Cannonball and Slingshot - The Essential Guide to HTML5 - page 137

Code Explanation

my = ev.layerY; ev.layerY for my

} else if (ev.offsetX || ev.offsetX == 0) { Does offsetX work?

Use it for mx

mx = ev.offsetX;

my = ev.offsetY; Use offsetY for my

} Close if true

cball.sx = mx; Position ball x

cball.sy = my; …and y

Position sling bx

mysling.bx = mx;

… and by

mysling.by = my;

drawall(); Draw everything

} Close if in motion

}

Close function

function finish(ev) { Function for mousedown

if (inmotion) {

In motion?

Reset inmotion

inmotion = false;

var outofcannon =
distsq(mysling.bx,mysling.by,
mysling.s1x,mysling.s1y)/700;

Base outofcannon proportional to square of
bx,by to s1x,s1y

var angleradians = -Math.atan2
(mysling.s1y-mysling.by,
mysling.s1x-mysling.bx);

Compute angle

horvelocity = outofcannon*Math.cos
(angleradians);

verticalvel1 = - outofcannon*Math.sin
(angleradians);