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motion-logo.jpgParticle systems are everywhere, and if you've dealt with motion graphics you've probably bumped into the concept before.  Today we'll try to flesh out the concept from the ground up.  While my practical exercise is laid out for Motion, the principles are universal.

I'll start by saying that particle systems are worth learning.  Know them, and you'll build more stunning graphics faster.  In all sorts of organic settings -- from smoke and flame effects, to snow and rain, to fields of grass -- particle systems, used artfully, allow for coherent and visually stimulating effects with far less effort and precision than frame-by-frame animation would require.  Particle systems animate "automatically" over time, but you still have tremendous control over how the systems behave at any given time.  Plus, you have complete control over each individual wisp of smoke or blade of grass: used effectively, this allows for systems with extremely clean alpha channels as well as allowing quick, basic 3D animation.

Particle-based animation is a type of procedural animation -- instead of describing the specific transformations that you want to happen to a specific element, you describe what a typical example of the "right" behavior and give the computer the freedom to pick the specifics.  For example, I could say "take this picture of a football, and 'throw' it across the screen from left to right."  But what if I wanted to throw a whole bunch of footballs at once? 

If I were using strictly keyframe-based animation, I would need to describe each individual football's path.  Sure, I could copy and paste keyframes to speed up that process -- but I'd still have to tweak each football's path individually, or they'd all look exactly the same and be timed similarly.

That's a shame.  The "throw" I want is a pretty basic concept in my mind, at least in aesthetic terms: The football will start somewhere to the left of the screen, then shoot toward the right of the screen.  Some throws might be a little stronger than others, but I can describe what would be a "too slow" throw and a "too hard" throw.  Some throws may be steeper or shallower than others, but I can describe what would be "too steep" or "too shallow."  And just like that, we've described an extremely basic particle system.

Conceptually, what we're doing is simple: basically, it's what I just described, except in computer terms.  We'll designate a "thrower" -- or emitter to be proper -- somewhere off to the left of the screen.  We'll tell the emitter, "look here, emitter: I want you to throw several footballs a second.  On average, I want you to throw them this fast -- but I want you throw some of them faster than others.  And I want the footballs to basically go in this direction, but throw some slightly steeper or shallower than others."

And believe it or not, this will give you many of the tools that will let us build really compelling particle systems down the line.


Practical [download finished project file]

Download this picture of a football - just right-click it (or Ctl+click) and save it to your Desktop.
football-image.png
Open Motion, and start a new project.  Drag the football picture off to the left side of your Canvas (this is where we'll put our emitter).  You might want to scale it down a little bit, too.

Click the "Make Particles" button in the top toolbar -- it has a bright orange icon.

There are two main ways to adjust your particle system.  The simplest is the HUD (Heads-Up Display): while your particle system is selected, click the HUD icon in the top toolbar to display the grey, semi-transparent Particle HUD.  Using the emission widget on the HUD (it's round and has gridlines), click in the middle of the circle and drag out to the right.  This sets how fast each football will be thrown on average.

Grab one of the little filled circles on the left outside border of the circle, and drag it around the circle.  Notice that the arrows in the HUD no longer point in all directions: these controls tell the computer what angles it should use to throw out particles.  Once you've "squished" the emission arrows, you can click and drag the arrows themselves to change the direction of the emission.  Adjust these emission range controls so that the arrows are mainly pointing to the right -- which means that the emitter is only throwing footballs to the right, across the screen.

Scrub back and forth along your timeline.  The system should be working, and you should see footballs flying.  Now let's refine the system a little bit.


The HUD is an easy way to adjust basic features of a particle system, but the Inspector offers you much more control.  Open the Particle Inspector (the Particle Inspector will show up in your regular Inspector pane whenever you have a particle system selected).  Adjust the "Birth Rate," and scrub across your timeline.  What happens?  Pick a birth rate that's aesthetically pleasing for you.

Now, let's adjust the system where some footballs throw faster than others.  Find the "Speed Randomness" slider, and move it upward.  You'll notice that the "Speed" slider -- which determines the average speed that footballs are thrown -- is already set (you set it using the Widget in the HUD).

Play around some if you'd like: the 3D checkbox does exactly what it says, if you happen to feel like working in 3D; also, you can apply behaviors to particle systems -- Simulation -> Gravity seems particularly appropriate for this exercise.  More on these and other techniques in future parts of this series -- and maybe some video tutorials too.


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