Creating falling particles
Click the Grid tool on the Create shelf tab to create the object from which the rain particles will fall.
Move the object above the scene (up in the y-axis).
With the grid object selected, click the Source from Geometry tool on the Create Particles shelf tab. This will create a particle network in a new object with a Source node, using the grid object to create particles. You can try playing the animation to see the particles emitted from the grid.
Once inside the POP network, alter the Const. Birth Rate parameter on the Birth tab to change the amount of particles emitted per second from each source point.
On the Drive Particles shelf tab, click Gravity to apply gravity to your particles. The Emission Type parameter on the Source tab changes the points of particle emission.
On the Gravity node, adjust the Force parameter value of the y-axis. Use a larger negative number to make rain fall faster, and a smaller negative number to make rain fall slower.
On the Drive Particles shelf tab, click the Wind tool to add a force to the particles that simulates wind. Wind simulation must be done manually through key frame animation. Refer to Animation Basics for more information on key frame animation.
Experiment with the parameters on the Noise tab to generate particle variations. For example, to create a noise effect that replicates snow, increase the Amplitude while decreasing the Frequency.
Rendering the rain particles
On the mantra node, click Properties > Sampling and turn on Allow Motion Blur.
Set the Geo Time Samples parameter to 2. This controls the length of the particle streaks. You can increase this value for longer streaks.
Experiment with Xform time samples to change the length of particle streaks.
Go back up to the object level and select the particle object. Click Render > Sampling and turn on Geometry Velocity Blur.
Normally mantra assumes the number of points in a geometry object stays the same over time. Since this is not true of the particle system, you need to turn this option on so mantra knows to blur newly created points.
Create a camera pointed at the rain.
Select the Camera object, click the Sampling tab, and increase the Shutter Time parameter. This makes the particle streaks appear longer during rendering.
Render the rain as a separate render pass.
Add lighting to the scene as required. For information on Lights and Cameras, refer to Lights and Cameras .
Adjust the Uniform Scale value on the particle emitter node at the object level to change the scale of rendered particles.
To create the effect of thinner rain, turn on Render As Points (Mantra) on the Particle object, which is located on the the Geometry subtab of the Render tab. Particles will render faster and as dots rather than spheres.
Using the procedure above, the particles continue to fall forever. You can use the Collision POP to remove the particles when they hit an object, such as a ground plane.
Create and position geometry representing the ground.
Click Collision on the Drive Particles shelf tab while making sure that nothing is selected in the viewport.
Click the particles you want to collide and press Enter.
Click the ground object you created and press Enter.
This will create a Collision POP inside the POP network. Play the animation to see what happens when the particles hit the ground object.
Select the Collision POP in the parameter editor and on the Behavior tab, change the Behavior parameter to Die on Collision.
Making particle splashes
In the network editor, select the Collision POP. On the Behavior tab, set the Behavior parameter to Bounce.
Provide a name for the Collision Group parameter (for example, “splash”). Particles that collide with the ground will be placed in this group.
Modify the Gain Normal parameter to adjust the particle bounce. Try a value of
On the Birth tab of the Split POP, set the Birth Probability parameter’s minimum and maximum values to 10.
The Split node kills particles in the collision group, replacing each with 10 new particles.
On the Birth tab, change the Life Expectancy to
1and turn on Kill original particle.
Click the Attributes tab and set the Initial Velocity to Add Inherited Velocity.
To add variation to the splashes, use the Variance parameter. For example, values of
2, 0.5, 2will randomize the direction and height of the split particles.
Enter the collision group name you used on the Collision node (for example, “splash”) to the Source Group parameter.
Move the Gravity node from above the Collision node in the POP network to below the Collect node. Making this change applies the gravitational pull to the particles created by the Split and Collision nodes.
Click the Behavior tab on the second Collision node and change the Behavior to Die on Collision. Aside from Behavior, the rest of the parameters on the second Collision node can be identical to that of the first.