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This week I worked hard on getting the fluid solver in the style of Jos Stam working. The basics were easy enough, but Stam makes some simplifying assumptions, so the continuation was not quite trivial. But combined with what I learned in my earlier work on the free-surface simulator, I managed to put together a fast, stable, flexible and pretty fluid solver that I’m more than a little proud of.
From my previous posts, it must now be clear that free-surface fluid dynamics is hard. This is mainly caused by the free surface. Without that, it is possible to write a fluid-in-a-box in a little over 100 lines of C code, as Jos Stam did in his paper Real-Time Fluid Dynamics for Games. However, I could not go down that road. I needed the free surface. I wanted my game to have waves, droplets, splashes, sprays, fountains! How these would make a game, I would figure out after I got the simulation to work.
Work on this project has been standing still for some time while I was working on another project. But this week I picked up work where I left off: making the fluid simulation even faster. Since the SOR solver I was using lends itself well to parallelization, and video cards are good at running parallel programs, I tried to run the solver on the video card (GPU).
The fluid simulation was beginning to approach results of decent quality. However, it was still far too slow. Most of the screenshots I’ve shown so far were done on a 64x64 grid, which barely ran in real-time even on my fast Intel i7 machine. For a full-screen game, I’d need at least 128x128 and preferably 256x256. As I noted before, a doubling of the grid size requires about ten times as much computational power. Clearly, some optimizations were in order.
I’m a very visual type of guy. A picture really does say more than a thousand words. It should come as no surprise that my way of coding and debugging reflects this.
Last week, I wrote to my fluid dynamics professor for advice on the free surface fluid simulation. It was a positive surprise to see that I had run into exactly the same problems as he had in his research. I must have been doing something right then!
It’s been a busy week with little to show for it. As I wrote last time, I more or less gave up on the SPH particle-based method, and opted to fix my grid method instead. That turned out to be harder than I expected.
I realized that the problems I was having with the tracking of the water volume were not as easy to fix as I thought. It seemed that grid-based (Eulerian) methods are very suitable for a continuous fluid, but not so good when a sharp boundary between water and air is needed.
To find the source of the instabilities, I pulled my code apart into more independent steps, that could individually be turned on or off. This did result in a speed hit, but allowed me to quickly trace the source of the problems to the advection routine. This is the part that moves the water along at its own speed.
Since no existing code fit my requirements, I started working on my own fluid simulator a few weeks ago. The idea was to try both a grid-based and a particle-based method, and see which worked better for my situation. I started with the grid-based version.