Actionscript Cookbook

5.14. Determining Points Along a Circle

Problem

You want to calculate the coordinates of a point along a circle given the circle’s radius and the sweep angle.

Solution

Use the Math.sin( ) and Math.cos( ) methods to calculate the coordinates using basic trigonometric ratios.

Discussion

Finding the coordinates of a point along a circle is easy with some trigonometry. So let’s look at the formulas you can use within your ActionScript code and the theory behind them.

Given any point on the Stage—a point we’ll call p0, with coordinates (x0, y0)—plus a distance and the angle from the horizontal, you can find the coordinates of another point—which we’ll call p1, with coordinates (x1, y1)—using some basic trigonometric ratios. The angle is formed between a conceptual line from p0 to p1 and a line parallel to the X axis, as shown in Figure 5-2. The opposite side (O) is the side furthest away from the angle. The adjacent side (A) is the side that forms the angle with the help of the hypotenuse (H).

Figure 5-2. The angle, adjacent side, opposite side, and hypotenuse of a right triangle

If you know the distance between two points and the angle to the horizontal, as shown in Figure 5-2, you can calculate the x and y coordinates of the destination point using trigonometric functions. The trigonometric sine of the angle is equal to the ratio of the opposite side over the hypotenuse:

sine(angle) = opposite/hypotenuse ...