nullprogram.com/blog/2011/11/28/
I’ve done all my video editing in a very old-school, unix-style way. I
actually have no experience with real video editing software, which
may explain why I tolerate the manual process. Instead, I use several
open source tools, none of which are designed specifically for video
editing.
The first three are usually available from your Linux distribution
repositories, making them trivial to obtain. The last one is easy to
obtain and compile.
If you’re using a modern browser, you should have noticed my
portrait on the left-hand side changed recently (update: it’s been
removed). That’s an HTML5 WebM video — currently with Ogg Theora
fallback due to a GitHub issue. To cut the video down to that portrait
size, I used the above four tools on the original video.
WebM seems to be becoming the standard HTML5 video format. Google is
pushing it and it’s supported by all the major browsers, except
Safari. So, unless something big happens, I plan on going with WebM
for web video in the future.
To begin, as I’ve done before, split the video
into its individual frames,
mplayer -vo jpeg -ao dummy -benchmark video_file
The -benchmark option hints for mplayer to go as fast as possible,
rather than normal playback speed.
Next look through the output frames and delete any unwanted frames to
keep, such as the first and last few seconds of video. With the
desired frames remaining, use ImageMagick, or any batch image editing
software, to crop out the relevant section of the images. This can be
done in parallel with xargs’ -P option — to take advantage of
multiple cores if disk I/O isn’t being the bottleneck.
ls *.jpg | xargs -I{} -P5 convert {} 312x459+177+22 {}.ppm
That crops out a 312 by 459 section of the image, with the top-left
corner at (177, 22). Any other convert filters can be stuck in there
too. Notice the output format is the
portable pixmap (ppm),
which is significant because it won’t introduce any additional loss
and, most importantly, it is required by the next tool.
If I’m happy with the result, I use ppmtoy4m to pipe the new frames
to the encoder,
cat *.ppm | ppmtoy4m | vpxenc --best -o output.webm -
As the name implies, ppmtoy4m converts a series of portable pixmap
files into a
YUV4MPEG2
(y4m) video stream. YUV4MPEG2 is the bitmap of the video world:
gigantic, lossless, uncompressed video. It’s exactly the kind of thing
you want to hand to a video encoder. If you need to specify any
video-specific parameters, ppmtoy4m is the tool that needs to know
it. For example, to set the framerate to 10 FPS,
... | ppmtoy4m -F 10:1 | ...
ppmtoy4m is a classically-trained unix tool: stdin to stdout. No
need to dump that raw video to disk, just pipe it right into the WebM
encoder. If you choose a different encoder, it might not support
reading from stdin, especially if you do multiple passes. A possible
workaround would be a named pipe,
mkfifo video.y4m
cat *.ppm | ppmtoy4m > video.y4m &
otherencoder video.4pm
For WebM encoding, I like to use the --best option, telling the
encoder to take its time to do a good job. To do two passes and get
even more quality per byte (--passes=2) a pipe cannot be used and
you’ll need to write the entire raw video onto the disk. If you try to
pipe it anyway, vpxenc will simply crash rather than give an error
message (as of this writing). This had me confused for awhile.
To produce Ogg Theora instead of WebM,
ffmpeg2theora is a great tool. It’s
well-behaved on the command line and can be dropped in place of
vpxenc.
To do audio, encode your audio stream with your favorite audio encoder
(Vorbis, Lame, etc.) then merge them together into your preferred
container. For example, to add audio to a WebM video (i.e. Matroska),
use mkvmerge from MKVToolNix,
mkvmerge --webm -o combined.webm video.webm audio.ogg
Extra notes update: There’s a bug in imlib2 where it can’t read PPM
files that have no initial comment, so some tools, including GIMP and
QIV, can’t read PPM files produced by ImageMagick. Fortunately
ppmtoy4m is unaffected. However, there is a bug in ppmtoy4m
where it can’t read PPM files with a depth other than 8 bits. Fix this
by giving the option -depth 8 to ImageMagick’s convert.