Dan's expression works great but I often parent my "emitter" null to another null for more complex animations.  In any case where the "emitter" Null is parented you'll have to use an expression that doesn't rely on the position property of the Null but rather the rate of change of its absolute world position.  Altering Dan's expression this would be:

minSpeed = 0;

maxSpeed = 500;

minParticles = 10;

maxParticles = 1000;

n = thisComp.layer("Null 1");

f = thisComp.frameDuration;

p1 = n.toWorld(n.anchorPoint);
p2 = n.toWorld(n.anchorPoint, time + f);
s = length(p2 - p1) / f;
linear(s, minSpeed, maxSpeed, minParticles, maxParticles);

Here's an animation preset that will apply Particular with the required expressions, as well as some expression controls to allow artists to change the variables of the expression on a per-layer basis:

http://dl.dropbox.com/u/1950339/SpeedEmit_Particular.ffx

This animation preset uses some error catching to avoid problems if the artist does not have an "emitter" null selected.

-D

On Thu, Jan 5, 2012 at 7:03 PM, Dan Ebberts <debberts@comcast.net> wrote:

There’s more than one way to do it, but something like this should work:

 

minSpeed = 0;

maxSpeed = 500;

minParticles = 10;

maxParticles = 1000;

N = thisComp.layer("Null 1");

linear(N.transform.position.speed,minSpeed,maxSpeed,minParticles,maxParticles)

 

 

Dan

 

From: Chris Biggs

Sent: Thursday, January 05, 2012 6:51 PM

To: After Effects Mail List

Subject: [AE] Attention Expression Gurus

 

I'm working on a project that requires a Base comp be built that all the artists can use.

it's a Trapcode particular effect that needs to add to amount for particles emitted depending apon

velocity of the emitter ( using a null to drive the emitter)

i.e. moving slower less particles, faster more particles.

 

any suggestions ?

 

Thanks

Chris

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R.Christopher Biggs
Digital Ronin