Finish Line Cameras and Slit Scan Photography

Yesterday we reported on how US Track and Field saw its first “photo finish” tie this past weekend in an Olympic qualifying race. If you thought the finish line photo looked strange, it’s not just you: it’s not an ordinary photo. Journalist Daniel Rutter has written up a great article on how finish line cameras work:

[…] most finish-line cameras aren’t super-high-speed movie cameras, but instead a kind of slit camera. A slit camera has a line-shaped lens, which exposes the film or electronic sensor line by line or column by column, not unlike the way a rolling shutter works. The critical difference, though, is that a slit camera can keep on going past the lens indefinitely. You can keep collecting image data, or keep spooling film past the slit, for as long as you have memory or film. The shutter never closes as long as the film or memory lasts, so it’s impossible to miss any action between the frames.

[…] imagine taking a flatbed scanner sensor and setting it up vertically, looking across a racetrack at the finish line. Start a “scan”, and it’ll authoritatively tell you when every body-part of every runner makes it to the finish, by simply showing that part of that person before any part of anyone else. The speed of the scan should be set to roughly match the speed of the runners, so they look generally the right shape, but any part of any runner that stays stationary relative to the scan rate – a foot on the ground, for instance – will seem long. Any part that’s moving forward relative to the scan rate – a hand or foot coming forward, for instance – will seem short. Even if you mess up the scan rate so everyone looks wide or narrow, whatever part of whatever runner shows up first in the scan is the first to cross the finish line.

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Image credit: Slitcam-Math by VanDerMouche

  • Carlos Garcia

    So I have a question…isnt the foot of runner 2 ahead of any other body part of runner 1? And if so, why did they tie? Or is this not the photo that caused the tie in the first place? 

  • Michael Zhang

    The races are determined by seeing whose torso passes the finish line first :)

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  • Kamil Sladek

    The cited article actually doesn’t describe the method very well since it confuses by randomly comparing terms like rolling shutter, “line shaped lens” (that’s non-sense), etc.

    You can make your own slit camera out of any video capable digital camera with a regular sensor and a regular lens. All you need to do is the following:

    1. record a video of your action
    2. extract each frame as an individual image (the opposite to what you would do for a time lapse)
    3. extract a vertical single pixel wide line from each image (for example a line from the center)
    4. stack those lines horizontally from left to right to form an actual “slit scan” image

    This can be automated by tools like e.g. ImageMagick and the longer your initial video was, the wider your image will be. In fact, the width of your slit scan image will have exactly the same amount of pixels as your initial video’s frame number.

    Now, to go one step further you can proceed for all the other vertical lines of your images and create one slit scan image for each particular set of vertical lines. This will give you a set of as many slit scan images as your initial video was wide in pixels. Combining that set of slit scan images to a video (this time exactly as in a time lapse) your result can look like this:

    Give it a try ;)

  • koferk

    check out my first try with slit scan
    this the same only with 60 lines..

    which one you liked better ?

    can anyone tell me why i’m getting those weird “waves” ?
    anyways, its cool try it out