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## Pixar in a Box

### Course: Pixar in a Box>Unit 7

Lesson 1: Introduction to particle systems

# Smooth collisions

To make realistic water, we'll need to soften the particle collisions. Smoothed particle hydrodynamics helps create realistic fluid simulations in movies like Finding Nemo. By adjusting parameters like collision softness and viscosity, animators can mimic water, honey, and other fluids. More particles increase realism but slow down computation. Experimenting with these parameters creates a wide range of fluid behaviors.

## Want to join the conversation?

• Why was it red on the corners when he put 800 particles
• It's the collision softness. A higher number indicates that the particles collide harder with each other.
• in which app we can make these effects or animations?
• At why does the circles turn red when they touch something?
• Oh, the circles turn red simply because of the pressure. When droplets fall against each other with a high collision softness, they turn red as the pressure increases. (There's no where to go, but other droplets keep falling on them.) This may also be because heat is produced when pressure is created. When the great pressure is too much, the droplets just burst out which you see later.

Hope that helped! : )
• The moment the balls hit the wall, they raise upward. How come?
• some of them move upward. that is because they are being pushed upward by the rest of the balls.
• How are these balls made into the water we know in the movies?
• these balls are much bigger than the ones they use to make water in movies. in the actual film, the water we see is made up of very tiny balls, like a ball is a millimeter high, so we see them as water because the balls are so small
(1 vote)
• why is it turning red??
(1 vote)
• A think its because when particles collide, the RGB colors mix together
(1 vote)
• what is with the red part when the balls cloud together?
(1 vote)
• hi everyone and how were you guys day today
(1 vote)
• what program are they using
(1 vote)
• why do the particles turn red in the video but in the simulation after this video they just get darker?
(1 vote)

## Video transcript

(springy bounce) - Hey, welcome back. You probably noticed that the ping pong ball stimulator looks a little more like sand than water. For instance, the balls can pile up on each other like this. We used a stimulation much like this in Finding Dory. For instance, the sand in this shot. And the East Australian current in this shot. To create more water-like stimulation, we need to remove the hard boundary of the balls. They don't just bounce off like ping pong balls. They kind of smush together to create a soft collision. The smooth collisions are an approximation of the pressure forces that water normally experiences. There's a fancy name for this kind of stimulation. It's called "smoothed particle hydrodynamics" or "SPH" for short. Hydrodynamics literally means the motion of fluids. Now let's look at how lots of smooth particles interact. We can control how the particles collide using this parameter called Collision softness. Okay, so this is getting pretty close. Another parameter we can control is called Viscosity. Viscosity is a measure of how easily things flow. Fluids with low viscosity like water, flow pretty freely as we see here. Fluids with high viscosity like honey, flow more slowly. Like this. (water rippling) The more particles you add, the more computation is needed and the slower the program responds. (slow motion water ripple) Here's an example of SPH from Finding Nemo. It takes place inside the mouth of a whale. We've drawn the particles as balls here so you can watch their motion. In the next exercise, you'll get some practice experimenting with these different parameters to get a wide range of behaviors. See you soon!