Overlaying the CG characters with photoreal drips through a skinny movie solver was simply one of many many water simulation challenges on the movie.
At one level in James Cameron’s Avatar: The Means of Water, the brothers Neteyam and Lo’ak scuffle with some Metkayinan youngsters amongst the shallow lapping waves of a seaside. It’s a surprising scene, not least due to the presence of effective water droplets on every CG character. The simulation for these would take days to compute.
With so many water scenes for each above and under the water, the group behind the work–which included artists on the VFX studio Wētā FX and the engineers who at the moment are a part of the Wētā Digital/Unity acquisition–wanted a complete water toolset for the job.
Certainly, such a toolset particularly for The Means of Water was embarked upon way back to 2017, with Loki, an in-house proprietary simulation framework, being at its heart. Right here, Alexey Stomakhin, principal analysis engineer at Wētā Digital x Unity, tells befores & afters concerning the intensive R&D course of for water simulation on the movie.
(Stomakhin and Steve Lesser, Sven Joel Wretborn and Douglas McHale simply took out the Rising Expertise Award of the Means of Water water toolset at this yr’s VES Awards, with a number of different VES Award trophies additionally going to those that labored on the movie in a lot of classes, together with straight referring to water).
The origins of the water R&D
Alexey Stomakhin: We had a particular Water Improvement Challenge established again in 2017. It targeted particularly on the upcoming water challenges. It was handled as a miniature present consisting of a lot of difficult pictures. The aim was that we we’d not merely attempt to closing these pattern pictures, however fairly use them over time to repeatedly refine the pipeline; in order that after we received 2000+ water pictures to work on for the film–really The Means of Water had 2,225 pictures with the water parts in them–and plenty of FX folks got here on board, the toolset would enable to ship these pictures with constant business main excessive constancy.
We received a touch of what was to come back in Alita: Battle Angel
Alita: Battle Angel got here out on the finish of 2018. Once I began at Wētā, I started working on this one shot within the film the place the principle character dives into the water to get inside a spaceship after which emerges again from the water proper subsequent to the digital camera. That shot was the place plenty of the water instruments had been battle-tested.
The supervisors, Joe Letteri, and James Cameron, had been very specific about how they wished the water to roll off the face and the way the material and hair wanted to be coupled with the water, the way it all wanted to look pure and natural collectively. It nearly felt like we had been proving the price of our strategies for a future manufacturing.
The Means of Water’s over-arching water structure
The toolset consists of a lot of distinct solvers. Initially, there have been makes an attempt to attempt to envision this single illustration water system that may simulate every part directly, nevertheless it seems it’s not fairly computationally possible as a consequence of scales starting from massive 10- or 100-meter splashes and wakes, all the way in which right down to sub-millimeter drips on pores and skin.
So despite the fact that the identical fluid equations get solved beneath the hood, we needed to make sure assumptions and simplifications for every of the completely different eventualities to make sure we hold compute instances inside cheap bounds.
On stage: procedural water for stage
Water began with a procedural GPU-based deformer that may be used on stage. It supplies an approximation to how waves would behave far out within the ocean. The answer is predicated on the idea that you would be able to mix a number of Gerstner/Stokes waves to create arbitrary wave patterns. It’s considerably simplistic, however on the identical time there’s a sure physicality constructed into it, with Tessendorf and TMA being well-liked decisions for frequency spectrums.
This method gave the precise thought to the director of how the water would appear to be, not solely from movement but additionally from the lighting viewpoint. Then, for us, it might present a bodily believable place to begin to drive extra advanced simulations.
Loki because the centerpiece and state machine
A lot of the water instruments we used are primarily based inside our proprietary simulation framework, Loki. Essentially the most distinguished one is the Loki state machine, our airborne spray system. It consists of a lot of distinct solvers that excel at completely different scales, however on the identical time, work and run in tandem. We have now solvers for a number of water states: bulk, spray, mist, all of that are coupled with the encircling air. Transitions between the states are dealt with in a mass and momentum conserving manner.
In apply, we begin with primary FLIP water–we name it bulk water–which represents “strong” volumetric fluid. Because it will get involved with air, the interaction between Reynolds stress and floor rigidity could trigger it to shatter into droplets, which we name spray. The spray is simulated utilizing SPH (smoothed-particle hydrodynamics) since we nonetheless need the droplets to work together to seize “tendrily” floor rigidity results. The spray additional transitions into even smaller mist particles, that are principally ballistic particles however absolutely coupled with the encircling air through a drag pressure, which is kind of essential, as a result of at that scale air has an important impact on the dynamics of water.
The most effective examples of utilizing the Loki state machine is the shot of Payakan and Lo’ak taking part in collectively within the water. In that shot there are a number of solvers operating in tandem. There’s bulk water. There’s SPH spray. There are ballistic mist particles. All coupled with the encircling air and accomplished in a single simulation go. So despite the fact that there are a number of simulation layers, they’re all computed concurrently with correct transitions and interactions between them. And that’s partly why the shot appears so pure and reasonable. I do imagine that the froth and the underwater bubbles for that shot had been performed in a separate go, however the entire airborne elements had been accomplished inside a single Loki remedy.
Bubbles, coupling and extra
We have now mentioned airborne water within the air, however equally air can get submerged within the water, creating bubbles. These get simulated with our Loki secondaries system. The secondaries method originated from an concept that despite the fact that you’ll assume that the dynamics of bubbles could also be considerably decoupled from the water, it actually isn’t. There are specific interplay results you could be after.
In an effort to seize reasonable bubble-water interplay we’d really re-simulate parts of bulk water to make sure correct coupling with the bubbles. This allowed us to seize so-called collective results, reminiscent of a gaggle of bubbles rising sooner than every particular person bubble inside that group, and it turned out to make a big distinction visually.
Because the bubbles rise to the floor, they transition into SPH foam.
For any belongings submerged into the water: fabric, hair, vegetation, despite the fact that you could not see the water floor, we’d run a completely coupled fluid remedy to verify we get a correct response from the encircling surroundings. As an illustration, the Creature division would closely depend on Loki for these sorts of interactions.
There’s not essentially a lot new to our bulk water FLIP solver, as a result of FLIP solvers have been round for some time. Nonetheless, we now have included narrow-band, spatial adaptivity and distribution over MPI to have the ability to deal with large excessive decision simulations.
Loki integration
Loki has been built-in into each SideFX Houdini and Autodesk Maya via the usage of particular bridge nodes that may enable the customers to deliver knowledge out and in of Loki. It may additionally run as a standalone product which is very helpful for multi-machine distributed functions.
Loki’s RenderGraph
Loki’s RenderGraph permits you to deliver all of the completely different water elements: spray, mist, bubbles, volumes, collectively and composite all of them with out compromising the standard or constancy of particular person parts. The result’s handed over to Manuka, our in-house renderer, to provide the ultimate picture.
Residual wetness
If you see characters standing within the rain with tiny droplets on their faces, slowly evolving and forming rivulets, that’s our residual wetness system at work. The residual wetness was really developed by the Look Dev group exterior of Loki. When the characters will not be too moist and the droplets are largely sitting in place, there isn’t a lot motive to simulate all of it as a result of it will be costly and actually, you could need to be very specific about how the droplets evolve. So it was extra of an art-directed fairly than a bodily answer. But, this further layer added an unbelievable stage of realism.
The key problem of dripping characters
For characters getting out of the water, we would have liked to create realistically trying drips shedding off of their pores and skin and clothes. We had been particularly after capturing effective skinny sheets and tendrils of liquid pushed by floor rigidity forces. The answer we got here up with was simulation-based and mimicked what has been performed within the beforehand talked about Alita: Battle Angel shot.
Below the hood, it’s primarily a FLIP simulation, however a really costly one, as a result of these wanted to be run on the decision of a fraction of millimeter to realize the specified stage of element. So if you happen to take a single FLIP particle, it might signify say a tenth or a fifth of a millimeter little bit of fluid; and so, that was after all a computational problem as a result of sheer variety of particles we needed to take care of. Additionally, the presence of floor rigidity forces launched a extreme timestep restriction. We would wish to run our simulations with 10-20 substeps per body (at 48fps), which led to extraordinarily lengthy simulation instances, in some instances as much as a number of days.
One other unexpected problem that needed to be addressed was making droplets stick with character surfaces. We had to make sure the contact angle computation and viscosity remedy was performed precisely, as in any other case the water would simply fall off, which is after all undesirable. We have now revealed a brief paper about this in 2019 however I suppose folks could haven’t related it to Avatar: The Means of Water, as a result of it occurred a number of years earlier than. As well as the characters as kinematic colliders and their velocities needed to be ready in a really particular manner in order that the water wouldn’t get disconnected and observe the surfaces exactly. We ended up constructing a complete set of Houdini HDAs round our FLIP solver to ensure that the inputs had been generated accordingly.
For the Alita shot, the skinny movie water solely lined her face, and even then we already needed to simulate near 10 million FLIP particles. Now, think about protecting the entire physique of a personality or protecting a number of characters with water. These particle counts simply attain tons of of thousands and thousands and the compute instances undergo the roof. So we needed to be extraordinarily cautious about how and the place we emit and cull these particles to maintain the complexity of our simulations beneath management.
As a personality would go away the water, we detected the place the character’s mesh crossed the water floor and we might “glue” new particles to the rising elements of their our bodies. And as quickly because the particles get again to the principle physique water, we’d take away them from the sim. This method was fairly environment friendly on the finish of the day, and we had been capable of simulate skinny movie water on a number of characters inside a single shot.
Even so, a lot of our skinny movie water sims had been painfully time consuming. One of the costly sims in your entire film was within the shot the place Na’vi youngsters combat on a seaside in Metkayina village. The water interplay has been simulated for each character, and a single iteration would take 8 days to finish.
Regardless of lengthy simulation instances, our skinny movie solver has been utilized in almost each shot with moist Na’vi characters, offering slow-but-steady options in a few of the most troublesome and dynamic eventualities, and resulting in excellent visible outcomes.
SIGGRAPH papers
Loki: a unified multiphysics simulation framework for manufacturing – https://dl.acm.org/doi/abs/10.1145/3528223.3530058
Underwater bubbles and coupling – https://dl.acm.org/doi/fullHtml/10.1145/3388767.3407390
A sensible information to skinny movie and drips simulation – https://dl.acm.org/doi/10.1145/3306307.3328141
Guided bubbles and moist foam for reasonable whitewater simulation – https://dl.acm.org/doi/abs/10.1145/3528223.3530059
Wave curves: simulating lagrangian water waves on dynamically deforming surfaces – https://dl.acm.org/doi/abs/10.1145/3386569.3392466
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