## Tuesday, October 25, 2011

### Time Lapse Fun with FFMPEG and Mencoder

I was in the office early this morning and saw the sun rising. I captured some video of it (my phone can do 30 minutes of 720p, though it doesn't look that good). However, the thirty minutes of video is rather boring to watch in real time. So I decided to speed up the video.

Speeding up the video was actually much more difficult that I thought it would be (and I am somewhat acclimated to the troublesome nature of FFMPEG and Mencoder). Anyway, in order to perform the speed up I used the FFMPEG filter select to only pass every nth frame. The large the n, the faster the speed up. At the same time I chose to convert to WebM (the phone puts out H.264+AAC and placed in an MPEG4 container format). This produces a video file with only every nth frame, but the video just hangs for the time it would have been displaying the other frames. To fix this, I then passed it through another program, mencoder. This program fairly similar to ffmpeg in purpose. The mencoder program has a special option, -speed, that will speed up the video stream by a factor you specify. This means that if we use -speed n then we should get the desired video frame rate. Refer to the man pages and FFMPEG manual for more insight for what is happening here.

ffmpeg -i VID_20111025_071454.m4v -vf select='not(mod(n\,50))' -an temp.webm
mencoder -ovc copy -oac copy -speed 50 temp.webm -o faster.webm


This would have been all it would take, however, since I am using Maverick Meerkat, the version of FFMPEG included doesn't have support for filters. This means that, ugh, you can't use the first command. Not unless you install a newer version. I avoid compiling libraries from source like the plague. Instead I added this PPA to my software sources and updated my software. This actually gave a lot of scary errors and almost tricked me into a dist-upgrade to Natty, (shame on you apt, I wouldn't do that to my worst enemy). But in the end, those commands did work (as evidenced by the videos).

The end result, we have a video where only every 50th frame show up in the output. This resulted in a pretty good effect. I didn't actually point the camera at the sunrise as that would cause problems with the contrast and I couldn't balance the camera on the window sill pointing that direction from my office. So it's not as dramatic as an actual sunrise.

I also captured another 30 minutes later that morning as the clouds forming as the air passed off the flatirons was a pretty cool dynamic effect.

Of course, all of this is kind of stupid (neat, to me, but stupid), as time-lapse is usually done by taking individual frames spaced seconds to minutes (or more) apart. Also, there are several options available on the Android Market. Though I wouldn't install them without researching the application and developers.

1. Hey there, thanks for your share.

What I ended using after some fairly extensive R&D was the following:

ffmpeg.exe -i $source_file -vf select='not(mod(n\,$frammes_to_skip))',setpts='N/(29.97*TB)' -an -y $target_file This gets the job done in one line and keeps the framerate in$target_file normal.

HTH

1. Typesetting screwed up some of your post and until I figure out how to fix it, this is what Kyle posted to anybody who wanders past:

...
ffmpeg.exe -i source_file -vf select='not(mod(n\,frames_to_skip))',setpts='N/(29.9*TB)' -an -y target_file

This gets the job done in one line and keeps the framerate in target_file normal.
...

the phrases source_file, frames_to_skip, and target_file need to be replaced.