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• Extracting subtitles of a mkv file using ffmpeg while downloading

  19 août 2020, par Tim Untersberger

  I am trying to stream a mkv file to a webpage, which is being downloaded using WebTorrent. The video file has ASS/SSA embedded subtitles. I am using electron to display the video, which uses chromium without proprietary codec support, so I have to use the following html :

  


  <video controls="controls">&#xA;  <source src="video.mkv" type="video/webm">&#xA;<video></video>&#xA;</source></video>

  


  Since chrome supports webm and they use the same container, I can watch the video with audio. Webm doesn't support embedded subtitles, so the subtitles of the video get ignored.

  


  I am extracting the subtitles using the following command :

  


  ffmpeg -i video.mkv -map 0:2 sub.vtt


  


  This works fine, but requires the file to be downloaded completely. Is it possible to extract the subtitles while the file is being downloaded, so that I can begin streaming the video together with the subtitles as soon as possible ?

  


• ffmpeg : Create a fake shadow below alpha channel webm/png sequence

  6 mai 2021, par Beneos Battlemaps

  Purpose : I'd like to render out animated 3D meshes as png sequence to use them as animated tokens for virtual tabletop games. To make the mesh looks more natural I'd like to create a fake show beneath the actual token.

  


  Problem : I have a png sequence 1 (as well as a webm file created with ffmpet out of this png sequence if it makes it easier) with alpha channel. To create the webm I use :
ffmpeg -framerate 24 -f image2 -i Idle_Top.%04d.png -c:v libvpx-vp9 -crf 25 -pix_fmt yuva420p Idle_Top.webm (If its relevant). I'd like to render out the png sequence to a webm file that have the current images as well as the transparent shadow beneath the token combined.

  


  Possible workflow : I think a good way to achieve the wanted shadow effect is to use the alpha channel image as a mask on a black picture with the same resolution as the source image 2. Then you have a complete black version of the image. Then you need to place this image beneath the colored image and make a offset of 10px left and 10px down to create the ilusion of perspective 3. At the end the black image below the colored image must have a transparency as well ( 30% visibility should be enough) 4.

  


  Workflow overview

  


  Assets : I've put the webm file and the png files on my gDrive https://drive.google.com/drive/folders/1wznGaPwhKc2UyPpSZBSISa1gs3oixsHR?usp=sharing

  


  Though I work with ffmpeg on a regular basis I have no clue where to start. Can you please help me out with this interesting problem ?

  


  Best regards
Ben

  


• run ffmpeg commands from my own project

  28 octobre 2013, par bruno

  I'm starting a project where I want ppl to upload videos of a talk and a the video of slides for that talk and want to merge them (to play at the same time) and then show results.

  My question is :
  Is it possible to do that from code ? if it is, can you point me to the right doc ?
  I was able to do it running command line, but as I want this to run on a server with different ppl uploading their videos I think this would not be the best approach.
  I have a preference for Java if it's possible to do it, but I can manage to use other languages what do you guys suggest ?

  The idea would be to have a service where I can point the urls of the videos stored in my server and it would merge them and save file where I can later stream. With different ppl uploading videos at the same time and being able to watch the result in a reasonable amount of time.

  I used this tutorial to test :
  https://trac.ffmpeg.org/wiki/Create%20a%20mosaic%20out%20of%20several%20input%20videos

  Thanks for your time