ActionScript for Flash MX: The Definitive Guide, Second Edition

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which again corresponds to the point (0,0). The point (0,0) in both cases is the origin point

(or simply origin) of the coordinate space being used to plot the location of the clip. We’ll

see how the localToGlobal() and globalToLocal() methods can convert between these two

coordinate spaces.

Flash’s coordinate system inverts the Y-axis of Cartesian coordinates;

that is, y values increase in a downward direction, not upward. For

example, a y-coordinate of 100 indicates a point 100 pixels below the

X-axis.

We’ll often use the coordinate-related properties and methods to move a clip, determine

where a clip is, or determine whether it intersects another object or point. The last tech-

nique is referred to as collision detection because it is often used to determine whether to

change the direction in which a clip is animating, as if it bounced off another object (see

the MovieClip.hitTest() method).

Note that ActionScript doesn’t have a native datatype to represent a point (i.e., an x- and y-

coordinate). See MovieClip.globalToLocal() for an explanation of how to create a point

object from a generic object.

In Flash 5, no direct API was available to create ActionScript subclasses of the MovieClip

class. As of Flash 6, the Object.registerClass( ...