4 ms·

A blank wall can show how many people are in a room and what they’re doing

81 points by isaacfrond 5y ago

twistedpair 5y ago
Pretty sure Plato wrote about this in his _Allegory of the cave_ (c 400BC),
donarb 5y ago
I recall that when the first Apple stores were opened that they used an overhead camera technology that recorded customers moving about the store. This included customers that walked up to the window, and those that walked inside. The images were rendered as black silhouettes so as to anonymize the customers, but the technology helped Apple understand traffic patterns in their stores.
- deleted 5y ago
  [deleted]
gene-h 5y ago
And the actual work including papers and video is here: http://wallcamera.csail.mit.edu/ http://wallcamera.csail.mit.edu/ I still don't understand how they're able to get a detectable optical signal through brick and concrete walls. It'd be nice if they actually measured the transmissibility of said walls in the paper.
- phdelightful 5y ago
  It's not through the wall, it's from reflections off the wall. Imagine being able to see around a corner because all walls were mirrors, except this works for walls that are not traditional mirrors.
antegamisou 5y ago
Is it any more advanced than this? https://youtu.be/H5xmo7iJ7KA https://youtu.be/H5xmo7iJ7KA
- gene-h 5y ago
  This work used regular cameras to see through brick and concrete block walls.
jimmyed 5y ago
Someone using their phone will throw this off too I'm guessing...
deleted 5y ago
[deleted]
nayuki 5y ago
Related work from over a decade ago: https://graphics.stanford.edu/papers/dual_photography/ https://graphics.stanford.edu/papers/dual_photography/ ; https://graphics.stanford.edu/papers/dual_photography/DualPhotography.pdf https://graphics.stanford.edu/papers/dual_photography/DualPh... ; https://www.youtube.com/watch?v=p5_tpq5ejFQ https://www.youtube.com/watch?v=p5_tpq5ejFQ
hughw 5y ago
See also Inference from inadequate and inaccurate data [1] [1] https://pubmed.ncbi.nlm.nih.gov/16591812/ https://pubmed.ncbi.nlm.nih.gov/16591812/
ackbar03 5y ago
My understanding is that these are all super fragile though. All these experiments are done in super controlled environments. Sounds cool and might be a step forward but never going to be applied realistically for a while I think
EamonnMR 5y ago
This reminds me of trying to explain Ambient Occlusion https://en.m.wikipedia.org/wiki/Ambient_occlusion https://en.m.wikipedia.org/wiki/Ambient_occlusion
kurthr 5y ago
It's unclear to me why the wall would need to be "blank". Maybe simply unobstructed and flat or moderately consistent reflectance Lambertian? A dark black wall would be noticeably more difficult, while one painted with several bright colors could be corrected fairly easily. Even a wall with fixed fine black on white patterns (e.g. text) should be comparable, while a white sheet with fans blowing on it incredibly difficult.
- dahart 5y ago
  It’s really a statement about what the researchers actually did for this paper, not a statement about what’s required or possible in general now or in the future, right? Because they qualify this is for blank walls both in the abstract and in the video, I’d first assume it’s possible they tried it and ran into substantial issues - even though it might logically seem easy. Maybe it will take a bit more math or engineering to handle non-blank walls, and it’s being saved for the next paper. Since non-blank walls will increase the dimensionality (and training time) of the problem, and likely come with reduced effectiveness in some cases, and since a result with blank walls is good enough for publication, maybe this is just a first step?
- hammock 5y ago
  >It's unclear to me why the wall would need to be "blank" The initial state of the wall (before people are introduced) should be known. With a solid color wall you can find shadows of people, objects, etc. But with a wall with objects on it, you need an image of it before the room was filled, in order to interpret the shadows. (From a machine standpoint. As humans we can process out the art or whatever is on the wall)

81 points by isaacfrond 5y ago

Pretty sure Plato wrote about this in his _Allegory of the cave_ (c 400BC),

I recall that when the first Apple stores were opened that they used an overhead camera technology that recorded customers moving about the store. This included customers that walked up to the window, and those that walked inside. The images were rendered as black silhouettes so as to anonymize the customers, but the technology helped Apple understand traffic patterns in their stores.

deleted 5y ago
[deleted]

gene-h 5y ago

And the actual work including papers and video is here: http://wallcamera.csail.mit.edu/ http://wallcamera.csail.mit.edu/ I still don't understand how they're able to get a detectable optical signal through brick and concrete walls. It'd be nice if they actually measured the transmissibility of said walls in the paper.

phdelightful 5y ago
It's not through the wall, it's from reflections off the wall. Imagine being able to see around a corner because all walls were mirrors, except this works for walls that are not traditional mirrors.

antegamisou 5y ago

Is it any more advanced than this? https://youtu.be/H5xmo7iJ7KA https://youtu.be/H5xmo7iJ7KA

gene-h 5y ago
This work used regular cameras to see through brick and concrete block walls.

jimmyed 5y ago

Someone using their phone will throw this off too I'm guessing...

deleted 5y ago

[deleted]

nayuki 5y ago

Related work from over a decade ago: https://graphics.stanford.edu/papers/dual_photography/ https://graphics.stanford.edu/papers/dual_photography/ ; https://graphics.stanford.edu/papers/dual_photography/DualPhotography.pdf https://graphics.stanford.edu/papers/dual_photography/DualPh... ; https://www.youtube.com/watch?v=p5_tpq5ejFQ https://www.youtube.com/watch?v=p5_tpq5ejFQ

hughw 5y ago

See also Inference from inadequate and inaccurate data [1] [1] https://pubmed.ncbi.nlm.nih.gov/16591812/ https://pubmed.ncbi.nlm.nih.gov/16591812/

ackbar03 5y ago

My understanding is that these are all super fragile though. All these experiments are done in super controlled environments. Sounds cool and might be a step forward but never going to be applied realistically for a while I think

EamonnMR 5y ago

This reminds me of trying to explain Ambient Occlusion https://en.m.wikipedia.org/wiki/Ambient_occlusion https://en.m.wikipedia.org/wiki/Ambient_occlusion

kurthr 5y ago

It's unclear to me why the wall would need to be "blank". Maybe simply unobstructed and flat or moderately consistent reflectance Lambertian? A dark black wall would be noticeably more difficult, while one painted with several bright colors could be corrected fairly easily. Even a wall with fixed fine black on white patterns (e.g. text) should be comparable, while a white sheet with fans blowing on it incredibly difficult.

dahart 5y ago
It’s really a statement about what the researchers actually did for this paper, not a statement about what’s required or possible in general now or in the future, right? Because they qualify this is for blank walls both in the abstract and in the video, I’d first assume it’s possible they tried it and ran into substantial issues - even though it might logically seem easy. Maybe it will take a bit more math or engineering to handle non-blank walls, and it’s being saved for the next paper. Since non-blank walls will increase the dimensionality (and training time) of the problem, and likely come with reduced effectiveness in some cases, and since a result with blank walls is good enough for publication, maybe this is just a first step?
hammock 5y ago
>It's unclear to me why the wall would need to be "blank" The initial state of the wall (before people are introduced) should be known. With a solid color wall you can find shadows of people, objects, etc. But with a wall with objects on it, you need an image of it before the room was filled, in order to interpret the shadows. (From a machine standpoint. As humans we can process out the art or whatever is on the wall)