The ability to emit particles from 3D layers in the composition can be used for example to make a logo burst out particles. In this tutorial both the Layer and Layer Grid emitters are covered.
When the Layer Emitter is keyframed and the keyframes are edited a lot, the cache sometimes works inappropriately. This will manifest as jerkiness in the animation (the emitter jumps from one frame to the next). To get rid of the erroneous frames from the cache, use Edit>Purge>Image Caches and re-render.
The layer used as Layer Emitter cannot be a text layer, instead create the text in a comp and use that comp as shown in the tutorial below.
The layer used as Layer Emitter must have "continous rasterize" switched off.
Step-by-step instructions on using the Layer emitter
Let's start by creating the layer that will be used to emit particles.
Start with a new comp (Ctrl/Cmd-N), make it 640*480 at 30 fps. 5 seconds long. Name it "Boom".
Use the text tool and type "Boom!", use the stroke and fill color settings to make it look something like this:
An image (or movie clip) could be used instead of the AE text tool. In that case there is no need to create a separate comp for the image/movie. However, the image should have an alpha channel, otherwise, if it's on a black background like our text here, black particles will be generated from the black areas.
Now, we'll create the main comp.
Make a new comp (Ctrl/Cmd-N), make it 640*480 at 30 fps. 5 seconds long. Name it "Main".
Create a comp-sized solid (Ctrl/Cmd-Y), make sure to click "Make Comp Size" before OK.
Apply Effect>Trapcode>Particular to the solid.
In Particular, set Emitter>Emitter Type to Layer.
Drag the Boom comp from the project window into the comp-window and let it snap to center.
Switch the Boom layer to 3D and edit scale to (50,50,50), then set X Rotation to 20 and Y Rotation to 320. It should look like this:
Now let's start emitting particles from this layer!
In Particular, set Emitter>Layer to "Boom". Notice that a new light named "LayerEmit [Boom]" is created. This is a control light that Particular uses to keep track of the layer. It is locked and shy and it's not supposed to be edited.
Set Emitter>Emitter Size Z to 0 (zero).
Try a RAM preview to see particles emitted from the text. Notice that all particles are behind the layer. To fix this, move the Boom layer to the bottom of the TLW. Then set in Particular Visibility>Also Obscure with to "Layer Emitter". Now try a RAM preview again. It should look like this:
It's still hard to see exactly what's going on. Let's try and make it more apparent. In Particular, set Emitter>Direction to Directional. Set Direction Spread to 0. Set Velocity to 200. Set Velocity Random to 0. Finally crank up Particles/sec to 500. Try a RAM preview again:
What happens is that particles are generated all over the layer and then the alpha of the layer at each particle postion is sampled. That alpha value is assigned to the particle. The RGB color at the position is sampled and that becomes the particle color. When Emitter>Layer Sampling is set to "Current Time" (that's what it is by default) that means the emitter layer is resampled each frame, so if it is animated, particles colors (and alpha!) will change as they fly. If it is set to "Particle Birth Time", that mans the particle will keep the alpha and color given at birth throughout its life.
Layer Grid emitter
Now let's have a look at the layer grid. In Particular, change Emitter>Emitter Type to "Layer Grid". Then click Options and change the values in the "Grid Emitter" section: Particles in X = 20 and Particles in Y = 15. Now try a RAM preview:
Layer RGB Usage
So far we've used Emitter>Layer RGB Usage at "RGB - Particle Color". There are other mappings available:
Lightness - Size
Lightness - Velocity
Lightness - Rotation
RGB - Size, Vel, Rot
RGB - Particle Color
None
To demonstrate the other mappings we need a different layer to use as layer emitter. Create a new 400*400 comp, name it "Radial". Create a 400*400 solid in the comp. Apply Render>Ramp to the solid. Set Ramp Shape to Radial. Set Start of Ramp to (200,200) and set Start Color to white and End Color to black. The Radial comp should look like this:
Go back to the Main comp. Delete the Boom layer. Drag the Radial comp from the project window into the comp-window and let it snap to center. Switch the Radial layer to 3D and set X Rotation to 20 and Y Rotation to 320. It should look like this:
In Particular, set Emitter>Layer RGB Usage to "Lightness - Size". Set Particles/sec to 100 and hit RAM preview:
As can be seen above, with this mapping, the particle size is affected by the lightness of the layer.
Try changing Emitter>Layer RGB Usage to "Lightness - Velocity". Hit RAM preview:
In this case the partcile velocity is affected by the lightness. If lightness is less than 50%, the particle will emit backwards as can be seen in the dark areas. If lightness is exactly 50% velocity will be zero, and at above 50% the particles will emit forwards.