The Write On behavior provides a quick way to draw a paint stroke or outline on the Canvas over time. Write On lets you create a handwritten text effect, build the ever-popular old-time serial travel map effect, create a hand-sketched alpha mask for a transition or reveal, and so on. The Write On behavior can be applied to a paint stroke created with the Paint Stroke tool or added to an existing shape.
You set the animation effect to draw, erase, draw and erase, draw in reverse, and so on, over time. When you apply Write On to a filled shape with an outline, only the outline of the shape is drawn.
You can adjust the Write On Shape behavior using the following controls in the Behaviors Inspector:
Shape Outline: A pop-up menu that sets whether the stroke is drawn, erased, or drawn and erased. Choose one of the following menu items:
Draw: The stroke is drawn over the duration of the behavior.
Erase: The stroke is completely drawn at the beginning of the behavior and is erased over the duration of the behavior.
Draw and Erase: The stroke is drawn, then erased over the duration of the behavior.
Erase and Draw: The stroke is completely drawn at the beginning of the behavior, is erased, then is drawn again.
Stroke Length: A slider that defines the length as a percentage of the drawn or erased stroke. A value of 100% uses the entire length of the stroke, bounded by the First Point Offset and Last Point Offset parameters. If Stroke Length is set to 50%, when 50% of the stroke is drawn on, it begins to erase (from the beginning of the stroke) so only half of the length of the stroke is ever displayed over the duration of the behavior.
Stroke Offset: A slider that offsets where the stroke begins on the shape. The value is expressed as a percentage of the total length of the shape from the start point defined on the shape.
Direction: A pop-up menu that sets the direction in which the stroke is drawn. There are two options:
Forward: The stroke is drawn in a forward direction.
Reverse: The stroke is drawn in reverse.
Speed: A pop-up menu that defines the stroke’s “draw-on” velocity from the first to the last point in the stroke. There are nine options:
Constant: The stroke is drawn at a steady speed from the first to the last point in the stroke.
Ease In: The drawing of the stroke starts at a slow speed, then reaches and maintains a steady speed through the last point on the stroke.
Ease Out: The drawing of the stroke starts at a steady speed, then slows down as it gradually decelerates to a stop at the last point of the stroke.
Ease Both: The drawing of the stroke slowly accelerates from the first point on the stroke, and then slows down as it gradually decelerates to a stop at the last point of the stroke.
Accelerate: The stroke is drawn with increasing speed.
Decelerate: The stroke is drawn with decreasing speed.
Natural: The speed at which the stroke is drawn along the path is determined by the shape of the path. For example, if the stroke is a U-shape curve, the stroke is drawn along more quickly as it moves toward the low point of the U, and more slowly as it moves up the edges.
Recorded: This option appears only if there’s a recorded time over which the stroke was drawn. In other words, if a shape is converted to a paint stroke, this option does not appear. If the paint stroke is created with the Paint Stroke tool in the toolbar (using a stylus or mouse), this option does appear.
Custom: Lets you draw the stroke along its path by setting keyframes for the stroke’s speed from 0 to 100%. In other words, you determine which portion of the stroke is drawn along its path in time.
Custom Speed: A slider that becomes available when Speed is set to Custom. You can modify the Custom Speed velocity curve in the Keyframe Editor. For example, you can keyframe custom values to draw a stroke forward to a specific percentage of its path, then backward, then forward, and so on before it reaches the end of the animation.
End Offset: A slider that offsets the end of the behavior inward from the defined Out point and holds the last value. In other words, it offsets the visible paint stroke from the end of the path of the stroke.