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Comment by tgb | Hacker News Reader

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?

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

Thank you!

Did someone achieve a picture with the sun as camera?

I would guess it may be blurry?

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).

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.

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).

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.

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!

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

s/analog/passive/

A speaker with an in-amplifier is still an analog speaker, but not a passive one.

Similarly, a typical LED can be used as a photometer.

>> 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.

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.

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

There is probably a more illustrative video but I can't find it at the moment.

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