drawPoligon.js

/**
 * Draws a poligon - can draw anything from a triangle to a star.
 * Source and demo: http://jsfiddle.net/tohan/8vwjn4cx/
 */
function drawShape(ctx, x, y, points, radius1, radius2, alpha0) {
    //points: number of points (or number of sides for polygons)
    //radius1: "outer" radius of the star
    //radius2: "inner" radius of the star (if equal to radius1, a polygon is drawn)
    //angle0: initial angle (clockwise), by default, stars and polygons are 'pointing' up
    var i, angle, radius;
    if (radius2 !== radius1) {
        points = 2 * points;
    }
    for (i = 0; i <= points; i++) {
        angle = i * 2 * Math.PI / points - Math.PI / 2 + alpha0;
        radius = i % 2 === 0 ? radius1 : radius2;
        ctx.lineTo(x + radius * Math.cos(angle), y + radius * Math.sin(angle));
    }
}