Hi,

I have a simulation of particles orbiting the Y axis looking at a point (think globe) which is then copied to a geometry of a box.

What happens is that when I run the simulation, the particles tilt in the direction of it's movement. The only tilt I want is for it to look at a point. Can anyone give me pointers as to how to achieve this effect?

you'll need to add a normal that is looking at your orbit, and an up vector that is the cross product of n and v

I'm not too sure how to add a normal in Houdini

but for now, I've add an up vector to the bottom of my POP Network, changed the up vector type to vector and added the following expression:

cross(vector3($NX,$NY,$NZ),vector3($VX, $VY, $VZ))

Which gives an error “undefined variable” I'll keep working on it, but any help will be appreciated

That would probably be because you have no normal
Are you trying to add this at the geometry level or during your simulation?

You can probably get away with doing this after the simulation.

You can use a point sop to add a normal, you'll probably want to do something like the beginning of http://download.sidefx.com/images/stories/tutorials/expressions/13_custom_attributes.mov [download.sidefx.com] to get a normal that is looking at your origin/centroid/specific point, only on a per $PT/$ID basis.

I'm trying to add this in my POP network. So I assume that it's simulation.

Tho I'm a bit confused now as I can't add a point SOP here. Still, will keep trying to figure out your instruction I've only been doing CG for about a week and it's been almost 10 years since I touched geometry at school

Unless you need the attributes *in* the simulation or for some reason can only acquire the attributes during the simulation, it is typically better to add them after the simulation. This way you can write the sim to disc, and do post processing that is easy to customize. Allows you to spend much less time redoing simulations.

You might be able to use the properties pop, or attribute pop to add normals if you insist on doing it in the simulation.