Tracking an Emitter to a Clip

After adding an emitter to a clip, a common task is to track it to a moving object in the clip. You do this using the Tracker, which is explained in The Tracker.

To track an emitter object or control point:

1. Make the Particles operator the current operator.
2. Select the emitter object(s) in the viewport or Workspace panel.

You can also select one or more control points of a line or deflector.

3. Show the Tracker (F6).

Above the Clip list, the heading of the Tracker panel indicates the number of emitters or emitter control points (for lines or deflectors) that are selected.

4. Track as usual. See Using the Tracker.

When a particle effect needs to be contained within a region, for example, you can use rotation tracking on an area emitter. Scale (size) tracking can also be used for area and circle emitters.

Note: You cannot use rotation or scale tracking for point emitters. You cannot use scale tracking for line emitters.

Simulating a Transform Manipulator

When you want an outside force to manipulate the transforms of particles, you can modify the transforms of the layer instead.

For example, to make wind affect a stream of smoke particles, you can make a motion path for the emitter in the Particles operator, then copy the keyframe data to the layer. After compensating for the difference in coordinate systems between the Particles and Composite operators, and reversing the data, the emitter remains stationary and the particles seem to blow in the wind.

This keyframing trick also works for rotation. This transform only needs to be reversed for the layer; the Composite coordinate system does not affect rotation.

You can also change keyframes using math operations. For example, you can add jitter and randomize them.

Note: Before performing these steps, you should try animating the emission angle and other emitter properties, which is easier and may give the results you want.

To simulate wind:

1. Create a solid layer in a composite with the Particles operator on it. Leave the layer at the center.
2. In the Particles operator, add an emitter. For example, add a point emitter that emits a short stream of smoke in one direction. Disable Speed, create a motion path for the emitter so it moves into the wind, then go to the first frame.

Note: The effect must stay within the frame size; you can make the solid layer's footage larger if necessary. Note the frame size; you will need it in steps 6 and 9.

3. Double-click the Composite operator so it becomes the current operator. Disable Animate.
4. In the Timeline, select the X Position channel for the emitter. This selects all keyframes automatically. Copy the keyframes.
5. In the Workspace panel, select the layer. The Timeline updates to show the layer's channels. Select X Position and paste the keyframes. Do not deselect the keyframes yet.
6. Double-click the X Position channel to open the calculator, then subtract half the width of the Particles frame size (for example, 320 for NTSC), then click OK. All the keyframes are offset by the entered amount.
7. Click Math Operations, click Reverse, then Apply. The X Position data is ready.
8. Repeat steps 4 and 5 for Y Position.
9. Double-click the Y Position channel to open the calculator, then subtract half the height of the Particles frame size (for example, 240 for NTSC), then click OK. All the keyframes are offset by the entered amount.
10. Play the clip. The emitter stays in the same position, while the particles seem to blow in the wind.

If you are curious why this works, read on. The Composite operator has (X=0, Y=0) at the center of the coordinate system, not the upper left corner like Particles (and Paint). To translate the X Position keyframes for the composite, you need to subtract half the frame width to move them over. For Y Position, you need to subtract half the frame height and negate the result. However, for this procedure, you also need to invert the values to "cancel out" the transform so the emitter position does not move. For X Position, you used the Reverse math operation to do this, but for Y Position, you can simply skip the earlier step where you negate the result.

Simulating an Attractor

You can simulate a black hole or other object that attracts particles by using the emitter as the attractor, and reversing the result clip.

To simulate an attractor:

1. In the Particles operator, set up one or more emitters at the same location.
2. Use properties such as Emission Angle and Emission Range to control the direction the particles travel.
3. As you create and modify the effect, play the viewport backwards.

Note: The Particles operator does not cache frames, so it must re-render the frames when you play the viewport. Rendering backwards can be slow since particles are always calculated from the first frame.

4. When the effect is ready, use the Commit to Disk feature to render the frames (and automatically import the result).
5. Select the committed clip in the Workspace panel and show the Footage Controls panel. In the Output controls, enable Reverse Footage.
6. Combine the reversed clip with the rest of the composite. If necessary, you can always modify the properties in the Particles operator and render again.

Showing View Modes

You cannot show view modes in the Particles operator. Instead, set up a second viewport that can show view modes. For example, add a Paint or Composite operator downstream from the Particles (if not already there), and show that operator in the viewport.