
The wow of water rendering - steinsiv
https://vimeo.com/120475526
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phireal
Does anyone know if this accurate when compared with observations of flows at
this scale? It certainly looks realistic, but I wonder if that's "just" a
trick of the eye rather than it accurately reproducing the processes which
generate these patterns.

~~~
thomasjames
The truth about all flow and especially turbulence is that it is not hard to
make it look convincing to the human eye. Capturing the motion on various
length scales and presenting it in a plausible way is not all that
computationally intensive (the algorithms to make them can still of course be
challenging). Making simulations that are numerically correct, however, is
incredibly computational intensive (in addition to being algorithmically
complex). This is why CFD is still run on massive clusters for transient
simulations. There is also a whole field in itself for "sub-grid" turbulence
models that allow for the spatial resolution of a simulation to remain coarser
than the Kolmogorov microscales
([https://en.wikipedia.org/wiki/Kolmogorov_microscales](https://en.wikipedia.org/wiki/Kolmogorov_microscales)).
(As a side note, Kolmogorov was a true polymath/genius in areas outside
information theory and complexity theory.) Cutting edge basic research on
turbulence will perform calculations at such scales (direct numerical
simulation). Suffice it to say, most graphics work does not waste too many
cycles on physical accuracy. Hard to know for sure, but since the video has
words "cinema" and "rendering" in the description as opposed to "simulation",
it is probably just realistic enough to pass muster in terms of human
perception. Also accurately simulating the formation of foam requires a two-
phase simulation of both gas and liquid as well as considering cohesive
forces.

~~~
martijn_himself
Small nitpick... my understanding is that these simulations are _numerically_
correct- they are, however, numerical approximations to simplified versions of
the Navier-Stokes equations. So the mathematical equations are incorrect in
that they do not accurately model parts of physical phenomena they represent.
They are _accurate enough_ if your objective is to create pretty (moving)
pictures. They are not accurate enough if your objective is to determine
forces on objects.

~~~
thomasjames
Point well taken. They are not numerically unstable in the sense that the term
is most often used with ill conditioned matrices or floating point weirdness,
but yes, they are significantly oversimplified and will not converge with
reality if you measure them by a meaningful metric.

~~~
martijn_himself
Excuse my curiosity- do you work in CFD? I did quite a bit of CFD work for my
degree- then I made the mistake of becoming a programmer :).

~~~
thomasjames
I worked for a CFD HPC company for about two years doing a mix of traditional
engineering and programming, and I realized that I like programming and
learning about computer architecture way more than the mechanics. So I went
back to school for CS/CE and just like you fell into the trap!

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beefsack
I can't help but feel the water in nearly all simulations I see is more
viscous than water in real life, it looks more sticky and solid to my eyes.

It still looks amazing and quite convincing though.

~~~
martijn_himself
This _may_ be because the viscous effects are not modelled or not modelled
correctly in order to simplify solving the governing equations for fluid flow
numerically. I'm not sure if these effects or the absence of them would be
visible to the naked eye.

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kijin
Incredible!

The only thing that seems to be missing is the movement of sand as water
sloshes over it. But I guess it would be more difficult to simulate tiny solid
particles suspended in turbulent water. For now, let's just say that this
beach is made of cardboard :p

~~~
mwilcox
The sand looks like simple geometry at the moment, I imagine it could be
(relatively) easily implemented as a second particle type as part of the same
simulation. The granularity of sand makes it pretty similar to liquid, it
would just need adjusted properties (weight, friction, appearance)

~~~
leni536
>The granularity of sand makes it pretty similar to liquid

Sand behaves _much_ differently than liquid. The friction between sand grains
is not like viscosity and it lacks surface tension too. I don't know any
continuous model that describes the movement of sand similar to Navier-Stokes
for liquid.

~~~
agumonkey
I lost touch with the CGI industry, but sidefx latest release of houdini
includes a dry and wet sand (snow, ...) simulator.

[https://www.sidefx.com/index.php?option=com_content&task=vie...](https://www.sidefx.com/index.php?option=com_content&task=view&id=3042&Itemid=66)

They used to be on the fore front of this kind of R&D, so I guess it's pretty
state of the art (as a CGI package of course).

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iMark
Beautiful bit of rendering.

Does anyone have any estimates as to how far we are from having this rendered
realtime?

~~~
Synaesthesia
Difficult to estimate as there are always optimizations which can make real-
time rendering orders of magnitude faster, for almost the same work.

But naively this is 30 min per frame render time, which is 1800 sec, wanna get
that down to about 1/25th of a sec, that's a 45000x improvement in computation
required. How long will that take? Well assuming GPU's get twice as fast every
year, then it will take still 16-17 years! Luckily we have those shortcuts.

However it's possible we may have a quantum leap in computer performance if
the new Memristor architecture ever takes off, which could speed things up.

~~~
theandrewbailey
Nvidia's next architecture (Pascal?) will move GPU memory into the same
package as the GPU itself, giving an instant ~3x boost.

[http://blogs.nvidia.com/blog/2014/03/25/gpu-roadmap-
pascal/](http://blogs.nvidia.com/blog/2014/03/25/gpu-roadmap-pascal/)

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novaleaf
when i look at it, it looks "fake"... I couldn't really describe why, until I
saw the foam from 2 waves collide between the rocks at 0:20. The foam shifts
down the screen (perpendicular to the two waves).

Funny how the brain can perceive "issues" without being consciously aware of
them.

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amelius
How much time did this take to simulate and render, and on how many machines?

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jstr
according to a comment on the video, 12.5 days, not clear how many machines.

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amelius
Ah thanks, I was watching with the sound turned off :)

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lgas
FWIW jstr was referring to the text comments below the video. There is other
info there too about e.g. rendering settings.

