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Comment by AndrewKemendo | Hacker News Reader

That is amazing. I think I've looked at every photogrammetry, desconstruction, hand modeling etc... technique for 3D reconstruction and this one takes the cake for ingenuity, quality and capability.

Not sure how practical it is right now, but I wonder if you could do this with air volume at a high enough delta measurement resolution you might get some amazing results.

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I think I've looked at every photogrammetry, desconstruction, hand modeling etc... technique for 3D reconstruction and this one takes the cake for ingenuity, quality and capability.

Elsewhere in this thread, 'proee' links to what might be an even more interesting technique, which restrains the object in a dodecahedral "cage" (to allow for precise angular positions) and then measures the amount of liquid necessary to create a set predetermined rise in liquid level. http://www.romansystemsengineering.com/hypothesis.html.

Combining some aspects of the two, it might make sense to start with the object at the bottom of an empty container (in a cage or otherwise restrained) and add liquid at a known constant rate (as for a titration). Then generate a 2D graph of time against liquid height for a number of known angles, and solve in the same manner as this paper describes.

> Combining some aspects of the two, it might make sense to start with the object at the bottom of an empty container (in a cage or otherwise restrained) and add liquid at a known constant rate (as for a titration).

I believe this is isomorphic to the draining mechanism described in their paper.

I was thinking that accuracy would be easier if you are starting with a dry container, so that one wouldn't need to use fluorinert. You could always combine the two: measure while filling, and then while draining, then average.

I also wonder if instead of using discrete angles and multiple fills (or drainings) one could just tilt the container, possibly even slowly rotating it continuously. Add a squirt, measure the liquid level for a 360 rotation, then add another.

Edit: just saw your other comment suggesting similar things!

I think the original link is much more elegant, though this example is certainly impressive.

How about firing air at an object through a series of nozzles, measure the resulting turbulence and then solve for shape ;-)

> measure the resulting turbulence

Just a quick solution of a million-dollar math problem (https://en.wikipedia.org/wiki/Millennium_Prize_Problems#Navi...) and you're on your way!

You don't have to solve the Navier Stokes equations analytical for this. You don't even have to solve them explicitly at all: I can see a grad student just getting a lot of experimental data and throwing it all at a neural net with good results.

> You don't have to solve the Navier Stokes equations analytical for this.

Yes, to be sure you are right. It was just a silly (and, as you point out, mathematically inaccurate) joke.

It doesn't solve the hidden cavity problem very well, though.

Indeed. At least not in a way I can think of within thirty seconds.

If you're going to allow infinite engineering effort then I propose going all thermodynamic and having a VERY large piston connected to a small volume, change the volume and therefore pressure of a large quantity of room temperature gas, perhaps a short hydrocarbon like butane or propane or chlorine or ammonia or any one of numerous refrigerants, then the piston can vary the pressure setting the fluid level without moving anything but the piston. It may not be fast but computers can be very patient. Given some pumps and mixing vanes it could generate a ton of thermodynamic data. So toss in a piston, hit start, come back a couple days later to a 3-d model AND chemical composition data.

A somewhat similar, much simpler process is in wide use in the manufacturing industry for form measurement:

https://www.google.de/search?q=air+gaging

Seems like the turbulence would present difficulties. How about lasers? Oh wait that's a known method.. :)

How about shining a normal light though and just inverting the shadow calculations.. hmm.. has that been done?

I think I might have done what you are talking about a while ago. See my write up on my website:

http://jack.minardi.org/software/whats-in-a-shadow/

Fascinating. I'd wondered a while back how feasible it is to reconstruct an object from a 3-view orthographic projection, e.g. https://s-media-cache-ak0.pinimg.com/originals/b2/92/a9/b292.... Your work seems related.

Cool!

I feel like this would only work in limited cases of laminar flow and even then the scattering of the air would be complicated af I think

If you can solve the math (even just numerically), you can in principle do it for non-laminar flow.

I think that's already a thing, right? They use that for corneal deformation at a minimum.

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