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The Motion Path feature makes it possible to precisely control the path that a particle takes after being emitted. This is done by setting keyframes for a comp Light and naming the Light in a certain way. This keyframed Light lets you control the path that particles travel after being born. Motion Path is useful when precise control of the particle path is needed. Plus it's kinda fun. Read our tutorial 'Setting up a Motion Path' at the bottom of this page.
Above: An example of motion path with a keyframed Light.
The popup for Motion Path is the only Particular parameter that you will set when creating a motion path. Each of the numbers corresponds a number 'X'. That number is used in the name given to the comp Light that acts as the motion source.
In the Motion Path popup, you will notice there is an 'HQ' variant of each number. This is because if the Motion Path Light moves extremely quickly, the path is notably linearly interpolated between frames. A path has edges that should be smooth, so if this issue occurs, you should select the HQ variant instead. A word of warning: The HQ setting is much slower, so do not use it unless needed.
Make a new comp (Ctrl/Cmd-N). Make it 640*480 at 30 fps. and 5 seconds long. Create a comp-sized Solid (Ctrl/Cmd-Y), choose Make Comp Size, and click OK. Apply Effect> Trapcode> Particular to the Solid.
To use the Motion Path feature, first create a comp Light (Layer>New>Light). This should be a Point or Spot Light. The other Light settings (like Color and Intensity) do not matter.
The name of the Light has to be 'Motion Path X', where X is a number between 1 and 9. Let's use 1 in this example.
Make sure the Light's position is where you want particles to be born. Also make sure the Emitter> Position, where particles are born, is in the location that you want. Then turn off Emitter> Velocity by setting the value to 0. This will keep the particles from spreading out over time. Set Particle> Life high enough so the particles live for awhile and don't die mid-path. Set Physics> Air> Motion Path to 1. (Or if you used another value for X, use that instead)