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• advanced ffmpeg compression control

  18 juillet 2017, par Daniel Mahler

  I am using very aggressive video compression, eg -crf 51. I am using this for ’artistic’ effect, so what I am doing may not make sense from a normal video compression point of view.

  So far I have only been using very basic compression control using only the -crf or -b:v flags. The results look like ffmpeg divides images into square patches and the makes smooth approximations within the patches. This gives 2 control dimensions to the process : the patch size and the aggressiveness of the smoothing within the patches.

  It have found that ffmpeg uses both parameters to some extent, but there appears to be an absolute maximum patch size in pixels beyond which it will not go regardless of the frame size.
  After that it will only increase compression by reducing the detail within the patches.

  This is suboptimal for high resolution video, where this becomes equivalent to reducing the resolution. The problem is particularly noticeable on fractal like images which have large featureless region as well as regions of high detail.

  How can I tell ffmpeg to increase the maximum patch size and retain more detail within the patches ?

• lavf/ftp: check for truncation in snprintf

  25 novembre 2021, par Anton Khirnov

  lavf/ftp: check for truncation in snprintf
  Silences e.g. the following warning in gcc 10 :
  
  src/libavformat/ftp.c : In function ‘ftp_move’ :
  
  src/libavformat/ftp.c:1122:46 : warning : ‘%s’ directive output may be truncated writing up to 4095 bytes into a region of size 4091 [-Wformat-truncation=]
  
   1122 |     snprintf(command, sizeof(command), "RNTO %s\r\n", path) ;
  
        |                                              ^  
  
  src/libavformat/ftp.c:1122:5 : note : ‘snprintf’ output between 8 and 4103 bytes into a destination of size 4096
  
   1122 |     snprintf(command, sizeof(command), "RNTO %s\r\n", path) ;
  
        |     ^ 
  

  • [DH] libavformat/ftp.c

• Low performance when extracting frames from video

  25 février 2016, par Rakatan

  I am writing a video processing application and have hit a bit of a snag.

  The idea is to extract frames (images) from a video file, process them on the GPU and then write them back to a new video file.

  So far i have been successful in doing this using JavaCV.

  The only problem is that extracting the frames takes a very long time (in the 200ms region). This is way to much, as added to the GPU processing time and the time it takes to write to a new file, it results in a total process duration of about 700ms per frame.

  This is how i grab the frames :

   FFmpegFrameGrabber frameGrabber = new FFmpegFrameGrabber(videoLocation);
  
   frameGrabber.start();
  
  
  
   frameGrabber.setFrameNumber(frameNumber);
  
   frame = frameGrabber.grabImage();

  Pretty standard, nothing special, and it works. I am able to process the frame, convert it to a bitmap, etc...

  As i have said before, the bottom two instructions (setting the frame number and grabbing the image) take a very very long time. Writing the frames also takes a fair bit.

  I would greatly appreciate if anyone has any input regarding this issue. Maybe i’m not setting up something correctly, maybe there is another solution that is faster.