
Visualizing Photons in Motion at a  Trillion FPS - Vinz_
http://web.media.mit.edu/~raskar/trillionfps/
======
ColinWright
Lengthy discussion:

[https://news.ycombinator.com/item?id=3346609](https://news.ycombinator.com/item?id=3346609)

Other submissions:

[https://news.ycombinator.com/item?id=3347267](https://news.ycombinator.com/item?id=3347267)

[https://news.ycombinator.com/item?id=3347793](https://news.ycombinator.com/item?id=3347793)

[https://news.ycombinator.com/item?id=3348210](https://news.ycombinator.com/item?id=3348210)

[https://news.ycombinator.com/item?id=3348427](https://news.ycombinator.com/item?id=3348427)

[https://news.ycombinator.com/item?id=3348679](https://news.ycombinator.com/item?id=3348679)

[https://news.ycombinator.com/item?id=3349278](https://news.ycombinator.com/item?id=3349278)

[https://news.ycombinator.com/item?id=3349614](https://news.ycombinator.com/item?id=3349614)

~~~
TeMPOraL
ColinWright, HN's own biological duplicate content detection bot ;).

(disclaimer: I find what you do very valuable, please keep doing that)

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stcredzero
One of the things that are unrealistic about "bullet time" as portrayed in
games and movies, is that the world doesn't dim as time slows down. Slow the
rest of the universe down, and the amount of light energy entering your pupil
per unit time slows down as well.

One of the things that came up in the criticisms of Khan Academy's teaching of
slope/rate is that lots of people don't internalize a conception of sensory
intensities as rates. To some people out there, things like _speed_ are
feelings of intensity divorced from the notion of X things happening in a
second.

This is probably especially true for things like brightness.

~~~
derefr
The eye normalizes for orders-of-magnitude difference in luminance.
Presumably, it makes for a good cinematographic choice to avoid the effect,
for the same reason it would be grating to be hit with a lens flare every time
a shot tracks from indoors to outdoors.

On the other hand, bullet-time would, and should, be _red-shifted_ , which is
a whole 'nother thing.

~~~
jerf
It depends on your "bullet-time" model whether it's red-shifted or not. Much
"bullet-time" is explicitly just a special effect, i.e., the idea is that the
action is still occurring in real time but we're having it slowed down so we
can see it, understand it, and enjoy it. In that case we are simply shooting
the scene with a high-FPS camera. "Technically" we should indeed get a dimmer
scene, just as real high-speed cameras do, but it's not _that_ unrealistic
that it's compensated for, again, just as in real life when the Mythbusters go
to a high-speed camera shot they've already compensated for that.

If you have a "real" time slow-down, exactly what happens depends on your
model, especially where the photons cross whereever the effect is occurring.
It is true that one model could be that a photon crossing into the effect
keeps the same "real" Hertz, in which case you'd see it red-shifted. However,
since in the end the entire idea of such sharply-localized time variance
without a corresponding gravity field is complete nonsense, the truth is that
no matter how you slice it the result is gibberish, so it's just as sensible
to let the field "keep" the old photon's frequency too.

However, I'd point out that "real" time slowing in the movies is fairly
unusual. Mostly it's the first kind. There's a number of sci-fi books with a
variety of "real" time slowing effects, though.

~~~
lcrs
Even cameras which vary speed during an unbroken shot automatically compensate
so the exposure stays the same, but doing so has other interesting effects,
depending how it's done:

a) Compensating with shutter so that the integration time is constant means
the quality of motion blur changes during the ramp. For example a shot that
starts at 24fps and ramps to 120fps keeping a shutter time of 1/240th would
start out with really crisp-looking "skinny shutter" motion, and you might see
flickering and strobing if there's much camera motion. The end of the ramp
would have normal looking amounts of motion blur. If there are CRTs or
fluorescent lights in shot they'd flicker at different rates throughout the
ramp. It can be a dead giveaway that something's about to happen if a shot
starts with really tight motion blur like that.

b) Compensating with lens aperture is much less common because the depth of
field changes, but starting with everything in focus, the focus becomes much
tighter with the background way out of focus at high speed. Reportedly,
mechanical problems tend to manifest as the aperture overshooting its mark or
bouncing back leading to brightness variations at the end of the ramp.

c) Compensating with ISO/gain is possible these days, which would avoid the
above problems but would end up with a noisier image for the high-speed end.

Recent bullet time shots are either fully CG, shot with an array of cameras,
or cheated by hanging everything on wires, but it's interesting to check if
any of these things from the days of 16mm skate films with the classic hang-
time dips are re-created :)

~~~
kibibu
I believe for the action scenes in 300 they recorded the whole lot at 120fps
and selectively dropped or merged frames to get back to 24fps

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tritium
This photography isn't really about "photons" or any amount of energy
measurements of radiation emission at the subatomic scale. This is more about
capturing macro-scale events at very precise time scales and exceedingly high
shutter speeds, and detecting how quickly an arbitrary amount of light will
traverse the components of a still life.

There's also a lot of distortion introduced into the final composited images,
enough that they cease to look like photographs of normal physical objects,
and start to look more like computer generated images of ray traced 3D models.

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e3pi
The camera filming itself in perpendicular mirror? Would we see an appreciable
delay for each diminishing image, or what?

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vilhelm_s
Does this have any practical applications, or is it just a (very cool!) art
project?

