Dual Photography

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.

in “How taking pictures through windows can inadvertently reveal your location” · View on HN

> 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 see. So the specific way the light rays interacts with the surface is what creates the 'information content'. For example, we perceive a blue-colored surface because the surface properties (i.e roughness) will specifically reflect wavelengths associated with blue. A specular surface will reflect light back to the eye, creating a 'shiny' surface. A very cool demonstration of how light conveys information is in dual photography: ▶ this video And the original paper: [link] . They shine a light source onto an object, take a picture, and then by processing the light rays in the photo, are able to generate the same scene but from the perspective of the light source. It's really astounding when you see it. They can do this reconstruction because the flow of light rays from the light source to the obj

in “In a photo of a black hole, a possible key to mysteries” · View on HN

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

in “TV Backlight Compensation” · View on HN

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

in “Reflected hidden faces in photographs revealed in pupil” · View on HN

I've never forgotten this SIGGRAPH demo from almost twenty years ago, in which the authors effectively switch camera and light source computationally (... in a static scene) [1] Ever since then, I have viewed scenes such as the "lingerie store scene" from Enemy of the State [2] with a little bit less eye rolling... [1] - ▶ this video [2] - [link]

in “Image-blaster: Creates 3D environments, SFX, and meshes from a single image” · View on HN

I was immediately reminded of the Dual Photography paper (well, I had to look it up first; I only remembered the "photographing a playing card that is facing away" demo at the end): - [link] - ▶ this video

in “Non–line-of-sight imaging over 1.43km [pdf]” · View on HN