 320k "converted.avi" 

  what is the problem here ?