A Beginner's Guide to Particle Rain in Maya

 

                by  Alan Fregtman  &  Erik Tryggvason,   11/20/2007

 

 


In this tutorial, we'll outline how to create simple rain using Maya's particles system. Read on.


 



Before we begin to even use particles, we'll need to be in Maya's Dynamics module.

This will reveal the Particles menu.


[We won't be using the Dynamics shelf because it can be accidentally modified or

destroyed, whereas menus are always there and have concise names.]











1. Creating The Emitter


First, we'll need an emitter. Rain comes from high above from random locations over a general area. To achieve this effect, we'll need to emit from an object.

 


Create a surface to emit from.  We can use any object, but for simplicity, we'll use a plane:


Create->Polygon Primitives->Plane
        - let's turn the subdivisions down to, say, 2 in each direction.

 



With the plane object selected, go to: Particles->Emit from Object

 

Create the emitter first, then we'll adjust its options in the channel box.


This will make our grid emit particles in every direction.



You'll notice that just clicking emit from surface causes particles to emit

from the vertices of the grid. That's because our emitter is an omni

emitter if we don't specify any options.



All of our options, and some more, will be in the emitter attributes page.







2. Emitter Settings


There should be tabs labelled particle#, particleShape#, and emitter#, where # will depend on however many emitters are in your scene.


particle# is your particles object (the actual particles),

particleShape# dictates their appearance, and

emitter# is where particles originate.






Other tabs are probably going to be lambert1, time1, and initialParticleSE.

[We won't need to change anything in these, or particle1.]






Without setting options, our particles will continue to emit without disappearing.

Eternal particles can be a problem as the more particles are in your scene, the slower the computation will be.


We want our particles to live long enough to do their job.


Under the particleShape tab (in the Attribute Editor) we have a lot of options.

A handy one right now is Lifespan Attributes.


 



     There are four different modes:

Live forever, Constant, Random range, and lifespanPP only.



Set the lifespan attributes to Random Range.








Here we've set our Lifespan to 1.000 and our Lifespan Random to 5.000.

This means that our particles die off after 1 second, give or take a random number
between 1 and 5.

This should make it look less robotic, more chaotic and thus more natural.





3. "Oh GRAVITY, I SUMMON THEE!! "


Right now, our particles are still flying in every direction.  If we want them to fall in a certain direction, we'll need to create Gravity.

In Maya terminology, forces are called "fields". All the fields are available to us under the Fields menu.


Select your particles (not the emitter) as an object and go to Field->Gravity     



The particles should now be falling downwards.

(Default gravity is set to a value of 9.8, which is Earth-like. Don't touch it.)



We haven't changed our emitter type yet, but it does
give us a good idea of how our particles are behaving right now.







[ TIP:  If your particles are hard to see, you can change the background color using Alt+B ]



LEFT: Particles emitting as Omni.               RIGHT: Particles emitting as Surface, and BG changed.




4. Making the Particles Look A Little Bit More Like Rain


To change the look of our particles, we must play with the options under particleShape# ยป Render Attributes.


There are a variety of display options.   "(s/w)" means it will render in software only, which we won't be using.




Let's choose Streak.


Don't forget to click on the "Current Render Type" button
(next to Add Attributes For).

If you don't, you won't see attributes for modifying the look
of the current render type.







Now, our particles are still falling directly downwards.






We can change this by using a Turbulence field.


This will create irregularities in the motion of our particles, akin to noise or jitter.








Magnitude: will increase or decrease the intensity of the field.
Attenuation: will affect how the force diminishes over distance. No attenuation means forces will remain constant.
Frequency: will let us change how often there are irregularities.

Unless it's really windy out, though, rain doesn't blow every which direction.
Let's set our magnitude between 5.000 and 10.000, and the frequency to 50.000

This will give our particles a bit of chaos in their movement, but not too much.






Unfortunately, while our particles have some more randomness to their movement, they still fall for the most part directly downwards.
A vortex field can help here.


While turbulence simulates disturbances in the air on a small scale by itself, a vortex field will pull our particles in a circular direction.

This may not seem fitting, but it actually makes them look more believable if the effect is gentle. (We're not looking for a tornado here.)






At high values, we can simulate tornadoes or whirlpools.
At smaller values, we'll be able to make our particles as a whole spin slightly.

This will reduce the randomness of our rain somewhat, but it'll also produce a more believable effect.
For the most part, it prevents our particles from looking like they're a square column of rain.


Let's set the vortex magnitude to 5.000 and attenuation to 0.500 for now.




Still, our particles will only be emitting from within the bounds of our polyPlane.
We can easily make them emit from around it by changing the Min & Max Distance values.

These options will be in the Distance/Directions Attributes, under the emitter# tab in the channel box.


Basic Emission Speed Attributes will let us make our particles behave even more random:

Values for Speed, Speed Random, and Tangent Speed set multipliers for the emission speed of our particles.
(ie  0.5 will reduce speed by half, while 2 will double speed.)

While Speed will remain our main speed, Speed Random will define the range of variation between particle speeds.

Tangent Speed and Normal Speed will control the angle at which particles are emitted from the surface.



Let's leave Speed at 1.000, set Speed Random to 0.900 and Tangent Speed to 1.500.

Now our particles emit from a much larger area around the original surface.

If your fields seem to be affecting the particles too strongly, now is a good time to adjust them.




5. Collisions

Now that our rain is falling and doesn't appear too mechanical, we want it to collide with something.

Create a new polyPlane large enough to see the particles colliding with it.

    


To make things collide:

1. Selecting your particle# object
2. Then select the object we want to collide with
3. and then go to Particles->Make Collide












However, default values will make our particles bounce quite dramatically.
This bounciness is controlled by our resilience.


Resilience can be changed through the geoConnector# tab in the channels box.

Generally, a lower value will be better.







Now, while our rain is bouncing off the ground, it doesn't look very accurate at this point,
because a raindrop should splash when it hits the ground, not bounce like a ball.

We want our raindrops to split into 3 on collision with the ground. (Could be more, but 3 seems like a good number.)

For this, we want to use the Particle Collision Event Editor, which you can find in the Particles menu.




To create particles emitting when the rain collides with the ground, we need to create a new event.

We want this to happen on all collisions, so we'll check the All Collisions box.


We can either emit new particles on collision or split existing particles, which will thus inherit velocity.



Now, while we have new particles emitting on collision, the new particles won't be affected by gravity!
While you could make a new gravity field, it's best to re-use the existing one.




For this, you'll need to select the gravity field,

and shift+click to select the new particles,

then go to Fields->Affect Selected Object(s)


 


(You can always use the Outliner if you have

difficulty selecting stuff in the viewport.)













If we want other fields to affect the new particles, we can do the same thing,

but we shouldn't need to use any of the other fields except for gravity for the split particles.












 



There's also Window -> Relationship Editors -> Dynamics Relationship Editor


This can show which fields or events are affecting scene elements and make or remove connections by simple clicking.



Select an item from the left pane,
and the right pane will show which dynamic fields are affecting it.







(Though the highlighting of connected events could be better.

  Blame Autodesk for choosing an invisible shade of yellowy white. Augh!)





We'll adjust the look of the new particles in the next chapter.




6. Improving The Look


Now that our particles are behaving like rain, we want to make them look even more like rain.  We have a few options here:

- Render Attributes will let us set basic attributes such as particle size and length.
- Using Per Particle attributes, we can specify opacity and rgb values using ramps
- We can also instance geometry and use an appropriate shader.


For now, it will be much simpler to adjust settings through the Render and Per Particle Attributes window.




Our particles will not be lit with scene lighting if we do not enable Use Lighting (at the bottom of the picture.)


However, they will not be shaded correctly for rain when lit and colored if we don't enable Color Accum as well.

Color Accumulation will add all of the particle's color together, making them bright white at the point of emission.



Enabling Depth Sort will force Maya to draw particles furthest from the camera first.

(This isn't important for us right now, but should you want to render the particles,
it would ensure that there were no overlapping colors or edges.)





Now, let's create some ramps that will control the particle opacity and color.  To do this, we'll need to add two attributes - Opacity, and Color.


These buttons are in the Add Dynamic Attributes section,
right below the Per Particle (Array) Attributes section for particleShape#.




When we click either of these buttons, another window will come up:


Since we want to add an attribute Per Particle, we want to check the Add Per Particle Attribute.

The windows appear slightly different, but are otherwise identical.





Now, your Per Particle (Array) Attributes Window should sport two new entries: opacityPP  &  rgbPP    (PP = Per Particle)


Hold the right mouse button in these new properties and pick Create Ramp.
This will create expressions in the two entries for opacity and color.





Hold the right mouse button again and this time we'll hit Edit Ramp.

Ramps similar to the following should be fine. (We assume you have working knowledge of how to adjust them. Ask a fellow classmate for help if you need it.)


LEFT: The ramp controlling the rgbPP value.                           RIGHT: The ramp controlling the opacityPP value.




Last but not least, for the new particle# object (the one of the collision particles),
you'll want to go to its render type and set it to MultiStreak.

This is good because it increases the complexity of the look without
bogging down its computation any more than any other type.

(Again, don't forget to click on the "Current Render Type" button afterwards.)


It should now look very splashy.






If all goes well, then the particles should look, and act, to a degree, like rain!

Thank you for taking the time to read this tutorial. We hope you learned something along the way. :)