Hey, me again.

So general question - I'm grouping particles with a popgroup DOP. I'd like to capture the frame, or the age, or some information relating to when the particular particle becomes part of a group. How could I go about doing this? If I can't do it with groups how else would I acheive this? It's like I need to flag an event per-particle so it can smoothly transition from one state to another.

Cheers

Do you mean you want to store the frame on which a particle changed some sort of attribute? Like a trigger frame?

Yes indeedy - since posting this I've been looking at collision detection POPs and I'm making some progress, but any pointers would be helpful.

you can either create custom attribute that accumulates the age particle has spent in certain group/stream
http://forums.odforce.net/topic/20755-wrangle-reset-particle-age-when-the-particle-enters-a-group/?p=124254 [forums.odforce.net]

or maybe even use f@hittime if it's really about collision
hittime attributes stores time of the last collision (generated by POP Collision Detect, or Add Hit Attributes on POP Solver)

Hey!

Sorry it took so long. This is a multi-purpose trigger technique I use all the time. Uses some simple VEXpressions. I hope you're comfortable with those. It's all commented for you.

The gist of it is this:
1. Store a ‘control’ and ‘active’ attribute, both equal to 0, on the source geometry.
2. Put your particles in a group, according to any method you like. In my case, I'm using a bounding sphere.
3. ‘Activate’ the particles by setting the attribute to 1 when they're in the group.
4. Once the particle is ‘active’, then it'll be greater than the ‘control’ value. At this point, store the active frame.

Now that the points have an ‘active_frame’ attribute, you can compare that with the current frame and do something cool. In my case, I just color the points red.

Feel free to ask any questions!

That is awesome Francisco, thank you very much! That looks like a trick I'll be using for years to come. Really cool. Thanks for your help as well, Tomas.

You're very welcome!

You can also do this using a multi-solver with a SOP solver tied in. But, that's a little convoluted. The vex snippet is much more compact and elegant and probably faster.