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.
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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.
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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.

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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.

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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.

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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.
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