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The controls here are used to help accommodate this by providing methods of calculating the intervening frames.
This control also works in 3D. Clicking on the Restart button will restart the particle system at the current frame, removing any particles created up to that point and starting the particle system from scratch at the current frame.
This control also works in 3D. Clicking on this button causes the particle system to recalculate, starting from the beginning of the render range up to the current frame. It does not render the image produced. It only calculates the position of each particle. This provides a relatively quick mechanism to ensure that the particles displayed in the views are correctly positioned.
If the pRender node is displayed when the Pre-Roll button is selected, the progress of the pre-roll is shown in the viewer, with each particle shown as point style only.
Selecting the Automatic Pre-Roll checkbox causes the particle system to automatically pre-roll the particles to the current frame whenever the current frame changes. This prevents the need to
manually select the Pre-Roll button whenever advancing through time in jumps larger than a single frame. The progress of the particle system during an Automatic Pre-Roll is not displayed to the viewers to prevent distracting visual disruptions.
About Pre-Roll
Pre-Roll is necessary because the state of a particle system is entirely dependent on the last known position of the particles. If the current time were changed to a frame where the last frame particle state is unknown, the display of the particle is calculated on the last known position, producing inaccurate results.
To demonstrate:
1 Add a pEmitter and a pRender node to the composition.
2 View the pRender in one of the viewers.
3 Set the Velocity of the particles to 0.1.
4 Place the pEmitter on the left edge of the screen.
5 Set the Current Frame to 0.
6 Set a Render Range from 0–100 and press the Play button.
7 Observe how the particle system behaves.
8 Stop the playback and return the current time to frame 0.
9 In the pRender node, disable the Automatic Pre-Roll option.
10 Use the current time number field to jump to frame 10, and then to frames 60 and 90.
Notice how the particle system only adds to the particles it has already created and does not try to create the particles that would have been emitted in the intervening frames. Try selecting the Pre-Roll button in the pRender node. Now the particle system state is represented correctly.