This example shows how you can use groups to selectively apply different behaviors to particles. The particles first fall through the collision sphere. Only after passing through the sphere do they exhibit a different collision behavior and bounce on contact.
If the particle has collided, we want the particles to bounce. If they haven’t collided yet, we want them to fall through.
To do this, create a group of all points that have collided. The same Group POP can be used to create the complementary group of points that have not collided. Note that the Collision POP creates the “numhit” attribute so when it is accessed by the Group POP on frame 1, it hasn’t been created yet. The default value of 0 is used, which is fine since on frame 1, none of the particles have collided.
Then two Collision POPs are used to collide the particles against the sphere. The first collision is applied to the “noncollided” group and those particles are allowed to continue on course. The second collision is applied to the “collided” group and those particles should bounce off the collision object.