Resizing html canvas using javascript
Question
I'm trying to smoothly resize an html5 canvas element. I've cobbled together bits of code and I think the following should work:
<body>
<header id='myheader' height='200'>
<canvas id='mycanvas' width='200' height='200'></canvas>
<script>
var mycanvas = document.getElementById('mycanvas');
var context = mycanvas.getContext('2d');
var centerX = mycanvas.width / 2;
var centerY = mycanvas.height / 2;
var radius = 70;
context.beginPath();
context.arc(centerX, centerY, radius, 0, 2 * Math.PI, false);
context.fillStyle = 'blue';
context.fill();
context.lineWidth = 5;
context.strokeStyle = '#003300';
context.stroke();
var animate = function(prop, val, duration) {
var start = new Date().getTime();
var end = start + duration;
var current = mycanvas[prop];
var distance = val - current;
var step = function() {
var timestamp = new Date().getTime();
var progress = Math.min((duration - (end - timestamp)) / duration, 1);
mycanvas[prop] = current + (distance * progress);
if (progress < 1) requestAnimationFrame(step);
};
return step();
};
animate('mycanvas.height', 10, 1000);
</script>
</header>
</body>
...but obviously it doesn't! The result I'm looking for is a canvas that shrinks to just show the middle part of the circle (something more interesting than a circle will be added later). Is there anything obvious that I'm missing, or am I just doing this the wrong way? Ultimately I want to to resize both the canvas and the header together, so getting the canvas resizing to work is stage 1. Any help appreciated...
(Edit: actually, I ultimately want to resize both the canvas and header in response to a scroll event - which I think means avoiding a css solution - but I want to get this bit working first!)
1 answers
solution1
1 ACCPTED 2015-11-04 11:22:03
Here are a few changes to your script that I believe do what you want:
var mycanvas = document.getElementById('mycanvas');
var context = mycanvas.getContext('2d');
var radius = 70;
function draw() {
var centerX = mycanvas.width / 2;
var centerY = mycanvas.height / 2;
context.beginPath();
context.arc(centerX, centerY, radius, 0, 2 * Math.PI, false);
context.fillStyle = 'blue';
context.fill();
context.lineWidth = 5;
context.strokeStyle = '#003300';
context.stroke();
}
var animate = function(target, prop, val, duration, action) {
var start = new Date().getTime();
var end = start + duration;
var current = target[prop];
var distance = val - current;
var step = function() {
var timestamp = new Date().getTime();
var progress = Math.min((duration - (end - timestamp)) / duration, 1);
target[prop] = current + (distance * progress);
action();
if (progress < 1) requestAnimationFrame(step);
};
return step();
};
animate(mycanvas, 'height', 10, 1000, draw);
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