A colleague wanted to edit a video (actually, three) for a presentation she intended to make in a laptop with Windows Vista and MS Office 2007. The video was a WMV, and the required edition included removing fragments, changing the speed of one of the fragments (and join it back with the others), and adding a soundtrack.
She could not, for the life of her, edit the damned thing on a Windows computer.
Why, Free Software, of course.
The motivation of this post
To help dispell two ideas: that “Windows is easy”, and that “With Linux, you waste your time finding out how to do things”.
Part I – Linux
OK, so I proposed her to use some Free Software called Avidemux. Our first problem was that apparently Avidemux was unable to cut the video in pieces (it crashed at the attempt). After much perusing, and using the humble file command, I found out the reason: the WMV had no playing FPS set. Players, like MPlayer would reproduce it by guessing 25 or 30 frames per second, but editors need a precise value to count on. I readily fixed it by reencoding the video to 25fps with MEncoder:
% mencoder in.wmv -ovc lavc -nosound -fps 25 -lavcopts vcodec=wmv1 -o out.avi
Once a proper FPS given, I used Avidemux to split the file. However, I encountered a second problem: I couldn’t split the file anywhere. I could only cut it at points 10 seconds appart. I had to sweat a bit more to fix that, but I also learned something more in the way. Most (all?) compressed video formats use at least two kinds of frames: normal frames and keyframes. The latter are the frames where any player can seek to in the video. According to the man page of MEncoder:
maximum interval between keyframes in frames (default: 250 or one keyframe every ten seconds in a 25fps movie. [...] Keyframes are also needed for seeking, as seeking is only possible to a keyframe – but keyframes need more space than other frames, so larger numbers here mean slightly smaller files but less precise seeking. 0 is equivalent to 1, which makes every frame a keyframe. [...]
So here you are: the problem was the default value of some variable called keyint. To make the video seekable to any frame (so it could be cut at any point), I set keyint to 1:
% mencoder in.wmv -ovc lavc -nosound -fps 25 -lavcopts vcodec=wmv1:keyint=1 -o out.avi
Once the movie was split into parts with Avidemux, and the unwanted parts were removed, the next step consisted on playing one fragment faster. The problem here is that I don’t know how to make a variable FPS video, so we had to make it so all the video played at the same FPS, but a part was faster. How? Removing frames, of course. I used MPlayer to deconstruct the relevant fragment into individual frames (in PNG format):
% mplayer -vo png:z=2 fragment
The command above generates a whole lot of 0000xxxxx.png files, with frames ordered by the number in the filename. Next, I deleted every second frame. How? With a stupid GUI I don’t know, but from the command line it is trivial:
% rm -f 00*.png
Now, I just re-constructed the video with half the frames, to get an effectively double-speed video, with same FPS as original:
% mencoder "mf://*.png" -mf fps=25 -o output.avi -ovc lavc -lavcopts vcodec=wmv1
If I am allowed to say it, the effect is really great. You wouldn’t tell the sped-up video from the original, except from the increased play speed.
Using Avidemux for joining the video fragments was a breeze, and it could even be done from the command line:
% avidemux fragment1.avi --append fragment2.avi --append fragment3.avi --save total.avi --quit
The last (Linux) part consisted on adding a soundtrack, which Avidemux can do, from a MP3, WAV, or another video. This was easy.
Part II – Windows Vista
OK, the last Linux step consisted on reencoding the video in some format that Vista could read. This was no immediate task, but after some tests, we made it. Windows Media Player could reproduce the movie with no problem.
Finally, we opened the wonderful Office 2007 in the shiny and new Vista laptop, and created a PowerPoint slide to insert the video (the rest of the presentation was already done). Everything seemed to work, but when we played the presentation, we discovered that either the video or the sound could be played (depending on how we had encoded the video in Linux), but not both simultaneously. WMP would play the videos just fine, but the embedded player in PowerPoint would not… go figure why. After at least 3 crashes of Office (yes, Office crashes), some bitching because we could not make any sense of the new Office interface (we are experienced pre-Vista and Linux users, and Windows is for idiots, right? We must be idioter than average) having to stand the fact that the semitransparent border of a window refused to disappear when we closed it (so we kept working with a blue-greenish stripe across a part of the desktop), and one Windows reboot (yes, Vista still hangs from time to time), we managed to insert and play the darned video. How? We just inserted two videos: one for which only the audio was playing, and another one for which only the image was showing. We then make these two objects to kick off at the same time, et voila!. Not the cleanest of solutions, but with Windows “everything just works”, right?
The moral of the first step (the FPS not being set) was that I had to play around for a while with my Linux tools, but the culprit was MS, and their lousy WMV. I have never produced a video with no FPS (and all other necessary metadata) set, because my FLOSS tools do it automatically. Secondly, I didn’t waste my time. Thanks to the usefulness of the FLOSS tools, I ended up learning something about movies, FPSs, and that they are required. I also learned about key frames, and seeking and cuting video streams.
On the other hand, for a much simpler job, we spent relatively (and maybe absolutely) longer with Windows, and we did lose our time with it. The problems we encountered with Linux were difficulties of the situation itself: the original WMV was flawed, the AVI we created had too high an inter-keyframe interval… and the FLOSS tools we used helped us fix them and learn in the process. In the case of Windows, the task was so simple, and all the problems we met were created by Vista. We didn’t learn anything from all of our struggle, because we only struggled against Windows (the GUI, the crashes, how to encode the video in Linux so that Vista could read it, why the darned Office would not play the audio or the video), not our problem (editing and embedding the file). All the time was devoted to learning how to overcome the limitations and errors of our tool, not to how to use our tool to perform some task, learning about the task itself in the process. Thus, it was wasted time.