Angle between points

angle-between-points.js

/**
 * Calculates the angle between two separate points. 0 degrees points up and 180 degrees points down
 * @link https://developer.mozilla.org/en-US/docs/JavaScript/Reference/Global_Objects/Math/atan2
 * @link http://stackoverflow.com/questions/3309617/calculating-degrees-between-2-points-with-inverse-y-axis
 * @param {object} start Beginning vertex formatted as {x, y}
 * @param {object} end End vertex formatted as {x, y}
 * @returns {number} Returns a counterclockwise angle, measured in radians
 */
function angleBetweenPoints (start, end) {
    // find angle
    var theta = Math.atan2(end[0] - start[0], -(end[1] - start[1]));

    // if negative flip it positive
    if (theta < 0) {
        theta += 2 * Math.PI;
    }

    return theta;
}