Sunday, March 6, 2011

Turn an online webcam into a video log

Opponents of the transport of huge "megaloads" of oil-refining equipment through the wild mountains of Idaho and Montana need a way to monitor the progress (or not!) of the loads.  The Idaho and Montana Departments of Transportation operate webcams around the state that are available on their websites.  Volunteers have been staying up all night watching these webcams to keep tabs on (and potentially capture images of) the megaload convoys.

I wanted to relieve the night watchmen with an automated capture.  The method I used is outlined below.

Note:  This method is Macintosh-specific, though the principles could be applied in Linux or Windows.  If you craft a solution for other platforms, please include it in a comment on this post.

My plan was to capture the image (or the whole web page) into a file four times per hour, since the web-available images are updated every fifteen minutes.  I figured I needed a capture program, and a way to call it at intervals.

Capture the Page

The first step was easy: Googling "mac webcam capture" lead me to Paul Hammond's excellent script, "webkit2png".  The script requires the "python" programming language and the "PyObjC bridge" software, both included in Mac OS X 10.5 and later, but you don't have to know a thing about python or PyObjC to use webkit2png.  When you run from the command line, it accepts a URL, fetches the web page, and converts the entire page to a "png" image file, which is widely supported by browsers and other graphics programs.

To run this in an automated way, each file generated needs a unique file name.  webkit2png offers an option to append the date to the output filename.  Looking at the script, it wasn't hard to find the function that fetches the date.  I googled that function in python and found out how to modify it to include the time as well as the date.  With that one six-character mod, webkit2png was good to go for me.  [Note that this was simplified for me by the fact that webkit32png is a script, not a compiled program.  It is often takes less knowledge to modify a script than a program.]

The command I ended up with is:
python ~/bin/paulhammond-webkit2png-9c4265a/webkit2png -Fd -o Lolo -D ~/Desktop/+Lolo 
  • python is the program which will "interpret" (run) the script
  • ~/bin/....webkit2png is the script to run
  • -Fd tells webkit2png to append the date-time to the output filenale
  • -o Lolo gives the "stem" (first part) of the output filename
  • -D /Desktop/+Lolo is the directory into which to put the output file
  • http://...lolo.shtml is the URL to capture.
Launch At Intervals

The classic way to launch a command at certain times in Unix-derived systems like Mac OS X is to use the built-in program cron.  [Though Apple now uses and recommends their "launchctl" program, citing extended features, I'm an old Unix hand, and I don't need extended features, so I went with what I know.]  cron "wakes up" once per minute, and reads a control file (called crontab) that tells it what commands to run at specific times.    

crontab has a somewhat arcane syntax, so rather than relearn it, I downloaded Cronnix, which provides a convenient dialog box (in a "simple" or "advanced" mode) to create the entries.  In Cronnix I use:
  • min: 8,23,38,53 - one minute after the image is updated by the webcam
  • hour: 0-6, 18-23 - only record from midnight to 6am and 6pm to midnight (the megaloads only roll at night)
  • mday:  * - any day of the month
  • month: * - any month
  • Wday: * - any day of the week
  • Command: - as above
I clicked Cronnix's "Save" button, and that was it!

  1. The particular webcam I am logging uses a static URL for the actual image, so I could have fed the img "src=" URL to webkit2png instead of the web page URL.  In fact, any URL that return a file type that can be interpreted by webkit could be used.
  2. There are many other ways to accomplish the same thing.  This one was pretty simple for me to put up.  YMMV.

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