Cartoon Liquid Simulation

Update June 2013: This program has been ported to WebGL!!!

The other day I came across this neat visual trick: How to simulate liquid (Flash). It’s a really simple way to simulate some natural-looking liquid.

I [made my own version][fun] in Java, using JBox2D for the physics simulation.

For those of you who don’t want to run a Java applet, here’s a video demonstration. Gravity is reversed every few seconds, causing the liquid to slosh up and down over and over. The two triangles on the sides help mix things up a bit. The video flips through the different components of the animation.

It’s not a perfect liquid simulation. The surface never settles down, so the liquid is lumpy, like curdled milk. There’s also a lack of cohesion, since JBox2D doesn’t provide cohesion directly. However, I think I could implement cohesion on my own by writing a custom contact.

JBox2D is a really nice, easy-to-use 2D physics library. I only had to read the first two chapters of the Box2D manual. Everything else can be figured out through the JBox2D Javadocs. It’s also available from the Maven repository, which is the reason I initially selected it. My only complaint so far is that the API doesn’t really follow best practice, but that’s probably because it follows the Box2D C++ API so closely.

I’m excited about JBox2D and I plan on using it again for some future project ideas. Maybe even a game.

The most computationally intensive part of the process isn’t the physics. That’s really quite cheap. It’s actually blurring, by far. Blurring involves convolving a kernel over the image — O(n^2) time. The graphics card would be ideal for that step, probably eliminating it as a bottleneck, but it’s unavailable to pure Java. I could have pulled in lwjgl, but I wanted to keep it simple, so that it could be turned into a safe applet.

As a result, it may not run smoothly on computers that are more than a couple of years old. I’ve been trying to come up with a cheaper alternative, such as rendering a transparent halo around each ball, but haven’t found anything yet. Even with that fix, thresholding would probably be the next bottleneck — something else the graphics card would be really good at.

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Chris Wellons