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We'll start modifying the particle's behavior by adding forces. Forces blow or move the particles around, and cause them to shimmy, swirl, and dance under your control. To speed things along, I've prepared a Nuke script. So let's go up to File > Open > Project Media > Lesson_06_Media, script number 03 Forces. Open that. I've got a Particle Emitter all set up. So all we have to do is a cursor, and the viewer, and type F to wake it up, and we will pull back a little bit.
How about a little more screen space, so we can see our particles better? Very good! Okay, and then we'll play. So we have a nice little stream of particles emanating from origin, moving vertically in line, with just a little bit of spread out, and not too great a velocity; that'll make it easier for us to see the action. Let's see about the ParticleDirectionalForce. I'll hook it up to this node; double-click. Now, the Particle Directional Force does exactly what you would think: it applies a force to the particles in any direction that you want. It's a vector.
You can think of it as sitting at origin, and then it points in a direction, and an amplitude. With the setting, I've created a gravity type of an effect. It goes down in Y by .01. If I increase that downward force, the particles fall faster, and farther. I'll undo that. You don't have to use it as a gravity. We can also apply a sideways force; 0.01, and now they're taking off horizontally, as well as a downward pressure.
So, think of this as a vector that is pointing to the right in X, and down in Y, and that's imparting this motion to our particles. The key concept of a force is that, without any wind resistance, the particles are going to continue to gain speed, and go faster and faster, because the force, like gravity, is going to continue to accelerate them. You can see here that the particles are moving rather slowly, but out here, they're moving much faster. Okay, I'm going to set the horizontal component to 0, so we just have a negative Y, or a downward gravity type effect.
Let's re-home the viewer here, and take a look at the Conditions and Region tabs. Most of the particle nodes have both a Conditions, and a Region tab. We'll take a look at Conditions first. The first slider, probability, is the probability that this force will affect each particle. With a probability of 1, you basically are getting 100% of the force on that particle. If I set it to 0, then none of the particles are getting any of the force.
But I could set it something like 0.5, and now the gravity is being applied with a probability of 0.5, or 50%, and we get a very interesting effect. I'm going to put that back to default of 1. So there's our 100% gravity effect. Now the min and max age is very important. These are relative to the age of the particle. 0 is the birth, and 1 is the death, regardless of the lifespan of the particle. So, this is relative to that particle.
Let me scoot in here little bit. If I say that my downward gravity effect will only effect the minimum age of 0.2, that means it will not start taking affect until that particle is through 20% of its lifespan. So we'll click here, and you see the result. Okay, so for the first 20% of the particle's life, there's no gravity effect, and it kicks in right about here, and starts pulling the particle down. We'll put that back to default.
The max age; this means the force will not take effect after this number. A max age of 1 means the full life of the particles, so it has no affect. But if I set the max age to, like, 0.4, so what's happening is, it's having an affect until the particle gets to 40% of its life, and then it stops affecting it, and so the particles, in this particular case, have whatever velocity is left over, the gravity has ceased to pull them down, and so they drift up gently.
Okay, let's put that back to default. The random seed allows you to change the starting seed for the random number generator. So it just gives you a little different look at the randomness. Channels, here, is something we're going to take a look at shortly. Now let's take a look at the Region tab. The function of the Region tab is to limit the force within a small volume: a region. We can pick from these different type regions here: sphere, a box, I have spaces like a card, and a cylinder.
Let's turn on the box to see what it does. I'm going to set the size of the box up to be 10, and I'm going to move it up in Y by 5. Maybe move it up to 8. Okay, then we'll go back to the Directional Force tab, and I'm going to add a horizontal component: 0.01. Now, you don't see the box unless you have the Region tab selected. So what's happening here is that particles are being generated, and the force is not operating on them until they enter the box. Then the force operates on them all the time they're in the box, and as soon as they leave the box, it's not affecting them anymore.
Let me move that up a couple more units. You can see the affect. Okay, let's take a look now at Particle Wind. I'm going to clear the property bin, and let's switch the viewer over to the ParticleWind node, and we'll open that up. The Particle Wind is also a vector force, and the point of origin -- right here, 0, 0, 0 -- you can change that, and then this gives you the vector, both in direction, and amplitude.
At this point, we're getting a wind horizontally in X, along the X-axis, of 0.4. The key difference between a wind, and a directional force is, the particles will get up to the speed of the wind, and go no faster than the wind. So they reach a maximum velocity, and go no faster, whereas the particle forces, we saw, the particle keeps accelerating faster and faster, as long as the force operates on it. We can see that difference here. I'm going to connect the viewer to this ParticleDirectionalForce node, and you can see the great difference in appearance. I have dialed in a force that's about the same as the wind. And back to the wind.
There's also a drag, and an air resistance. Enabling air resistance means that the drag starts to take effect. With a high drag value, the particles are very quickly brought up to the speed of the wind. With a low drag value, like this, the particle only slowly reaches the maximum speed of the wind. I'm going to turn off the air resistance, and reset drag to default. Now, there're some additional force nodes. I'll stop this; come over here.
Let's push in, and take a look at the ParticleTurbulence node. This node, of course, will impart turbulence to your particle system. The ParticleVortex node will give you tornadoes, and whirlpools. And the Particle Point Force, now, this is interesting: it allows you to put a point in space, and then you could either repel or attract your particles to this point. So if you wanted them, for example to fall into a sun, you could put a Particle Point Force at the center of the sun, and all the particles would bend, and fall into your sun.
Once you have your particles moving the way you want, you may want them to bounce off some surfaces. Let's take a look at that next.
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