Flow Flow

In an early scene establishing the post-diluvial landscape of the Golden Globe-winning animated feature Flow, the hero cat prowls a sunlit forest when a threatening rumbling begins. Suddenly, a tumultuous wave smashes through the trees, chewing up terrain, churning mud, engulfing the lens, and almost drowning the cat that scrambles to survive what is left of a drowned world eerily bereft of humans.

Whereas, only decades ago, flood and water work remained some of the most challenging vfx simulations only seen in Hollywood blockbusters –– The Perfect Storm (2000), Pirates of the Caribbean (2003), Battleship (2012), or San Andreas (2015) –– the water simulations of Flow were achieved entirely in open-source Blender, using Physical Open Waters. The plug-in (or ‘add-on’ in Blender terminology) is now available to purchase for as little as $50 on Blender Market.

The technology was developed by Physical Addon co-founder Mārtiņš Upītis, an animator and visual effects artist in Latvia who joined the production of Flow early in the film’s development.

“When I started developing Flow,” writer and director Gints Zilbalodis told Cartoon Brew, “I was looking for tutorials on Youtube for how to create water in Blender. I stumbled upon a tech demo video which looked perfect for what I hoped the water would look like in Flow. Much to my surprise, I realized that it was made by someone from my hometown Riga, of all places. We later met and ended up co-renting an office space together. Mārtiņš and his team develop Blender add-ons right next to our Dream Well Studio team, which was very useful because we often ended up asking for advice to each other. Luckily, I was able to convince Mārtiņš to come on board and work on Flow and develop further the tools that I’d seen in his tech demo.”

Upītis had been developing his experiments in fluid simulations since 2010 and posted updates of his early tests on Youtube, demonstrating procedural water wave propagation including ocean models of water surface behavior, buoyancy, wave force, meniscus, and water particle effects. “The water system Gints found was actually an old demo from a game engine,” Upītis noted. “I adapted it for production use in Blender. The original system has been adapted to Unity and Unreal Engine, and used in some popular computer games.”

Developing techniques for choreographing water for Flow, “was a back-and-forth kind of process,” said Zilbalodis. “In scenes where the characters interact with water, sometimes we did a first pass of the water only, then the animators would create the character movement and finally the water was adjusted and fine tuned to fit with the finished animation.”

As animation reference, the production team analyzed real water behavior in tests captured on an iPhone. “I took massive amount of video and photo references of waters in different depths and weather conditions,” Upītis explained. “I filmed some of those in slow motion to get the characteristics of smaller details as the viewpoint of a cat is much closer to the surface of water. IPhone tolerates filming under water, so I used that.”

Adaptability was key in developing the aesthetic of water simulations that set the mood for scenes following the cat’s journey in the film. “The problem with water is that it behaves very differently in every kind of situation,” Zilbalodis observed. He further added:

There isn’t just one tool that fits all cases. We had to develop different systems for all kinds of water – calm lake, stormy sea, river, puddle, etc. In many cases, we could rely on the custom tools that Mārtiņš was developing which didn’t require any heavy simulation. We could instead generate the water procedurally which was much faster. In shots with very specific character interactions, we had to do simulations using Blender add-ons, like Flip Fluids and Cell Fluids. Konstantīns Višņevskis did most of those simulations. Mārtiņš then combined those with his systems and made sure that the seams are invisible. In most big studio vfx films there would be dozens of artists working on effects like this, but in case of Flow it was just two people – Mārtiņš and Konstantīns.

What Do You Think?

Read an interview with 'Flow' director Gints Zilbalodis

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