Scene Setup:
Box to spawn particles inside, and a Geosphere to use as the parent particle (This will have a smoothing group setup on it).
Method:
To do this all you need to do is plug your particle group (for whose surface you want to spawn off of) into a MatterWaves node. Usually this node can be used to spawn particles on the surface of geometry in the scene, but by using the particle input you can spawn on them instead. This also lets you use all the features, such as spawning only on a certain smoothing group or selection.
Groups:
Spawn_Parent_Particles:
Spawn_Child_Particles:
Parent Particle Setup:
I set the top half of a Geosphere to Smoothing Group 2, and the bottom half to Smoothing Group 1. I then used this as a GeomInstance for my parent particles. From here in the MatterWaves you can set it to spawn from Smoothing Group 2.
Dynamics:
Spawn_Parent_Particles:
Spawns particles to use as the objects to emit particles off of. These are spawned inside a volume, and assigned the Geosphere that has been prepared earlier.
Spawn_Child_Particles:
Spawns particles off of the Spawn_Parent_Particles' surface using MatterWaves, and Smoothing Group 2.
As a side note, by checking "Use Active Emitter only" you will spawn your full amount of particles from only a selected region. For example if you are using a pistol shot of 10, you will only get 5 particles spawn on the selected surface, as it has 10 particles places across the entire sphere, but only 5 are within the selected area. With this checked it will spawn those 10 particles only within the selected area and so you will get all the particles you set in the spawn options.
