Apple Motion 3 - Manual (Page 715)

Chapter 8 Working with Particles 715
If a particle system has two or more cells, the Emitter tab looks much different. The list
of parameters is much shorter, and the majority of the cell parameters are replaced
with a smaller group of master controls.
Changes made using the master controls modify the effect of each cell’s parameters
relative to the other cells in the system. This means that for a particle system with three
cells that have different Scale values, increasing the Scale parameter in the Emitter tab
multiplies the Scale value of all three cells by the same percentage. This has the result
of increasing or reducing the size of every particle in the system, while keeping the size
of each particle relative to one another the same.
For this reason, the master control parameters of multi-cell particle systems appear as
percentages.
Emitter Shape Parameter
The first parameter in the Emitter tab is the Shape pop-up menu. When 3D is turned
off, nine options are available. When 3D is turned on, two additional shapes become
available. Different shapes significantly alter the distribution of generated particles.
When you choose an emitter shape, different Emitter tab parameters appear that are
unique to that shape. For example, when Rectangle is the selected Emitter Shape,
Outline, Tile Fill, and Random Fill become available in the Arrangement options. When
Spiral is the selected Emitter Shape, the Arrangement parameter goes away and new
parameters such as Radius, Number of Arms, and Twists become available. These
different parameters provide additional control over the distribution of particles.