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Comet tails and other streaking effects are created by spawning additional particles from the main particles. For these effects, we will need to use the ParticleSpawn node. To see how it works, we will need a Nuke script. So let's go up to File > Open > Project Media > Lesson_06_Media, script number 06, streaking, Open. And we will push in and get a little bit more screen space here.
Okay, let's take a look at what we have got. Cursor in the Viewer, type F, orbit around, push in, and over, and play. So you have got a little particle emission set up here. We are getting our color and our graphics from this Read node; this Grade node turns it yellow. We are using the card as an emitter to vault them up into the air, and we have a little particle gravity here to make them bend back down. I will stop this. The mission is, then, to put streaks after these.
I am going to start by cutting down on the point density. I am going to set the emission rate from the particle emitter from 0.5 to 0.1. So the particle emitter will emit one particle every 10 frames, which will mean fewer particles. This will give us room for our comet tails. Okay, let's push in here. So let's take a look down here at the ParticleSpawn node. I am going to open that up in the property bin, and do a few settings for you.
First of all, you want to turn off the conservation of mass, and the conservation of momentum. Those introduce some complicated physics that we don't need. Next, let's set the lifetime from 10 frames, to 1 frame. That means the spawned particle will only live for one frame. Last, lets set the velocity from 1 down to 0. A velocity of 0 means the particle is sort of just left behind the main particle as it walks away from it. So the spawn particle has no intrinsic velocity or speed of its own.
And then let's set the emission rate for 4. This means we are going to create four spawned particles for every frame. Okay, now let's hook up the viewer to the ParticleSpawn, and see what we got. We only got one little spawned particle. Actually, we don't. We have four spawned particles based on this emission rate. The problem is, they are all stacked up on top of each other right here, and the reason they are stacked up is because the emission is happening only on even frame intervals.
So on Frame 1: 1, 2, 3, 4 spawned particles, and they're all sitting right on top of each other. The way we solve that is with the ParticleSettings node. The ParticleSettings node causes the particle system to process particles in between frames. So if I set this to 4, matching the emission rate in my ParticleSpawn node -- we will hook it up to take a look -- I now have four particles. There they are.
1, 2, 3, 4 behind the main particle. Now, I would like a bit of a longer tale, so let's go the ParticleSpawn, and set the lifetime from 1 frame, to 4 frames. There we go, and now my tail is longer. Alright, so let's give it a treatment, a look, and for that, we will need the ParticleCurve node. We saw this in the last movie, so let's open up the ParticleCurve node, and let's start by changing its size over its life.
So I am going to turn on the size parameter, select size curve, and I want the size to be 0 at the end of the life of each spawn particle. Keep in mind, this is the life time of the spawned particles; not the main particles. Second of all, I want the initial size to be a little bit smaller than the main particle. Now, a critical issue: we have to tell the ParticleCurve node to affect our spawned particles, and not the main particles.
And the way we do that is we go to the Conditions tab, and tell it, the channels that we want to work on are only the b channel particles, because our spawn particles, as it turns out, are all in the b channel. Here is how. Let's go back up here to the ParticleEmitter, double-click to open it up. The main particle emitter is throwing all of its main particles into channel a, right here. ParticleGravity; the ParticleGravity condition is set to only operate on channel a. So far so good.
The ParticleSpawn Conditions tab is set to operate on the a channels. However, the ParticleSpawn output is the b channel. So the main particles come in on channel a, and the spawned particles go out on channel b. That's how we can operate on them separately from the main particle. Okay, back to our story. Let's get the ParticleCurve at the top of the property bin by double-clicking. Alright, so we are now talking to only the spawned particles with the ParticleCurve node. Back to our curves.
So if I now connect the viewer to the ParticleCurve node, I get a size change over the length of the tail. Now that we have a ParticleSpawn setup roughed in, we can start dialing in more attributes.
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