Hi is there any way with an expression to control at which frames I want particles to birth at. I know that the impulse birth rate is on a per frame basis, so I would of thought to define a range of specific frame numbers would be pretty simple.
Can anyone point me in the right direction where to find the information?

cheers rob

maybe “$FF > 5 && $FF < 10” in the impulse rate field (without quotes) would give you the result? (and dont forget to put “0” in the constant activation"

'5' and ‘10’ are for the frame range…. use your own desired values.

hope this helps.

cheers.

I cannot get it to work via your method … also another question reguarding a Transform SOP and particles , I can pipe my sim into a copy sop and then into a transform SOP , I have a sphere piped into my copy SOP , If i emit 1 particle and then in the transfrom SOP use $ CEX , $CEY , CEZ I get a nice rotation of the ball in its z axis , if I use this with more particles its gets ignored and all the 5 particles end up rotating around there world pivot not there own local ….what gives there ? its on page 151 of will cunninghams book !

R

Hello,

The Transform SOP itself works as intended. Transform SOP doesn't do per-point transformation here. What the $CEX, $CEY, $CEZ is doing is that it's taking the center of the bounding box of that whole particle system you just fed in, and perform transformation to that whole system. You probably want to use Point SOP or setup your own VOP SOP to handle per-point transformation.

If you want to do per-particle rotation, I'd highly advice you to transform the point's Normal and Up Vector intead. Of course, unless you want to rotate along some other arbitrary axis (which is another big can of worm…), setting up normal and up vector should work just fine. The up vector here is just primarily use to prevent the particle from flipping and it should always be perpendicular to the Normal vector. That will allow you rotate your particles however you wish. This is something you can setup in AttribCreate, Point Sop, or VOP SOP (highly recommended).

hope that helped

Alex that sounds great but means absolutly nothing ….to me ?I understand the use of the point SOP but thats as far as it goes .. how would I set his up ? even the guy who helped write the Houdini Help in the office is a bit stuck

Rob

maybe “$FF > 5 && $FF < 10” in the impulse rate field (without quotes) would give you the result? (and dont forget to put “0” in the constant activation"

If this isn't working, it might be because the terms are being used loosely (and incorrectly). You want to put the expression
$FF >= 5 && $FF <= 10
into the field called Impulse Activation, not Impulse Rate.

In the Impulse Rate field, you want a number that represents the number of particles per frame (or per impulse, really), so that obviously should be a non-zero number

Sorry, hope I didn't throw you off base… I actually haven't read the book myself eventhough I should…

Is this perhaps what you're looking for?


hope this helped.

Im having a detailed look now Alex .. I will let you know, but I do not understand how a VOP would help me with this ? could you explain the whole point of using it.

craig thanks for putting the expression into the correct contex, So far the biggest difficulty Im having is knowing what I want to do , but having nothing to refer against , no wonder people are scared of this application, when a guy who helped write the documentation doesnt know where to look


Rob

ops.. sorry .. the expression was ment to be in the Activation .. yeah … my bad..

cheers

Can you introduce an attribute inside that expresion?

For example:


$FF >= @startframe && $FF <= @endframe



Someboy knows if it is possible?