particle velocity problem.

[cloudfx]

Hi all,

I 've done particle set-up for fireworks and trying to make motion blur.

I used trail first time and it created strange velocity..

It looked okay in pop network but when I came out to obj network, the velocity start to go crazy..

So, I created attribute to export velocity of particle itself in pop network:

And, I was trying to read that attribute in obj network, and I got error. I need help on this:

Scene file:syndrome_fx_fireworks_002.hip

Thanks.

-j

Edited July 28, 2008 by Jae Yoo

[SpencerL]

your attribute "pv" is declared in POPs. You cannot access "pv" in SOPs using $PVX, $PVY, $PVZ. These are LOCAL variables which mean they are variables that only work within the context that they are created. In order to access this attribute in SOPs you would need to use a point expression, ie point("../popnet1", $ID, "pv", 0)...

Edited July 28, 2008 by SpencerL

[cloudfx]

your attribute "pv" is declared in POPs. You cannot access "pv" in SOPs using $PVX, $PVY, $PVZ. These are LOCAL variables which mean they are variables that only work within the context that they are created. In order to access this attribute in SOPs you would need to use a point expression, ie point("../popnet1", $ID, "pv", 0)...

Hi, SpencerL.

Thanks for replying.

I did what you told me.

I got this strange velocity.

I know that it should work that way but dont, know what it the problem.

Thanks,

p.s scene file is provided up there.

Edited July 28, 2008 by Jae Yoo

[edward]

I'm not sure why you're trying to put the velocity into a separate attribute and then copy it back in the first place. You just need to use the parameters in the Attribute tab of the Copy SOP. Here's my attempt (without rendering). Some of your velocities at certain frames do look off but I think that's coming from your POP network itself.

syndrome_fx_fireworks_002_edw.hip

[Dani F]

I'm not sure why you're trying to put the velocity into a separate attribute and then copy it back in the first place. You just need to use the parameters in the Attribute tab of the Copy SOP. Here's my attempt (without rendering). Some of your velocities at certain frames do look off but I think that's coming from your POP network itself.

Just let you know a workaround I've found when using trail SOP on particles to get a new velocity.

The problem is that the trail SOP is using the point number, not the particle's ID to compute v. Also Houdini recycles that point number when a particle dies by life, hence you'll get that crazy values since $PT is inconsistent.

The workaround I've found is to set a very high lifespan and kill the particles in any other way, in my case using expressions. This will force Houdini to keep the point number, so you can use the trail SOP safely.

So for SideFX guys, would be nice to be able to choose $ID as the trailing attribute, next version...

D.

Edited August 6, 2011 by Dani F

[gonzzzo]

There is a match by attribute in the point sop that allows you to calculate velocity from id. But that is only useful to recalculate velocity if you change the particles with a lattice or noise or something like that.

Edited August 7, 2011 by gonzzzo