Adobe After Effects CS3 - Manual (Page 516)

AFTER EFFECTS CS3
User Guide
511
negative Strength value does not suck the pieces into a black hole; instead, the pieces fly through each other and back
out the other side of the sphere. Setting Strength very low causes the pieces to break up into shapes, creating cracks
in the layer, but it doesn’t actually blow the pieces apart. If gravity is set to anything other than 0, the pieces are pulled
in the direction of gravity after they break up.
Note: A shatter piece is made up of vertices (points or dots that define the corners of the shape), edges (lines that connect
the dots), and planes (walls of the shape). Shatter determines when a shape has come in contact with a force sphere based
on when a vertex comes in contact with the sphere.
Gradient controls
Gradient controls specify the gradient layer used to control the timing of an explosion and the pieces that are affected
by the blast.
Shatter Threshold Specifies which pieces in the force sphere shatter according to the corresponding luminance of
the specified gradient layer. If Shatter Threshold is set to 0%, no pieces in the force sphere shatter. If it is set to 1%,
only the pieces in the force sphere corresponding to white (or very nearly white) areas on the gradient layer shatter.
If it is set to 50%, all the pieces in the force sphere corresponding to white-to-50%-gray areas on the gradient layer
shatter. If it is set to 100%, all pieces in the force sphere shatter. Because there are 256 shades of gray (including black
and white), each percentage point represents approximately 2.5 shades of gray.
Animating Shatter Threshold influences the timing of the explosion. If you leave it set to 0%, the layer never
explodes. However, if you set a Shatter Threshold keyframe at 50%, the pieces of your layer in the force field that
correspond to areas of your gradient layer that range from white to 50% gray explode. If you then animate Shatter
Threshold up to 100%, the remaining pieces in the force sphere explode.
Gradient Layer Specifies the layer to use to determine when specific areas of the target layer shatter. White areas
shatter first; black areas shatter last. Shatter determines which pixels correspond to which pieces by subdividing the
layer into pieces, each with a center point or balance point. If you superimpose the shatter map over the gradient
layer, the gradient layer pixels that are precisely under each balance point control the explosion.
Note: Some shapes have a balance point that falls outside the actual area of the shape—for example, the letters C and
U. When designing a gradient layer in such a situation, avoid using grayscale versions of letters. Instead, use larger
shapes that cover the balance point of each character.
Invert Gradient Inverts the pixel values in the gradient. White becomes black, and black becomes white.
Physics controls
Physics controls specify the way the pieces move and fall through space.
Rotation Speed Specifies the speed at which pieces rotate around the axis set by the Tumble Axis control, allowing
you to simulate different rotation speeds for different materials. In nature, similarly shaped pieces spin at different
speeds based on their mass and air friction. For example, a brick spins faster than Styrofoam.
Tumble Axis Specifies the axis that the pieces spin around. Free spins the pieces in any direction. None eliminates all
rotation. X, Y, and Z spin the pieces only around the selected axis. XY, XZ, and YZ spin the pieces only around the
selected combination of axes.
Note: Any application of z-axis rotation appears only when a second force hits the layer. The pieces do not rotate from
the first blast if only z-axis rotation is selected.
Randomness Affects the initial velocities and spins generated by the force sphere. When this control is set to 0,
pieces fly directly away from the center point of a blast (assuming a positive force). Since real explosions are rarely
this orderly, Randomness allows you to vary things a little bit.