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I have a background, let's say it's green grass. On top of the background I have a black overlay. What I want now is to make a movable hole in the overlay so that you can see the background like in the image below.
I am pretty new to canvas so I'm not sure what I'm supposed to look for. Alpha mask?
So my question is how can I achieve the effect demonstrated in the image above?
If it were HTML I would have two images of the grass, one as the background and one above the overlay in a div with a border radius that can move and just calculate positions.
Thanks.
900
Are you looking for a moving "flashlight" kind of effect?
If so, you can do that by drawing a circular path and then using it as a clipping region with: context.clip();
Anything drawn after the .clip() will be viewed through the clipping path.
Here is code and a Fiddle: http://jsfiddle.net/m1erickson/pRzxt/
<!doctype html>
<html>
<head>
<link rel="stylesheet" type="text/css" media="all" href="css/reset.css" /> <!-- reset css -->
<script type="text/javascript" src="http://code.jquery.com/jquery.min.js"></script>
<style>
body{ background-color: ivory; }
canvas{border:1px solid red;}
</style>
<script>
$(function(){
var canvas=document.getElementById("canvas");
var ctx=canvas.getContext("2d");
window.requestAnimFrame = (function(callback) {
return window.requestAnimationFrame || window.webkitRequestAnimationFrame || window.mozRequestAnimationFrame || window.oRequestAnimationFrame || window.msRequestAnimationFrame ||
function(callback) {
window.setTimeout(callback, 1000 / 60);
};
})();
var radius=50;
var x=100;
var dx=10;
var y=100;
var dy=10;
var delay=10;
var img=new Image();
img.onload=function(){
var canvas1=document.getElementById("image");
var ctxImg=canvas1.getContext("2d");
ctxImg.drawImage(img,0,0,img.width,img.height,0,0,canvas.width,canvas.height);
animate();
}
img.src="http://lh3.ggpht.com/_Z-i7eF_ACGI/TRxpFywLCxI/AAAAAAAAAD8/ACsxiuO_C1g/house%20vector.png";
function animate() {
if(--delay<0){
// update
x+=dx;
if(x-radius<0 || x+radius>=canvas.width){dx=-dx;}
y+=dy;
if(y-radius<0 || y+radius>=canvas.height){dy=-dy;}
delay=10;
// draw stuff
ctx.clearRect(0, 0, canvas.width, canvas.height);
ctx.save();
ctx.beginPath();
ctx.arc(x,y, radius, 0, 2 * Math.PI, false);
ctx.clip();
ctx.drawImage(img,0,0,img.width,img.height,0,0,canvas.width,canvas.height);
ctx.restore();
}
// request new frame
requestAnimFrame(function() {
animate();
});
}
}); // end $(function(){});
</script>
</head>
<body>
<p>Image clipped by moving circle</p>
<canvas id="canvas" width=300 height=200></canvas>
<br/><p>Unclipped image</p>
<canvas id="image" width=300 height=200></canvas>
</body>
</html>
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