HTML5 Game Physics Moving Elastic Collisions

HTML5 Game Physics Bouncing Elastic Collisions

This tutorial is the third in a series about HTML5 game physics. Game character movement becomes more realistic when multiple collisions are utilized. At each collision, a character will need to reverse or lose some momentum.

The third tutorial will move the game character in different directions after each collision. The bounce effect will be achieved by using a constant acceleration. The canvas sides will act as a non-moving object for collision with the moving game character.

This series is intermediate and assumes basic knowledge of physics and 2D graphics. The acceleration will hold steady but the velocity will be the exact opposite after each collision. Elastic collisions conserves momentum and kinetic energy which is possessed due to motion.

This tutorial uses the HTML5 canvas tag and JavaScript.

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HTML5 Moving Elastic Collisions File

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<!DOCTYPE html>
<html xml:lang="en" lang="en">

<head>
	<title>Ojambo.com HTML5 Game Physics Moving Elastic Collisions</title>
	<meta charset="utf-8" />
	<style type="text/css">
		canvas#myCanvas{
			width: 80%;
			height: 80%;
			background: green;
		}
	</style>
</head>

<body>
	<canvas id="myCanvas"></canvas>
	<script type="text/JavaScript">
		// Position, Size & Velocity
		var rect = {
			x: 0,
			y: 0,
			w: 20,
			h: 20,
			vx: 2,
			vy: 3
		};
		
		// Find Canvas Element
		var myCanvas = document.getElementById("myCanvas");
		// Call getContext() method for 2D drawing
		var ctx = myCanvas.getContext("2d");
		
		// Update Every 20 milliseconds
		setInterval(function() {
			// Draw Function
			draw();
		}, 20);
		
		// Draw Function
		function draw(){
			//Clear Display
			ctx.save();
			ctx.clearRect(0,0, myCanvas.width, myCanvas.height);
			ctx.restore();
			
			// Apply velocity to position (vx -> x)
			rect.x += rect.vx;
			rect.y += rect.vy;
			
			// Collision with canvas horizontally
			if ( rect.x > myCanvas.width - rect.w ) {
				rect.x = myCanvas.width - rect.w;
				rect.vx = -Math.abs(rect.vx);
			}
			else if ( rect.x < 0 ) {
				rect.x = 0;
				rect.vx = Math.abs(rect.vx);
			}
			// Collision with canvas vertically
			if ( rect.y > myCanvas.height - rect.h ) {
				rect.y = myCanvas.height - rect.h;
				rect.vy = -Math.abs(rect.vy);
			}
			else if ( rect.y < 0 ) {
				rect.y = 0;
				rect.vy = Math.abs(rect.vy);
			}
			
			// Draw on the canvas
			ctx.save();
			//Fill style red colour
			ctx.fillStyle="#FF0000";
			//Rectangle
			ctx.fillRect(rect.x, rect.y, rect.w, rect.h);
			ctx.restore();
		}
	</script>
</body>

</html>

The game character will bounce of the top, bottom, left and right sides of the canvas. The velocity will be change by an equal amount every time. Movement from left to right or right to left will be at a constant acceleration.

Velocity will reverse when the game character bounces off a side of the canvas. The first if statement deals with the left to right and right to left collision. The second if statement deals with the top to bottom and bottom to top collision.

How to Use:

Observe the canvas.
Observe the character moving down the canvas.
Observe the impact of the collision.

Demonstration:

Ojambo.com HTML5 Game Physics Moving Elastic Collision Tutorial

Image Missing — Ojambo.com HTML5 Game Physics Moving Elastic Collisions

Conclusion:

Game characters have constant acceleration caused by position, velocity and time. The velocity changes by an equal amount every equal time period.

Elastic collisions with constant acceleration maintain their momentum. The exact point of collision impact can be determined by the character’s position and dimensions.

Determine which objects moves after a collision.
Give the moving objects constant acceleration.
Constant acceleration requires velocity to change in equal increments over equal time.