canvas html scaling

how does the canvas react to html scaling?

by rob Davis

HTML

<div id="container">
    <div>
        <canvas id="starfield" />
    </div>
    <div>
        <canvas id="tunnel" />
    </div>
</div>
<input type="button" id="btn-shrink" value="shrink"/>
<input type="button" id="btn-grow" value="grow"/>

CSS

#container { background-color:yellow;
        width:500px;
    height:500px;
    position:relative;
}
#tunnel    {position:absolute; top:0px; left:0px; width:100%; height:100%; }
#starfield {position:absolute; top:0px; left:0px; width:100%; height:100%; }

JavaScript

setStarfield('starfield');
setTunnel('tunnel');
$('#btn-shrink').click(function() {
    $('#container').animate({
        width: "100px",
        height: "100px"
    }, "slow");
});
$('#btn-grow').click(function() {
    $('#container').animate({
        width: "500px",
        height: "500px"
    });
});

function setTunnel(id) {
    var canvas = document.getElementById(id);
    var context = canvas.getContext('2d');
    var width=400;
    var height=400;
    
    setCanvasSize(canvas, width, height);
    //clearCanvas(canvas, context, '#000');
    //drawDot(context, 200, 200, 200, '#00f');
    var gradient = context.createRadialGradient(1, 1, 0, 1, 1, 1);
    gradient.addColorStop(0.5,'rgba(255,255,0,0.0)');
    gradient.addColorStop(1.0,'rgba(0,255, 255,1.0)');
    drawEllipse(context, 200, 200, 200, 200, gradient);	
}




function drawEllipse(context, cx, cy, rx, ry, fill){
    context.save();
    context.beginPath();
    context.translate(cx-rx, cy-ry);
    context.scale(rx, ry);
    var ellipseFill;
    if (typeof(fill)==='function') {
        ellipseFill=fill(context);
    } else {
        ellipseFill=fill;
    }
    context.arc(1, 1, 1, 0, 2 * Math.PI, false);
    context.fillStyle = ellipseFill;
    context.fill();
    context.closePath();
    context.restore();
}




function setStarfield(id) {
    var canvas = document.getElementById(id);
    var context = canvas.getContext('2d');
    var width=400;
    var height=400;
    var maxStars=1000;
    
    setCanvasSize(canvas, width, height);
    clearCanvas(canvas, context, "#000");
    addStars(context, canvas.width, canvas.height, maxStars);
}
function setCanvasSize(canvas, width, height) {
    canvas.setAttribute('width', width);
    canvas.setAttribute('height', height);
}

function clearCanvas(canvas, context, style) {
    context.fillStyle = style;
    context.fillRect(0,0,canvas.width, canvas.height);
}
function addStars(context, width, height, maxStars) {
    for (var i=0;i<maxStars;i++) {
       ...