I am somewhat confused as to how to use geometry as particles. Basically nothing is happening. Particles are not showing up, nothing is really working.

Here is how I set up the project.
I have geometry node name Emmission Surface. In this node I have two SOP. An edited NURBS surface with appended Point SOP.

Two other geometry nodes, named Part-Obj1 and Part_Obj2 each contain one Edit SOP with simple polygonal geometry I want to use as particle objects.

An additional geometry node, named Particle System is where POP SOP is called.

Under Particles, I have a POP network with following POPs
Source POP that is using Emmission Surface under Object and Point SOP under SOP tab.

To Source POP and Instance POP is appended. It is referencing Part_Obj1 and Part_Obj2 under Particle Geo tab.

To Instance POP I have appended a Wind Force POP.

So to recap POP network, I have set up Source-Instance-Wind in that order. When I scrub or play forward in Particles I can see instanced geometry used as particles.

Now back to Geometry SOP.
In a new Geometry node called Part_Sys I have set up a POP SOP, that is calling Popnet1 under POP Network tab, and using CookPOP under POP Name tab. However the particle geometry is NOT displayed. Only lines or points, depending on Particles Display options. Even if I have POP SOP call Part_Obj1 and Part_Obj2 as Input Geometry I still see nothing. Its not working. Test renders just render little default spheres.

So what I am doing wrong. How do I use geometry as particle objects?

David Rindner

To see it in your renders, you have to turn it on in the Geoemetry object parameters underneath the Render tab. Make sure that the Point Instancing toggle is turned on and leave Point Geometry set to No Object so that it will pick up on the instance attribute inside.

This is one of those friustrating moments, where the instructions make no sense at all. Turn WHAT and WHERE? Details please. I do not understand what you are refering to.

Dave R

the parameters he's talking about are found in the render tab of the geometry object.

Sorry, let me try again. Right-click on your Part_Sys object in the Network Editor. From the menu, choose Parameters… This will bring up a parameters window for your Part_sys object. Flip to the Render tab (aka page). Turn Point Instancing on and leave the Point Geometry parameter set to No Object.

Hope that helps (more .

Thank you. But still nothing. Instances show up in Particles preview, but not in SOP, nor in Objects, nor do they render. This can't be this complex. Theres gotta be something that I forgot along the way, but I can't locate what it is.

What I really wish and what I can't find, is a simple example, of a geometry being used as emmitting surface, that is emmiting two separate geometry instances as particles.

Dave Rindner

I hope this will lead you to the right spot. If you go all the way up to the objects state. If in sops press u. Select the object that you have the pop sop in. On the parameters page go to the render tab and turn on point instancing.

Residnt

I just tried this i works. Try having a seperate object node for just the pop sop if it doesn't.

I do have point instancing turned ON for SOP Node where POP SOP is located , and its also the only SOP in the node. Point Geometry is to No Object. This was advice I received earlier, but it simply doesn't work. The geometry instances do show up in Mantra redners, but nothing I can do to make them show uo in the viewports. All I can do is to make them appear as lines, point, or sprites, as in Particle display options. What I want to see is the particle geometry in viewports. How is that done?

Dave Rindner

Hi David,

When you use the Instance POP, the design is such that you'll see the Instanced geometry in the POPs viewport but not the SOP's or Object's viewports.

If you would like to see the geometry, you can use an Object Merge SOP to read the selected geometry in the PartSys Object then append a Copy SOP. Next, connect the Pop SOP to the second input of the Copy SOP. Now, you should see the geometry in the viewport.

However, the above defeats the purpose of using Point Instancing, IMHO.

I hope the above helps!

Cheers!
steven

Steve

Thank you, thank you, thank you. This is exactly what I wanted. So from now, if I want to particle system state I'use this procedure.

So to recap this procedure, which in hindsight is quite simple.

In the Geometry node where POP SOP is called, create two nodes.
Object Merge, to read instance particle object, and Copy SOP.

Append POP SOP into 2nd slot and Object Merge into 1st slot.

Once again thank you.

Dave Rindner

Hey David,

You're welcome!

One thing though & quoting myself:

stevenong
When you use the Instance POP, the design is such that you'll see the Instanced geometry in the POPs viewport but not the SOP's or Object's viewports.

However, the particles will render as the Instanced geometry if you turn on Point Instancing for the Object.

Cheers!
steven

Steve.

Once again thank you. I understand now, why the system is implemented the way it is. If you have very large number of particles instancing, say isosurfaces or metaballs, then you just want to have the motion preview.
For small number of particles, at times geometry view is preffered. In my case, I was experimenting with flocking of birds, which brings me to my next set of questions.

For a bird particle, I created a low polygon bird that is flapping its wings through Blend SOP (which I am controlling through CHOPs). The bird is instanced for 50 individuals by Source POP (a cube) in volume. Particles are emmitted only on 1st frame (Impulse Activation set to 1 at Frame1, 0 on all subsequent frames), so the flock count never varies. The birds are given an initial velocity through Point SOP appended to the emmission volume cube. They then fly to the Attractor, the position of which is controlled via simple F-Curves in Channell Editor. I am using Attractor POP only becouse I could not figure out how to use Follow POP (see below). My problem is that they all beat their wings identically. This makes sence sence I am instancing geometry which uses Blend SOP for its Render and Display. My question, which POP, or an option in Source POP, or Instance POP, that would randomly offset the timing of Blend SOP in the instanced individual, so the flock members don't beat their wings in unison? My experince comes from MAX's particle systems, which have explicit variable for randomly offsetting animation channels for individual particles. I am looking for same thing in Houdini's POPs.

My next question concerns Source Group option found in many POPs. What is it used for? In my working with POPs, depressing the Source Group arrow button brings up no options as when working in SOPs. So there is never anything that POPs identify as potential source groups.

Follow POP. On this POP I am confused as to how to define the leader.
I append the Follow POP to network (1st input). Then Instance POP an object which I want the particle flock to follow. That POP is piped into 2nd input of Follow POP. My logic is that I can then animate the leader object normally through CHOPs or F-Curves. But it doesn't work that way. As soon Follow POP gets the 2nd input, it issues Red Arrow error.

Thank you

David Rindner