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• ffmpeg audio and video out of sync when using the raw format

  29 juillet 2016, par Mikhail Novikov

  I have 2 .webm files, one contains video, the other one audio. I need to produce a single mp4 file containing both video and audio, in sync. Here is what i try to do :

  ffmpeg -ss 00:00:00.33 -i user_17624_1-1762.webm -i user_17624_2-1767.webm -r 15 output_good.mp4

  -ss by 0.33s is because video started 0.33s before audio - 1762 and 1767 are frame numbers, so they are 5 frames off, and 15 fps.

  Doing this, i get a perfect recording where video and audio are in sync over the entire file.

  But the thing is, i need to do some alterations of the video frames, and i do it my a C app manipulating the bytes in a raw video stream, so i can’t do it this way, i need to transcode to RAW first, then manipulate, then encode mp4 back from the raw. Removing my C app from the chain to make sure it’s not the problem, it boils down to :

  ffmpeg -ss 00:00:00.33 -i user_17624_1-1762.webm -an -r 15 -f rawvideo -pix_fmt yuv420p -s 1920x1080 ./video.pipe &
  
  ffmpeg -i user_17624_2-1767.webm -vn -ar 44100 -ac 2 -f s16le -acodec pcm_s16le ./audio.pipe &
  
  ffmpeg -an -r 15 -f rawvideo -pix_fmt yuv420p -s 1920x1080 -i ./video.pipe -vn -ar 44100 -ac 2 -f s16le -acodec pcm_s16le -i ./audio.pipe -r 15 output_bad.mp4

  resulting in yes, recording where audio and video are way off from each other, and seems to be also fluctuate over the file duration.

  What am i doing wrong with these video transcodings ? Help me please :)

  Mikhail

• Extract list of specific frames using ffmpeg

  8 juillet 2016, par John Allard

  I’m trying to use ffmpeg on a video to extract a list of specific frames, denoted by their frame numbers. So lets say I want to extract just one frame from ’test_video.mp4’, frame number 150 to be exact. I can use the following command

  ffmpeg -i test_video.mp4 -vf "select=gte(n\, 150)" -vframes 1 ~/test_image.jpg

  But what if I want to exact a list of frames, such as [100, 110, 127, 270, 300] ? I have been looking at this page (https://ffmpeg.org/ffmpeg-filters.html#select_002c-aselect) and can see that there are ways to extract multiple frames based on parity of frame number, timestamps, etc, but I can’t find the syntax for doing what I need to do.

  Ideally I would extract the list of given frames under the naming convention out_image%03d.jpg where the %03d is replaced by the given frame number, although a timestamp would also work.

  Is there a way to do this ?

• FFMPEG, Blur an area of a video using Image Select Areas Plugin

  12 juillet 2016, par Drupalist

  I am building an online video editor. I need to allow the users to blur an area of movies. This must be done graphically, I mean user should be able to select an area of a video, using an screenshot, then the selected area must be blurred. Some thing like this

  

  Is there anyway to map this selected area dimension and its distance from borders to the real values that must be applied to the video ?

  I mean four numbers, width, length, top, left will be provided using this plug in and I need to use these numbers to blur an area of videos.

  I take an screenshot of video. In order to keep the aspect ratio, I will fix the width to 800px and let the height to be scaled up/down. It is clear that the width, length, top, left of the selected area of the screenshot is a factor of the width, length, top, left of the area that must be blurred in video. but I don’t know how to get this factor. Besides that I don’t know how to get the video resolution.

  Thanks in advance.

  ---

  Update

  This is my PHP code that gets the selected area dimensions and offsets

  $iLeft = $_POST['left'];
  
  $iTop = $_POST['top'];
  
  $iWidth = $_POST['width'];
  
  $iHeight = $_POST['height'];
  
  exec('ffmpeg -i '.$url.' -filter_complex "[0:v]scale=iw*sar:ih,setsar=1,split[bg][bb];[bb]crop='.$iWidth.'*iw/800:iw*'.$iHeight.'/800:'.$iWidth.'*iw/800:'.$iHeight.'*iw/800,boxblur=10[b0];[bg][b0]overlay='.$iLeft.'*W/800:'.$iTop.'*W/800"" '.$name.' > block.txt 2>&1');

  $iLeft, $iTop, $iWidth, $iHeight are the left, top, width and height of the selected area via the image plugin.

  It blurs many selected areas very well, but areas like this

  

  The Image Left, Top, Width, Height is 257,  39.26666259765625,  10,  391

  don’t get blurred. Also a video with Dimension 207x207 didn’t get blurred as well.