Environmental Animation - Blender Foundations: The Essential Guide to Learning Blender 2.6 - page 368

one second of time. The number specified in the Amount field is
the total number of particles that will be emitted over the lifetime
(start to end frame) of the system. The particle engine distributes
this number evenly across the frame range. The default is 1000,
which is probably plenty for our purposes. It's trivial to adjust the
number up or down though.
The Emit From controls are set to a decent default, and don't
require adjustment for basic usage. They control how the system
determines where on the mesh each particle is born. In most cases,
you want your particles randomly emitted over the entire surface,
but by using the Verts or Volumes options, you can force
them to be emitted only from vertices or from the space inside the
mesh.
The Velocity panel is the next most important, as it controls the
actual motion of the particles. Particle motion is calculated with two
factors: initial velocity and environmental forces. The initial veloci-
ties are assigned in this panel. The Normal control in the Emitter
Geometry section causes particles to shoot directly away from the
surface of the mesh, following the mesh's normal at the point of
emission. This is appropriate for explosions or any object that will
need to “radiate” particles. The X , Y , and Z controls in the
Emitter Object section shoot the particles in the direction of the
object's local axes. For example, setting the Y value to 1.0 would
give all particles a starting velocity of 1 along the object's y axis. If
the object were to rotate during the emission period, the direction
of the emitting axis would change like a rotating sprinkler shooting
water from its nozzle.
The Object control gives the particles a portion of the object's
actual motion. If the object is animated (like ours!) setting this value
to 1.0 will make particles come to life with the same velocity as the
object at the time of emission. In practical terms, this means that a
particle will seem to obtain some momentum from the object, fol-
lowing after it a bit. Setting the value below 0.0 will cause the
particles to appear to shoot away from the object's motion vector,
appearing like rocket thrust. The Random field adds randomness
to whatever the total result is from all the other controls. In fact,
Random will assign random velocities even when there is no other
velocity set. We want our particles to seem to follow the cube a
bit, but also to have a bit of randomness, so Object is set to 0.4
Figure 13.11 The particle system properties.