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x-particles in a null that moves question

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chad demoss
x-particles in a null that moves question
on Jul 18, 2018 at 11:06:56 pm

I made a rocket and put the x-particles system in a null along with the rocket and moved the null up on the y axis. The movement of the null is effecting the timing of the emmiter. Ive attached a screen grab. these are the same exact emmitters. the one on the left is in a null moving on the Y axis. the one on the right is just a stationary emmitter. The emmitter in the null I don't want it to stretch out the particles like it is. I want it to be the exact same distance/timing as the one on the right. is there a button somewhere that keeps the emmitter from having world events effect it? I want the emmitter to be relative to the null not the world.

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Jim Scott
Re: x-particles in a null that moves question
on Jul 19, 2018 at 12:38:02 am
Last Edited By Jim Scott on Jul 19, 2018 at 3:06:23 am

Since the emitter is moving, and emission is all a matter of particles per second (or frame), there is naturally going to be a larger distance between particles. And since the particles are no longer controlled by the emitter once they have been emitted I think the only way to get a similar particle density (in what has essentially become an elongated emission area) is to increase the birthrate. The emitter is always relative to the null, but it's the emitted particles which are not.

Just thinking about a scenario where the emitter and its particles are separate from the world hurts my brain, but I'm old and slow (and I am certainly not an X-Particles expert), so perhaps a brighter mind will prevail. Please post if you find an answer elsewhere.

Good luck.

Edit: Perhaps you could work out an XPresso set-up that linked birthrate to emitter velocity.

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Steve Bentley
Re: x-particles in a null that moves question
on Jul 20, 2018 at 5:51:46 pm

Jim is exactly right.
Think of it less as emitting particles and instead as the rocket simply leaves them behind as it moves faster and faster forward.
I would leave the birth rate at a constant higher level. With rockets it's usually full-on until the tank is empty - in fact the solid boosters usually can't be controlled or even turned off.
The bonus with a much higher output level is that you get that lovely exhaust maelstrom before the rocket moves because the birth rate of the particles is the same as when its moving, but because the rocket isn't moving you get huge billows building up around it. Have a look at static engine tests from NASA.

For rockets we normally use a number of emitters.
One for a flame element where the size of the particles diminishes over time and changes color over time.
One for the cloud element, where the particle size increases dramatically over time and those particles spawn children that also increase in size. These also roll quite a bit as the convection and the jet exhaust start them turning. The rolling also amps up the illusion of volume (go look at the Columbia Pictures logo at the start of a film - those clouds are just a bunch of flat stills that rotate but they sure look fluffy). Also add in some air density for these ones - they shoot out fast but then the air brings them to a slow roll pretty quickly.
Another emitter sends out streaks that create detail in the flame trail and fade over time.
The final one is some smoke or dirtying element. It works like the clouds but is thin and dark and sooty. This one can be heavy handed and we see this over-done in B movies or TV shows with missile trails etc. Use some finesse here.

BUT- if you are using a volumetric shader - start small and work your way up. It doesn't take many particles to build up to a dense cloud and one that is much larger than the area you were expecting to fill.

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