Autodesk 62205-051408-9001 User Guide - Page 393

Compositing with Particles ❚❘❘ 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. 377