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Emitters control the size and the shape of the area that emits the particles. The idea is to connect 3D geometry to the Particle Emitter node, and the particles will be emanated from its polygons, vertices, and edges. We have a Nuke script to load, so we'll go to File > Open > Project Media > Lesson_06_Media > 02 Emitters Nuke Script. Open that, and cursor in the viewer. Type F to wake up the 3D viewer, and we'll play.
So I've got a basic little setup here. We are using our snowflakes here. I'm using Frame 4, because it's a little nicer shape. And the ParticleEmitter node just has a couple of little settings in it: the maximum lifetime of 20, the size of 0.1, and the spread of 0.25. Now, we already talked about setting up the particles, and now we're talking about the emitters. You don't have to do it in that order; you could do the emitters first, and then the particle second. So let's take a look at how we'd set up an emitter.
The ParticleEmitter node has an emit input; you connect this to your 3D geometry. I am going to use the sphere. Now, we don't see anything; the particles jump, but we don't actually see the sphere. I'll double-click on the sphere, so that you can see it, and we'll set it to a wireframe, just to make it more visible. So when you attach the geometry, you're now emitting from each of the polygons, or edges, or vertices. Without any geometry, it emits from a single particle at origin shooting straight up in Y.
Let's put the ParticleEmitter property panel at the top of the property bin by double-clicking on the ParticleEmitter node, because I wanted to show you the emit from features. These are the rules for how the particles would be emitted from the geometry. You can choose points, edges, or faces; we'll leave it on points. And the rules of emission, which can be randomly, or uniformly, or in order. Randomly means it's going to randomly choose amongst the points in order to achieve an emission rate of 10 per second.
The next option, uniformly, means it's going to actually emit particles from every single point uniformly. We can see what that looks like here. Little more screen space, and orbit around. So every single point is uniformly emitting 10 particles per second, and a spread of 0.25. If I change that to 0.01, you can see the spread is much narrower, and you can really make out how they're emitting from each vertex. In order means it's going to emit them from the points in their numerical order.
Sometimes that might be useful. Here we go; there, you see? So it's walking around the points in numerical order to emit the particles. Okay, we'll stop that, and jump to Frame 1, and let's set the emit from back to randomly, and we'll pull the playhead out a little bit, so we get a little more particle action here. Alright. So we're done with the sphere, so let's pick up our emitter input to the ParticleEmitter node, and hook it up to the card. Now, the sphere is still in our face, so all we have to do is select it, and disable that.
Let's double-click on the card, so that we can see it in the viewer. Now, this is a low poly card. I just set it for 4 and 4. That way it's not too dense, so we don't have particles in our face. This is the kind of the emitter surface that you might want, if you were doing like rain or snow; things falling from the sky. Of course, you can take the card, and orient it any way you want, or change its scale; rotate it. So any of the transformations of the geometry here will be applied to the card, and the particles will emit accordingly.
Now, I'd like to hook up this ramp to the card to show you something completely new. That ramp has applied a gradient to our card, and it simply goes from black to white. This is a texture map on the geometry, and this texture map can be used to control the particles. Let's double-click on the ParticleEmitter node to put the ParticleEmitter property panel at the top of the property bin. Bet you can't say that ten times real fast. So here's what we can do with that texture map.
You'll notice that several of the parameters have a channel; here is a mass channel, a size channel; a velocity channel. We can, for example, take the size channel, and select the red channel of that texture map. So we're getting the red channel from whatever image is connected to your Particle Emitter geometry. This now controls the size of our particles. You'll notice that along this edge where it's black, the size is zero. And as we get to a dark gray, they're smaller, and they get larger as you move up to the white.
Another example -- we can, by the way, just turn that off. Let's go up to the velocity channel, and select the red channel again. This means the velocity of the particles as they're admitted down here in the black part will be zero velocity. Here, they'll move more slowly; there, a little faster, and so on, up to the white part, where they move the fastest. So let's take a look at that. We back off a little bit, and I'll hit Play. So you can see that they're moving faster and faster as you go from the dark up to the light. They're also getting taller, and that's because everybody has a maximum lifetime of 20, but the particles that are moving faster get further before they die.
And by the way, you can use any 3D geometry as an emitter; it does not need to be a Nuke geometric primitive. We'll stop that, turn off our velocity channel, and conclude by saying, with the Emitter node all set up, we can now add additional particle nodes to modify both the particle behavior, and appearance.
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