Jurassic Park had just six minutes of computer-generated dinosaurs in it, compared to modern blockbusters which can have thousands of shots requiring complex visual effects. It's no surprise that animators are eager to embrace any shortcut they can — which is why realistic-looking CG mud could be a game changer.
Visual effects studios are not only under massive time constraints as they race to finish shots for movies with release dates scheduled more than a year in advance, their work also has to look perfect and often be invisible, otherwise audiences will scream "fake!" and complain about a film on social media.
But some real-life phenomena are very difficult to recreate in a computer. It's only in recent years that computer-generated water has started to look believable, and Pixar's latest short film, Piper, made huge strides in simulating realistic sand. But combining the two? Computer-generated mud — mixing water and sand — poses big challenges when it comes to making it look and move realistically, but a team of computer science researchers from UCLA, DreamWorks Animation, Jixie Effects and the University of Pennsylvania, might have just had a breakthrough.
In a paper that will be presented at the upcoming Siggraph 2017 conference in late July and published in ACM Transactions on Graphics, the team details their research into altering the cohesive properties of simulated sand when a simulated liquid is introduced.
You might not give it much thought, but flipping over a sand-filled hourglass starts a complex chain reaction as millions of tiny stones bump and rub up against each other until they all eventually pour through a tiny opening. To simulate that on a computer requires millions of calculations every second to determine each grain's movements, and millions more to render it realistically. Add water to the mix, and the sand behaves entirely differently. Think of a sand castle that's slowly collapsing as it's saturated by the rising tide. The simulated sand will slowly lose its ability to maintain its structure as it mixes with the liquid, until it eventually succumbs to the destructive forces of gravity.
The researchers behind the new paper have come up with a way to merge the calculations that govern simulated sand and water so that each still behaves realistically, but are affected by each other's unique characteristics. The sand increases the water's viscosity, and essentially becomes a temporary dam until it's slowly eroded away. And the water, in turn, reduces the sand's ability to maintain a solid structure, as the cohesiveness and friction between the tiny individual particles is reduced.
Assistant Professor Chenfanfu Jiang of the Computer and Information Science Departments at the University of Pennsylvania, and co-founder of Jixie Effects Inc., describes the research as an advancement of the technologies used to bring films like Frozen and Moana to life.
"The novel aspect of this new work is the modelling of detailed physical interactions between water and sand," he told Gizmodo. "Technically, we use particles to represent individual grains of sand and water droplets. In each simulation time step, individual material responses as well as the interaction force between sand and water are computed," with the aid of a voxel grid, which is essentially a 3D bitmap. It's the combination of particles and grids, Jiang says, "that allows elegant modelling of material interactions, which was very challenging with many other methods."
Hollywood has always had an obsession with disaster movies, and as improvements in computer graphics have been introduced that make simulating the effects of volcanoes, tornadoes and hurricanes easier (AKA more cost-effective), blockbusters featuring those natural disasters haven't been far behind. Perhaps one day, someone will want to produce a winter blockbuster about catastrophic landslides and levees breaking apart. At least now they will have the visual effects tools to make that look suitably epic.