ActionScript: The Definitive Guide

Name

Math.atan2( ) Method — determine an angle based on a point

Availability

Flash 5; may be used when exporting Flash 4 movies

Synopsis

Math.atan2(y, x)

Arguments

y

The y-coordinate of the point.

x

The x-coordinate of the point.

Returns

The angle, in radians, of the point (x, y) from the center of a circle, measured counterclockwise from the circle’s positive horizontal axis (i.e., the X-axis). Ranges from π to -π. (Negative values indicate angles below the X-axis).

Description

The atan2() method, like atan( ), performs an arc tangent calculation but uses x- and y-coordinates, rather than their ratio, as arguments. That is, calculating an arc tangent with atan2( ) as:

Math.atan2(9, 3);  // Yields 1.24904577239825

is equivalent to calculating the arc tangent with atan( ), using the ratio of 9/3 (or 3), as follows:

Math.atan(3);      // Same thing

Usage

Note that the y -coordinate is passed as the first argument to atan2( ), whereas the x-coordinate is passed as the second argument. This is intentional and required. It mirrors the structure of tangent, which is the ratio of the side opposite an angle (y) divided by the side adjacent to the angle (x).

Example

The atan2( ) method can be used to make a movie clip point toward a moving target. The following example, available at the online Code Depot, shows code that rotates the current clip toward the mouse pointer. It can be used to orient an enemy spaceship toward a player’s spaceship:

// Rotate movie clip toward mouse onClipEvent (load) { // Convert radians ...