While on vacation near Devil’s Thumb Ranch I decided to try some time lapse photography. I wrote a bash script that took pictures on a laptop a few years ago. I ran this script using cron, but was looking for something with finer grained control. I found a Tenderlovemaking post that shows how to take web cam photos using Ruby with the AVCapture framework. Note that this appears to only work on OSX.

I modified the script from Tenderlove’s av_capture gem’s github page:

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require 'av_capture'

session = AVCapture::Session.new
dev     = AVCapture.devices.find(&:video?)

session.run_with(dev) do |connection|
  10000.times do |i|
    counter = i.to_s.rjust(4, "0")
    File.open("img_#{counter}.jpg", 'wb') do |f|
      f.write connection.capture
    end
    sleep 5
  end
end

I left fill with zeros so that the files will be img_0000.jpg, img_0001.jpg … If you want to increase the number of times we take photos (let’s call it N) then change the 4 on line 8 to be log_10(N).ceil. So if we want to take 15,000 photos:

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    counter = i.to_s.rjust(5, "0")

Now that we ran the script and generated a ton of sequentially numbered photos, how do we turn them into a movie? I used FFMPEG on a few projects in graduate school a few years ago and figured that it would be the right tool for the job.

To install it on OSX (requires brew):

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brew install ffmpeg

Now to turn the photos into a movie:

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ffmpeg -r 60 -i img_%04d.jpg output.mp4

The -r 60 flag sets the video frame rate to 60 frames per second.

I ran this script for about 3.5 hours starting in the early afternoon on October 6th facing towards Devil’s Thumb, a local geological feature. Here is the result:

I started with 30 second intervals, but found that there was too much movement between frames. I changed the script to use a 5 second delay, which seemed to result in smoother transitions. Also I used YouTube’s image stabilization post-processing feature. This is why the video appears to pan around a little. Occasionally the web cam took pictures that appeared to be too light. These appear as a white flash in the video. I identified these and copied the next image in the sequence over the white image.