Billiard Ball Physics - HTML5 Animation with JavaScript - page 264

bounce = -1.0;
for (var radius, ball, i = 0; i < numBalls; i++) {
radius = Math.random() * 20 + 15;
ball = new Ball(radius, Math.random() * 0xffffff );
ball.mass = radius;
ball.x = Math.random() * canvas.width;
ball.y = Math.random() * canvas.height;
ball.vx = Math.random() * 10 - 5;
ball.vy = Math.random() * 10 - 5;
balls.push(ball);
}
function rotate (x, y, sin, cos, reverse) {
return {
x: (reverse) ? (x * cos + y * sin) : (x * cos - y * sin),
y: (reverse) ? (y * cos - x * sin) : (y * cos + x * sin)
};
}
function checkCollision (ball0, ball1) {
var dx = ball1.x - ball0.x,
dy = ball1.y - ball0.y,
dist = Math.sqrt(dx * dx + dy * dy);
//collision handling code here
if (dist < ball0.radius + ball1.radius) {
//calculate angle, sine, and cosine
var angle = Math.atan2(dy, dx),
sin = Math.sin(angle),
cos = Math.cos(angle),
//rotate ball0's position
pos0 = {x: 0, y: 0}, //point
//rotate ball1's position
pos1 = rotate(dx, dy, sin, cos, true),
//rotate ball0's velocity
vel0 = rotate(ball0.vx, ball0.vy, sin, cos, true),
//rotate ball1's velocity
vel1 = rotate(ball1.vx, ball1.vy, sin, cos, true),
//collision reaction
vxTotal = vel0.x - vel1.x;
vel0.x = ((ball0.mass - ball1.mass) * vel0.x + 2 * ball1.mass * vel1.x) /
(ball0.mass + ball1.mass);
vel1.x = vxTotal + vel0.x;
//update position - to avoid objects becoming stuck together
var absV = Math.abs(vel0.x) + Math.abs(vel1.x),
overlap = (ball0.radius + ball1.radius) - Math.abs(pos0.x - pos1.x);
pos0.x += vel0.x / absV * overlap;