Actionscript Cookbook

4.6. Drawing an Ellipse

Problem

You want to draw an ellipse (oval) at runtime.

Solution

Create a custom MovieClip.drawEllipse( ) method using the Drawing API and invoke it on a movie clip.

Discussion

You can create a method of the MovieClip class to draw an ellipse that is very similar to the drawCircle( ) method in Recipe 4.5. In fact, the drawCircle( ) method is merely a degenerate version of drawEllipse( ), in which the radii in the x and y directions are the same.

The custom drawEllipse( ) method accepts four parameters:

xRadius

The radius of the ellipse in the x direction (major axis).

yRadius

The radius of the ellipse in the y direction (minor axis).

x

The x coordinate of the center of the ellipse.

y

The y coordinate of the center of the ellipse.

Refer to Recipe 4.5 for those aspects of the drawEllipse( ) method that are not commented on here.

MovieClip.prototype.drawEllipse = function (xRadius, yRadius, x, y) {
  var angleDelta = Math.PI / 4;

  // While the circle has only one distance to the control point for each segment, 
  // the ellipse has two distances: one that corresponds to xRadius and another that
  // corresponds to yRadius.
  var xCtrlDist = xRadius/Math.cos(angleDelta/2);
  var yCtrlDist = yRadius/Math.cos(angleDelta/2);
  var rx, ry, ax, ay;
  this.moveTo(x + xRadius, y);
  for (var i = 0; i < 8; i++) {
    angle += angleDelta;
    rx = x + Math.cos(angle-(angleDelta/2))*(xCtrlDist);
    ry = y + Math.sin(angle-(angleDelta/2))*(yCtrlDist);
    ax = x + Math.cos(angle)*xRadius;
    ay = y + Math.sin(angle)*yRadius;
    this.curveTo(rx, ry, ax, ay);
  }
}

Once you have defined and included the drawEllipse( ) method in your Flash document, you can draw an ellipse rather easily. Use the drawEllipse( ) method the same way you used the drawCircle( ) method in Recipe 4.5 but provide both x and y radii instead of just a single radius. Remember that you still need to define the line style before you call the drawEllipse( ) method.

// Create an ellipse with minor and major axes of 100 and 200, respectively.
this.createEmptyMovieClip("ellipse", 1);
ellipse.lineStyle(1, 0x000000, 100);    // Use a one-pixel, black, solid border
ellipse.drawEllipse(100, 200);

Having defined drawEllipse( ), we can rewrite the drawCircle( ) method, as follows:

MovieClip.prototype.drawCircle = function (radius, x, y) {
  // Call drawEllipse(  ) with the same radius for both x and y.
  this.drawEllipse (radius, radius, x, y);
}

See Also

Recipe 4.5