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script.js
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script.js
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// window dimensions
const WIDTH = window.innerWidth,
HEIGHT = window.innerHeight;
var starCount = 20000;
var starDis,
starRadius,
starAngle,
starRate,
starAlpha,
starMaxAlpha;
window.onload = function(e) {
// canvas setup
var canvas = document.getElementById("canvas");
var c = canvas.getContext("2d");
canvas.width = WIDTH;
canvas.height = HEIGHT;
// initial setup
init(c);
// main logic loop
var loop = function() {
update();
render(c);
window.requestAnimationFrame(loop, canvas);
}
window.requestAnimationFrame(loop, canvas);
}
function init(c) {
// initialize star data
starDis = new Array(starCount);
starRadius = new Array(starRadius);
starRate = new Array(starCount);
starAngle = new Array(starCount);
starAlpha = new Array(starCount);
starMaxAlpha = new Array(starCount);
// initialize stars
for (var i = 0; i < starCount; i++) {
resetStar(i);
}
}
function update() {
for (var i = 0; i < starCount; i++) {
// update star attributes
starRadius[i] *= 1 + starRate[i] * .001;
starDis[i] *= 1 + starRate[i];
if (starAlpha[i] < starMaxAlpha[i]) {
starAlpha[i] *= 1 + starRate[i] * .5;
}
// reset star on exit
if (starDis[i] > WIDTH) {
resetStar(i);
}
}
}
function render(c) {
// draw background
c.globalAlpha = .7;
c.fillStyle = "black";
c.fillRect(0, 0, WIDTH, HEIGHT);
// draw stars
c.fillStyle = "white";
for (var i = 0; i < starCount; i++) {
c.globalAlpha = starAlpha[i];
c.beginPath();
c.arc(
WIDTH / 2 + Math.cos(starAngle[i]) * starDis[i],
HEIGHT / 2 + Math.sin(starAngle[i]) * starDis[i],
starRadius[i],
.1,
Math.PI * 2
);
c.closePath();
c.fill();
c.globalAlpha = 1;
}
}
function resetStar(i) {
// reset star attributes
starDis[i] = Math.random() * WIDTH;
starRadius[i] = Math.random();
starAngle[i] = Math.random() * Math.PI * 2;
starRate[i] = Math.random() * .05;
starAlpha[i] = .035;
starMaxAlpha[i] = Math.random() * .95;
}