# Ball Bouncing Off Walls Example

Today I learned about utilizing angle of reflection in physics to simulate bouncing. Basically the idea is that the angle of incidence (incoming angle) = angle of reflection (outgoing angle). This is usually used for calculation of light rays, but it can also be applied to simulate an object bounce.

This example isn’t perfect – you’ll notice the ball goes halfway into the wall before the bouncing occurs – but it captures the basic concept quite well.

## The JavaScript Code

$(document).ready(function() { // Uses Modernizr.js to check for canvas support function canvasSupport() { return Modernizr.canvas; } canvasApp(); function canvasApp() { // Check for canvas support if (!canvasSupport()) { return; } else { // Grab the canvas and set the context to 2d var theCanvas = document.getElementById('canvasOne'); var context = theCanvas.getContext("2d"); } // Variables var speed = 5; var position1 = {x:20, y: 20}; var angle = 35; var radians = 0; var xUnits = 0; // represents pixels to move along the x-axis var yUnits = 0; // represents pixels to move along the y-axis var ball = {x: position1.x, y: position1.y}; updateBall(); // Drawing interval setInterval(drawScreen, 33); // Functions function updateBall () { // Called every time we set a new angle for the ball. // Recalculations radians and sets new xUnits and yUnits values. radians = angle * Math.PI/180; xUnits = Math.cos(radians) * speed; yUnits = Math.sin(radians) * speed; } function drawScreen() { // Reset canvas context.fillStyle = "#ffffff"; context.fillRect(0, 0, theCanvas.width, theCanvas.height); // Outside border context.strokeStyle = "#000000"; context.strokeRect(1, 1, theCanvas.width - 2, theCanvas.height - 2); // Update the position of the ball and draw it ball.x += xUnits; ball.y += yUnits; // Draw the ball object context.fillStyle = "#000000"; context.beginPath(); context.arc(ball.x, ball.y, 15, 0, Math.PI * 2, true); context.closePath(); context.fill(); // Check if ball has hit a wall. If so, then call updateBall if (ball.x > theCanvas.width || ball.x < 0) { // new angle = 180 - angle of incidence angle = 180 - angle; updateBall(); } else if (ball.y > theCanvas.height || ball.y < 0) { angle = 360 - angle; updateBall(); } } } });

## The Example App

You can find the code running live here: https://www.zesix.com/html5/ballBouncingOffWalls/