I am running through a tutorial (from 2012, so ancient) for making motion trails. It uses (old school) POP network to generate particles that will be connected into curves, and it uses a PartitionSOP to group the particles by their point of origin (set in the SourcePOP.)

The origin attribute exists and has the correct values, but I cannot create a rule that effectively uses the origin attribute in the connected PartitionSOP, even after adding an AttributeCreateSOP to “activate” the attribute outside of the popnet.

I have tried groupname_@origin, groupname_$origin, and groupname_$ORIGIN but none of them do anything, the group is just “groupname_” with all the points in it.

How can I get this to work, or what is the “new” way of doing this. Basically there should be a group of points for each emitting point that can be dropped into an AddSOP to make the lines.

This thread has some example files for making trails emit from geometry.
http://forums.odforce.net/topic/25909-how-to-add-continuously-generated-geometry-into-simulation/#comment-151177 [forums.odforce.net]

That is some cool stuff but a little beyond what I am asking…

What I want to know is more general: How to dynamically create particle groups based on a particle attribute. I have an “origin” attribute that holds the emission source id and I want group those particles based on that “origin” value.

The tutorial, which I guess is outdated, uses old style POP network and a PartitionSOP. That isn't working. What is the best way to dynamically group particles now?

Well, I rebuilt the particles using a DOP network (starting with Source Particles from shelf tools) and the Partition -> Add method of creating groups and adding a line per group now works!

The major difference I see now is that with the old POP network I was able to get a consistent number of points in the emitted trails. With the new DOP POPs the emission is not perfectly even like before. Cranking up the substeps in the popsolver helps but it still has gaps.

So why does the new POP source, emitting from points and using the same emit on impulse with the count set to the number of emitting points, not emit consistently one particle per point per frame? I turned off probabilistic emission.

I may be misunderstanding what you need but couldn't you use a for each driven by the @id attribute from the particle then use a trail SOP to create the geometry?

Probably? Sorry, getting up to speed again with Houdini. Seems like there have been some major changes in this area so the tutorial I was using is probably outdated.

The method was to generate particles emitting from an object (well, from points on the object) and then make curves by connecting them. Wasn't able to get the Partition node to recognize the particle source attribute. It works with the DOP POP though.

A side note: Using the old POP network it would lay down points in a very regular manner with the right impulse count and substeps settings. With the new DOP POPs the particles are emitting with irregularity even with high substeps. I cannot find a way to get the perfectly regular emission of the old POPs.