Particle group> Streaklet group

the Streaklet particle type can be used to create a long-exposure light painting effect. There are several other controls available in the Streaklet subgroup to change the appearance of the Streaklet. For more information on particle types, see the Streaklet particle in the Particle Type page.

The full Streaklet group controls.

The Streaklet option in Particle Type.

Random Seed

The random seed will vary the orientation of the lines that make up the appearance of the streaks.

Different Random Seed values result in a different appearance of the streak in the two images.

No Streaks

This control sets the number of streaks ("No" is an abbreviation of "number.") High values make more streaks which can create a denser rendered line. A low streak count will reveal the streak as a collection of dots in 3D space.

No Streaks set to a low value.

No Streaks set to a high value.

Streak Size

This controls sets the size of the overall streak. A low value gives the streak a thinner, more separated appearance, while high values give a thicker, brighter streak. A value of 0 turns off the streak.

Left to right, Streak Size set low, mid-range and high.

How to create a Streaklet 'iPod effect'

Following is a tutorial to create the Streaklet effect, which you could loosely term the light streak effect.

Start with a new comp (Ctrl/Cmd-N). Make the comp 640*480 at 30 fps and 5 seconds long. Add a Solid layer that is comp sized and apply Particular. Go to Particle> Particle Type and choose Streaklet.

The Particle Type set to Streaklet

First we need to create a 3D path for the Streaklet to follow. Let's create a composition Point Light and name it 'Emitter'. Set Emitter > Emitter Type to the Light(s) option. Both actions tie into the Options Panel and its Light Emitters > Light Name, where the default name is already set to 'Emitter'.

Create a Point Light in After Effects and name it Emitter.

The Emitter Type should be set to Light(s) to use the newly created light as a particle emitter.

In your Timeline, select the light named Emitter and type 'P' to reveal the Position property for the light. One way to set the Streaklet path is to hand keyframe the X, Y, Z movements of the Position. This is a little labor intensive, so let's instead enter a simple expression to wiggle the Position of the light in 3D space.

Option-Click (Alt-click) the stopwatch icon next to the Position name in the Timeline to open Expression area in the After Effects timeline. Type in the expression transform.position.wiggle(3,200). This sets the number of wiggles to 3 per second and sets the extent of the position change to 200 pixels in each of the three dimension X, Y, and Z. Now the Streaklet particles will move a long a path in 3D space as the expression calculates a new random offset for the position of the light at each frame.

The light-emitting particles after the wiggle expression is added to the Light Position property. Note that the particle still have velocity and are spheres, so this doesn't look anything like a streak yet.

To make our effect work, we need the particles to stand still or have 0 velocity. To do this, return to the Emitter group. Set the Velocity, Velocity from Motion, and Emitter Size X, Y Z all to 0. This makes the particles just omit from the light and they stand still from where they were omitted. This will assume that the particles that are emitted simply follow the light in 3D space.

The result now look like a set of dots being traced in 3D space.

We're getting closer now to the Streaklet effect, but there is a problem. Particular interpolates each frame linearly, causing straight lines of particles to be drawn between each new emitter position. This is fine for a slow-moving emitter. But if the emitter is moving quickly from frame to frame, you need to go to the Emitter > Position Subframe control. Set the menu option to 10x Linear to make the line created by Particular look smoother.

The Position Subframe option in Particular set to 10x Linear.

Right now there are gaps between the particles, but a light streak should look like a continuous or unbroken line. Still in the Emitter group, increase the number of particles created by setting the Particle> Particles/sec value higher so those gaps are filled in. Set the value to 500. This will create enough particles to fill in the gaps without impacting render times too much.

Now the particles look like a painted light streak effect. In the Particle group, let's make a few cosmetic changes. Change the Color to red, the Size to 12, Transfer Mode to Add, and Opacity to 40. You've just created a light streak effect, but remember that you can use this same technique to trace an outline around text or other objects.

The initial set-up of the Emitter motion in 3D space.

The streak effect after changing the Particle/sec and Position Subframe controls to fill in the gaps.

The final streak effect after setting the Color, Transfer Mode, and Opacity for the particle.