User Guide > Effects > Collisions   

Particle Sliding

When particles collide with a surface, you can set them up to slide along the surface until they drop off the edge. This behaviour is available as an option in the Collision POP and can be controlled using the Cling attribute in the Property POP.

The workflow explored in this lesson can be used to set up sliding in your own scene or you can use the following Houdini file to explore your options.

Load Start File


The start file opens with a stream of particles passing through two surfaces. The particles are being emitted from a Location POP and are being affected by a Force POP acting as gravity. The particle popnet has been set up with a 2 second pre-roll which creates a stream of particles at frame 1.

Playback the simulation to see the particles passing through the two surfaces.


Step One: Collide the Particles

Select the water object and press i to go to the geometry level. Press i again to go into the popnet network node. In the Viewer pane, press tab > Filter > Collision. In the Parameter pane, click on the plus sign (+) next to SOP and choose the ledge object. That object's display SOP will be used as the colliding geometry.

Playback the simulation. The particles now hit the surface and disappear. This is the default behavior used by the Collide POP.


Step Two: Set the Particle Sliding

In the Parameter pane, click on the Behavior tab and set Behavior to Slide on Collision. Now when you playback the simulation, the particles hit the surface and slide down until they roll off the side.


Step Three: Add Clinginess

In the Viewer pane, press tab > Filter > Property. In the Parameter pane, click on the Misc tab and click on the checkbox next to Cling. By default there is an expression which is designed to pass through any cling attributes that might exist higher up in the network. RMB on the text field and select Delete Channel then set Cling to 0.5.

Playback the simulation paying special attention to the right view. Change the value to 2 and observe how the particles stick to the surface longer before dropping off. Next set the value to 5 (shown below) to see the particles hanging on until they shoot off the edge of the surface. At the end set Cling back to about 0.3.


Step Four: Add another Collision Object

The sliding capability can be applied to more than one Collidion POPs. In the Viewer pane, press tab > Filter > Collision. In the Parameter pane, click on the plus sign (+) next to SOP and choose the ground object. Next, click on the behavior tab and set Behavior to Slide on Collision.

Playback to see the results. Now the particles are sliding along both surfaces.