Houdini 20.0 Particles

Reacting to surfaces

The default behavior for particles that hit a Static Object DOP is to bounce off. If you want them to react in another way, you can use the POP Collision Detect DOP.


If you are colliding particles with a Grid SOP, select the Static Object DOP and open the Collisions tab. From the Collision Detection dropdown, choose Use Surface Collisions.

The POP Limit DOP is very similar to the Collision POP, but instead of detecting collisions with geometry, it detects particles “colliding” with an imaginary boundary within a cube in 3D space. This is useful to automatically cull or bounce particles that fly too far.

The POP Soft Limit DOP is like the POP Limit DOP, but gives softer bounces. It slows particles down when they hit the boundary, giving a less jerky response than a limit or collision.


You can use a POP Property DOP node to add randomization to the Mass and Bounce so that particles don’t all bounce to the same height.

Using Collision Detect

  1. Create a particle system using the Location or Source shelf tools.

  2. Create an object for the particle system to collide with.

  3. Click the Collision Detect tool on the Particles tab.

  4. Select the object for collision detection and press Enter.

    For specific parameter help, see the Collision Detect POP node help. You can choose what happens to the particles once they hit using the options on the Behavior tab.


    Turn on Move to Hit if you want to trigger an effect from a particle’s collision. For example, if you want to birth more particles, you want the particle at the hit location, not where it ended up at the end of the frame. This will move the particle back to it’s hit location.

    In the following example, particles that hit the grid are sticking to it and are colored red.

  5. If you see particles, passing through the collision object, select the POP Solver DOP and increase Min Substeps and Max Substeps to improve collision detection.


Standard POP collision modes support the hittotal attribute that tracks the cumulative number of hits. See the Attributes page for a full list of Collision attributes.


Getting started


Next Steps