Particle density emission by speed or distance to object...

[Bog]

Hi!  First question here....

I've got a bunch of particles which are streaming from their emission, to a destination.  I'm controlling them with a POP curve force.

What I need the particles to do is emit density, but I need them to do so either based on speed (faster = more density emission) or on distance (starting a 0, ramping up to full emission as they reach the end of their course).

I'm sure this should be straightforward, but I'm not sure how to convey my wishes to Houdini.  Can anyone throw me a hint?

Edited February 8, 2018 by Bog
Clarity.

[Atom]

You can drop down a wrangle before you stamp out your density into the fluidsource.

This will allow you to use a ramp to shape density based upon age.

float index_into_ramp = fit(@age,0,@life,0,1);
f@density = chramp("shape",index_into_ramp);

 

You could also examine velocity and construct something based upon that value as well. Assume particles are falling.

// NOTE: -50 is determinend by looking at the spreadsheet for the fastest value.
float index_into_ramp = fit(v@v.y,0,-50,0,1);
f@density = chramp("shape",index_into_ramp) * 100;
f@pscale = chramp("shape",index_into_ramp);

 

You may want to create a second ramp to control the @pscale attribute for size of life perhaps?

 

For distance to object plug your object into the second input of the wrangle and determine the length of the vector from the current particle to the center point of the object on the second input.

Typically...

float l = length(@P-OpInput1_P);

 

ap_desnity_stamp_by_speed.hipnc

Edited February 8, 2018 by Atom

[Bog]

Thanks!  I'll see if I can get closer to where I need to be with this.