Gloop.
Interactive color-bending physics-based experience powered by volumetric indirect rendering. Writeup by Ben Congdon.
There is just a little bit of magic behind the scenes, as we progressively apply a physical linear drag to particles that are close to the controllers and currently being attracted. This helps the particles combine into a nice appealing glob! Without, with nothing to damp all the force being added, particles were more scattered as they bounced off one another before returning with higher force. Ultimately it meant the system needed a long time to settle. Adding the drag made the particles settle quickly and also provided an illusion of thickness or friction that the rigidbody physics system was not able to simulate.
On a technical level, the overall approach is to draw the particles into a volume which we then use to generate a new mesh. This all happens from scratch every frame. The advantage is that we can create the mesh at a low resolution and rely on rasterization and interpolation to make the result sharp at the user's render resolution. We can also use the mesh for shadow casting as in the longer video above. As the entire volume is one big mesh, we cannot sort it correctly for transparency, so we use order-independent transparency to create the water or glass-like effect in the second video.
This project was a bit of an experiment to see how far we could get with fluid rendering in virtual reality. VR typically needs a high output resolution, and we need to maintain a high framerate at all times as even short framerate dips can make people feel ill. So the aim here was to decouple the output resolution from the rendering of the liquid as much as possible to get something that was very fluid and fun to interact with using physical body motion.
I think my favourite part of the project was discovering how motion-tracking changes the feeling of interacting with the liquid particles. The control scheme is really simple; pulling the trigger attracts the particles, and the greater the trigger pressure, the more strongly the particles are attracted. But with just that simple input you can get some very fun behaviour out of the liquid by waving your hands around to influence their individual momentum, breaking them up and joining them together again.