Dual Photography Lets You Virtually Move a Camera for Impossible Photos

Want to see some mind-blowing research into photography (from the mid-2000s)? Check out the video above about “Dual Photography,” a Stanford-developed technique that allows you to virtually swap the locations of a camera and a projector, allowing you to take pictures from the perspective of the light source instead of the camera sensor.

The idea is so crazy that it almost seems like an April Fools Joke — except it’s real. By using science — specifically a principle called Helmholtz reciprocity — scientists are able to calculate what the light would have been like had the positions of the camera and projectors been switched.

Here’s an example they give: first, they start with a simple subject with a camera on one side and a projector at a roughly 90-degree angle:


Here are the two photographs that they were able to capture from the scene without touching anything in the setup:


What what? Your eyes are not mistaken: those are two different perspectives captured with a single fixed camera. By carefully projecting and calculating light from the projector, the scientists are able to generate what the projector sees!

The photograph on the left is called the “primal image,” or what a traditional camera would capture. The one on the right is called the “dual image.”

Just in case that example wasn’t mind-blowing enough, here’s an even crazier one: the scientists created a setup in which a camera could only see the back of a playing card and the bounced light coming from an open book.


A standard camera (and human eyes) would never be able to see the face of the card, since… you know… it’s turned away:


However, by controlling and calculating the light given off by the projector and bouncing off the book, the scientists are able to recreate a photograph showing the face of the card!


Additional applications of this technique include taking pictures without a camera (using only a “photo resistor”), and relighting a scene after it has been captured.

As we noted earlier, this research is already nearly a decade old, so it will be interesting to see if major real world applications start arriving soon.

(via Stanford via Reddit)

  • Brad Trent

    Mind. Blown.

  • Bryan Hanna

    I’m pretty sure thats witchcraft…

  • Dan Spencer

    Wait… what???

  • Arunwong Opastpongkarn


  • Peter Neill

    okay, this is utterly insane – there is no way this tech is not in use already by the NSA etc, it surely must have been picked up, this is truly scary and incredible at the same time

  • Brian Lee

    I love these kinds of posts. The only way I would improve upon this is by making the process of pixel scanning faster. Let’s say you had 2073600 pixels to scan (1920×1080). With a 120 fps camera using brute force (like how the video mentioned), it would take 4.8 hours! With their specific algorithm, it would be less than that!

  • pokerbri

    Isssss THIS your card?!?

  • Gabriel

    It is really cool, especially the flexibility to control lighting in the reconstructed image. But this technique won’t make it possible to virtually move the camera to an impossible angle, just swap it with the projector. Also, it’s important to use a (corrently focused) projector that can project an arbitrary image. A single light bulb won’t do.

  • superduckz

    You beat me to it. I can’t believe that this is not being used in forensics today.

  • Hunter

    What’s NSA going to use it for? You can only do the technique if you can take inputs from the light source. It would be crazy though if scientists could figure out an algorithm for natural light, but this seems highly unlikely given all the constantly changing variables in the real world. My guess is Pixar/Disney and EA are the highest bidders.

  • Pryere Coleman

    Errr, can’t compute.

  • Nick Miners

    What it doesn’t show is that the card was actually the queen of clubs

  • SteveO

    Wasn’t this supposed to be posted April 1???

  • Jason Kessenich

    I’m with Dan Spencer below….. what???