5 ms·

Relatedly, the fact that light is reversible (behaves the same forward in time/direction as backwards) means you can swap the light source and camera of an imag

by tgb 6y ago

Relatedly, the fact that light is reversible (behaves the same forward in time/direction as backwards) means you can swap the light source and camera of an image (if just one point lightsource). I.e. you can render the scene from the perspective of the light source as if light were emitted by the camera. I recall seeing a neat demo of this but I can't find it on google. Anyone know it?

murgindrag 6y ago
The demo I saw had a projector scanning a room, pixel by pixel, and a single light sensor. This was adequate for taking lousy photos for 10x the cost of a camera. It was neat, though!
BurningFrog 6y ago
Every analog headphone or other speaker can also be used as a microphone.
- segfaultbuserr 6y ago
  s/analog/passive/ A speaker with an in-amplifier is still an analog speaker, but not a passive one.
- lynndotpy 6y ago
  Similarly, a typical LED can be used as a photometer.
tim_hutton 6y ago
e.g. Dual Photography: https://graphics.stanford.edu/papers/dual_photography/ https://graphics.stanford.edu/papers/dual_photography/
- tgb 6y ago
  Thank you!
- espadrine 6y ago
  Did someone achieve a picture with the sun as camera? I would guess it may be blurry?
  - akdas 6y ago
    I haven't fully read through the paper, but it looks like an important part of the technique is that the light acts like a point light at each point in time. For example, the projector will emit certain patterns of light, like in Figure 6 of the paper, to illuminate a localized group of pixels in the camera. Varying this pattern over time, it's possible to reconstruct how a small amount of light exiting the light source affects a small amount of light captured by the camera. And even with "point" light sources and the parallelized light patterns, the whole scanning takes a while: > This image is 578x680 pixels and was acquired in a little over 2 hours. The swapped image has the same resolution as the projector, so it seems yes, you'd get a very low resolution image if you used a large light source like the sun. You can see this in the "Accidental Pinhole" supplemental video. The reconstructed image is very low resolution, but the quality is slightly better when the light comes in via a smaller area (a mostly closed window).
- joshvm 6y ago
  Figure 12 is wild. They take an image from overhead a scene - a box which contains a mirror (perpendicular to the camera so you can't see the surface) and some other objects. They illuminate from the side (shining into the mirror) and are able to reconstruct that view. That is an image facing the mirror whose surface the camera can't see and it correctly shows reflections.
  - laughinghan 6y ago
    Did you see Figure 16?? The camera is pointed at the back of a playing card (like, from a 52-card deck), and the front is faintly reflecting light onto a book, and they're able to reconstruct a shockingly clear image of the front of the card (King of Hearts, spoilers).
deleted 6y ago
[deleted]
Miraste 6y ago
This is how path-tracing graphics engines like Blender's Cycles work. They fire rays from the camera and record whether they hit light sources, which allows simulating far fewer rays.
sandworm101 6y ago
>> you can render the scene from the perspective of the light source as if light were emitted by the camera A slide projector. A 35mm slide is exposed from one side, in a camera, to create the image. Then it is lit from the other side, in a projector, to render the scene. Do this with polaroid-type film and one device could be both camera and projector, possibly with only one or two moving parts.
- smogcutter 6y ago
  That was my first thought as well, that analog media are generally reversible, but if I understand the parent post it isn’t really what he’s talking about - rather that the image from a camera is enough to render the scene from the perspective of the lamp that lit the initial image. That’s very different from a projector, or say the way a mic and speaker are essentially the same thing.
tdeck 6y ago
This reminds me of how early mechanical television cameras worked. Scenes like these [1] were recorded with a flying spot camera that scanned a dark room with a narrow beam of light. Photocells placed around the room picked up the reflected light. Such a photocell appears to "illuminate" the area around it in the footage. [1] https://youtu.be/2W8SrgJ1hVM https://youtu.be/2W8SrgJ1hVM There is probably a more illustrative video but I can't find it at the moment.

by tgb 6y ago

murgindrag 6y ago

The demo I saw had a projector scanning a room, pixel by pixel, and a single light sensor. This was adequate for taking lousy photos for 10x the cost of a camera. It was neat, though!

BurningFrog 6y ago

Every analog headphone or other speaker can also be used as a microphone.

segfaultbuserr 6y ago
s/analog/passive/ A speaker with an in-amplifier is still an analog speaker, but not a passive one.
lynndotpy 6y ago
Similarly, a typical LED can be used as a photometer.

tim_hutton 6y ago

e.g. Dual Photography: https://graphics.stanford.edu/papers/dual_photography/ https://graphics.stanford.edu/papers/dual_photography/

tgb 6y ago
Thank you!
espadrine 6y ago
Did someone achieve a picture with the sun as camera? I would guess it may be blurry?
- akdas 6y ago
  I haven't fully read through the paper, but it looks like an important part of the technique is that the light acts like a point light at each point in time. For example, the projector will emit certain patterns of light, like in Figure 6 of the paper, to illuminate a localized group of pixels in the camera. Varying this pattern over time, it's possible to reconstruct how a small amount of light exiting the light source affects a small amount of light captured by the camera. And even with "point" light sources and the parallelized light patterns, the whole scanning takes a while: > This image is 578x680 pixels and was acquired in a little over 2 hours. The swapped image has the same resolution as the projector, so it seems yes, you'd get a very low resolution image if you used a large light source like the sun. You can see this in the "Accidental Pinhole" supplemental video. The reconstructed image is very low resolution, but the quality is slightly better when the light comes in via a smaller area (a mostly closed window).
joshvm 6y ago
Figure 12 is wild. They take an image from overhead a scene - a box which contains a mirror (perpendicular to the camera so you can't see the surface) and some other objects. They illuminate from the side (shining into the mirror) and are able to reconstruct that view. That is an image facing the mirror whose surface the camera can't see and it correctly shows reflections.
- laughinghan 6y ago
  Did you see Figure 16?? The camera is pointed at the back of a playing card (like, from a 52-card deck), and the front is faintly reflecting light onto a book, and they're able to reconstruct a shockingly clear image of the front of the card (King of Hearts, spoilers).

deleted 6y ago

[deleted]

Miraste 6y ago

This is how path-tracing graphics engines like Blender's Cycles work. They fire rays from the camera and record whether they hit light sources, which allows simulating far fewer rays.

sandworm101 6y ago

>> you can render the scene from the perspective of the light source as if light were emitted by the camera A slide projector. A 35mm slide is exposed from one side, in a camera, to create the image. Then it is lit from the other side, in a projector, to render the scene. Do this with polaroid-type film and one device could be both camera and projector, possibly with only one or two moving parts.

smogcutter 6y ago
That was my first thought as well, that analog media are generally reversible, but if I understand the parent post it isn’t really what he’s talking about - rather that the image from a camera is enough to render the scene from the perspective of the lamp that lit the initial image. That’s very different from a projector, or say the way a mic and speaker are essentially the same thing.

tdeck 6y ago

This reminds me of how early mechanical television cameras worked. Scenes like these [1] were recorded with a flying spot camera that scanned a dark room with a narrow beam of light. Photocells placed around the room picked up the reflected light. Such a photocell appears to "illuminate" the area around it in the footage. [1] https://youtu.be/2W8SrgJ1hVM https://youtu.be/2W8SrgJ1hVM There is probably a more illustrative video but I can't find it at the moment.