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• Capturing FFmpeg output in python before storing it

  14 juin 2012, par tarrasch

  I am trying to capture output from ffmpeg and storing that in memory, deciding at a later stage if I want to write it to disk.

  Is it possible to analyze the output of ffmpeg before writing it onto the HDD ?

  Since I do not really know how to approach the thing, I cannot yet provide a code example.

  EDIT :
  Yes, I am trying to write small chunks of the captured avi/mp4 to RAM and analyze them before writing to disc.

• Giving Thanks For VP8

  25 novembre 2010, par Multimedia Mike — VP8

  It’s the Thanksgiving holiday here in the United States. I guess that’s as good a reason as any to release a first cut of my VP8 encoder. In order to remind people that they shouldn’t expect phenomenal quality from it — and to discourage inexperienced people from trying to create useful videos with it — I have hardcoded the quantizers to their maximum settings. For those not skilled in the art, this is the setting that yields maximum compression and worst quality. When compressing the Big Buck Bunny logo image, the resulting file is only 2839 bytes but observe the reconstructed quality :

  
  
  It really just looks like a particularly stormy day in the forest.
  

  First VP8 File From An Independent Encoder
  I found a happy medium on the quantizer scale and encoded the first 30 seconds of Big Buck Bunny for your inspection. I guess this makes it the first VP8/WebM file from an independent encoder (using FFmpeg’s Matroska muxer as well).

  Download : bbb-360p-30sec-q40.webm ( 13 MBytes)

  I think the quality makes it look like it was digitized from an old VHS tape.

  For fun, here’s the version with the quantizer cranked to the max : bbb-360p-30sec-q127.webm ( 1.3 MBytes)

  Aside : I was going to encapsulate the video in this post using a bare HTML5 <video> tag for the benefit of the small browsing population who could view that (indeed, it works fine in Chrome). But that would be insane due to the fact that supporting browsers preload the video with no easy (read : without the help of JavaScript) method for overriding this unacceptable default.

  The Code
  I’m still trying to get over my fear of git. To that end, I have posted the code on Github :

  https://github.com/multimediamike/ffvp8enc

  I still don’t like you, git. But I’m sure we’ll find some way to make this work.

  Other required code changes in the basic FFmpeg tree :

  • Of course, copy vp8enc.c into libavcodec/
  • In libavcodec/allcodecs.c, ’REGISTER_DECODER (VP8, vp8);’ turns into ’REGISTER_ENCDEC (VP8, vp8);’
  • Add ’OBJS-$(CONFIG_VP8_ENCODER) += vp8enc.o’ to libavcodec/Makefile

  Further Work
  About the limitations and work yet to do :

  • it’s still intra-only, no interframes (which is where a lot of compression occurs)
  • no rate control or distortion optimization, obviously
  • no intra 4x4 coding (that’s close to working but didn’t my little T-day deadline)
  • no quantization control ; this should really be hooked up to the FFmpeg command line but I’m not sure how
  • encoder writes into a static-sized, 1/2 MB memory buffer ; this can overflow
  • code is a mess (what did you expect at this stage of the game ?)
  • lots and lots of other things, surely

• bash script to start/stop ffmpeg x11grab

  28 juin 2012, par lunacafu

  A application is storing its status in a file on the server /temp/status.txt. There are 4 status possible : wait, ready, recording, finished. With a cronjob I would like to start a shell script that checks this file and take action as follows :

  • wait -> do nothing
  • ready -> execute ffmpeg -f x11grab -s cif -r 25 -i :0.0 /tmp/out.mpg. Now while ffmpeg captures, the status.txt file still has to be checked for a change to the status,
  • recording -> do nothing
  • finished -> send q command to ffmpeg process to quit capturing process and finish script.

  I tried to solve it with expect and xdotool to realize that this absolutely the wrong way. My problem is that I acutally have no experience in bash and handling processes, child processes etc.