Style the particles
- Move the playhead to frame 150, so you can see the results of your changes, as you make them.
- In the Inspector, go to the Emitter tab and set the Scale to 60%.
- Open up the Opacity Over Life gradient editor, give it three gradient tags, and set them thus:
Play back your project:
The opacity gradient has our particles fading in a tiny bit at the beginning, and then softly fading out towards the end. Since we're trying to make this look a bit more like smoke, we need some irregularity. All the particles come out the same size, which looks unnatural. We'll use a parameter behavior to randomize the particles' size.
Add the parameter behavior
- With the emitter selected, go into the Emitter tab of the Inspector.
- Right-click (or Ctrl-click) on the Scale parameter name and choose 'Randomize' from the menu.
- In the Behaviors tab, set the Randomize Amount to 15%.
- Set the Apply Mode to 'Add and Subtract'.
- Set the Frequency to 1.6.
- Set the Noisiness to 0.3.
- Enable the 'Link' checkbox.
The behavior will randomly add and subtract up to 15% of each particle's scale, as it gets emitted. This will give us some variance in particle size, over the length of the project:
The only thing missing is to make our particles break up as they fade out, similar to the way smoke dissipates. We'll achieve this through the use of the Drag simulation behavior. Let's take a moment to talk about Drag:
The Drag behavior adds drag to objects. It removes energy from the simulation, causing objects to lose speed (energy). Take this ball, for instance:
If we were to add the Drag behavior to it, and set the Amount to 1, you would get this:
The Drag behavior sucks the energy out of the ball, slowing it down and eventually stopping it. This is the typical use of drag: to slow things down. In our case, though, we want to use negative drag values, which will add energy. Here is the same ball with a drag value of -2.0:
As you can see, the ball begins accelerating as the Drag behavior injects energy into the ball's speed. We'll use this same effect to cause our skywriting particles to start drifting away as they age.
Add the Drag behavior
- With the emitter (not the emitter cell) selected, go to the toolbar and select Add Behavior->Simulation->Drag.
- In the Inspector, go to the Behaviors tab and set the Drag's Amount to -1.5.
Go ahead and play back your project. Nothing changed, right? That's because the drag is adding energy to the movement of the particles, but the particles have no movement to add energy to. Remember when we set the emitter's speed to 0? We did that to have the particles sit still when emitted, creating our smoke trail. But for the Drag to have something to affect, we'll need the particles (at least some of them) to have a speed greater than 0. Instead of setting the speed of all the particles to the same value, we'll use the Speed Range parameter to give them some variety.
Adjust the emitter speed
- With the emitter selected, go to the Emitter tab in the Inspector and set the Speed Range to 0.10.
Why such a small value? Remember, our negative Drag is acting as an accelerant: if the particles have a large speed value, they'll take off at warp factor 10! By having them start with incredibly small speed values, it takes time for the Drag to speed them up noticeably, which is what we want. Play back your project:
Our smoke particles now begin to drift away at the end of the trail, just as they fade out. Keep in mind that there is no direct relationship between the Opacity Over Life gradient, the emitter's Speed Range and the Drag behavior, so increasing the drag amount (negatively) will cause them to disperse faster, increasing the emitter's Speed Range will have them disperse sooner, but you'd need to adjust the opacity gradient to make them fade out earlier.
With the basics of the dispersing skywriting smoke, you can start tweaking and customizing to your heart's content. Here's an example where five Motion Path behaviors (one for each letter) were sequenced together to drive the emitter:
Click here to view the "hello" movie. Click here to download the "hello" project file.
This tutorial has given you a mere glimpse of the cornucopia of effects you can achieve with a shape and a few behaviors. As usual, no keyframes were harmed in the making of this tutorial.
Now go mess around with this stuff and have fun!