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particle birth rate with velocity


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I have a popnet set to emit from points coming from the first input connection

I am trying to define my birth rate according to the input points velocity

the faster the more particles, the slower the less

but I can't find how to use length($VX, $VY, $VZ) in the birth rate

it tells me:

"Variable is not defined. Using '$VZ = 0'."

"Variable is not defined. Using '$VY = 0'."

"Variable is not defined. Using '$VX = 0'."

I checked the "attributes" tab of my source node and it inherits all attribute.

I even tried to "create local variables" but this does not help.

do you guys have any idea how to set this up?

many thanks!


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The source pop doesn't really accept local varaibles in that way.

One way of doing what you need is to color the source points based off the length of velocity (ie fill the red channel with the length). And then in the birth probability use $CR, $CR. That way you'll get more points where the red value is high and none where there is no red value.

There are other ways, but that's the easiest to explain :).


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thanks for your answers!

Macha, I should have mentioned I am using point emission so I can't use your trick.


your solution works fine for a surface, but the result is visually not very obvious for a point

I can see it stops emitting when $CR==0 but I can't see the decrease in the rate

if you could have a look into my file and let me know where I am wrong, would be great!

thanks again for your help guys :)


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My recommendation would be to create more points at the point you're emitting from...

If you have two points for example, one not moving, and the other with some velocity, you could do some sort of a copy stamping based on length($VX,$VY,$VZ) to create more points at the moving point. Then inside pops, you set it to emit npoints("path to source geometry here") on each substep.

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