Dual Photography
Why Hacker News recommends it
Cited whenever threads discuss recovering hidden images or non-line-of-sight imaging, as a clear demonstration of light transport.
What it is
The SIGGRAPH 2005 demo of dual photography from Stanford, which swaps the roles of projector and camera to render a scene from the light's point of view. It includes the reveal of a hidden playing card.
Recommended by 4 HN commenters across 4 different months, 2013 to 2026.
What Hacker News said
Strong rec uptown · 2014-05 · on “How taking pictures through windows can inadvertently reveal your location”
Dual Photography has always been the optical "trick" that's blown me away. Here's the demo video from Siggraph in 2005: [this video] Skip ahead to 4:19 for a succinct demo.
Recommends evanb · 2013-12 · on “Reflected hidden faces in photographs revealed in pupil”
This reminds me of dual photography [1], in which you trade many projectors and one camera for one projector and many cameras using time reversal symmetry. The video [2] gives a good explanation, and the revelation of the playing card is what strikes me as similar to revealing reflected hidden faces. [1] [LINK] [2] [this video]
Recommends pdkl95 · 2020-02 · on “TV Backlight Compensation”
It sounds like "Dual Photography". [LINK] "We present a novel photographic technique called dual photography, which exploits Helmholtz reciprocity to interchange the lights and cameras in a scene. With a video projector providing structured illumination, reciprocity permits us to generate pictures from the viewpoint of the projector, even though no camera was present at that location." I recommend watchign the video from the paper for good explains and demonstrates the technique. It's very impressive work. [this video]
Recommends saeranv · 2020-06 · on “In a photo of a black hole, a possible key to mysteries”
> Does anyone assert that any photon carries information about what it interacted with in the past? Does a photon have any information content other than its energy/wavelength? So "each one carries a bit of the image of whatever it hits" is total (ok, be nice now) poetic blarney. An analogy to how we perceive things seems appropriate. The light from some source (i.e the sun) hits an object, the light rays are then reflected/transmitted/refracted through the surface based on various material properties. Your eye, acting like a pinhole camera, captures all the light rays from that surface, and then projects it onto the sensor reception surface (I think the retina?) creating the 'image' that we
Cites uptown · 2014-08 · on “Extracting audio from visual information”
Reminds me a little of "Dual Photography" as presented at Siggraph in 2005. All the data you need to construct a new view is available if you know where and how to look: [this video]
Cites evanb · 2015-01 · on “An Algorithm That Unscrambles Fractured Images”
This reminds me of dual photography: [LINK] in the sense that I imagine unscrambling the glittered image will be significantly easier if you can plug in known sources to the transfer matrix. Edit: the video there didn't work for me any more, here's the youtube link [this video]
Cites evanb · 2017-10 · on “Turning Corners into Cameras: Principles and Methods”
This reminds me of dual photography, where one can recover hidden images from diffuse-diffuse light transfer. video: [this video] page: [LINK]
Cites rzzzt · 2019-01 · on “Shadows used to peer around corners”
Reminds me of this work, aiming to "reverse" the light path between a projector and a camera/photosensor: [this video]