<!DOCTYPE html>
<html>
<head>
<style>
*{margin:0;padding:0}
</style>
</head>
<body>
<canvas/>
<script>
(function(bodyStyle) {
bodyStyle.mozUserSelect = 'none';
bodyStyle.webkitUserSelect = 'none';
var img = new Image();
var canvas = document.querySelector('canvas');
canvas.style.backgroundColor='transparent';
canvas.style.position = 'absolute';
img.addEventListener('load', function(e) {
var ctx;
img.width=window.innerWidth ;//2000;
img.height=window.innerHeight;//2000;
//img.width=document.documentElement.clientWidth;
//img.height=document.documentElement.clientHeight;
var w = img.width,
h = img.height;
var offsetX = canvas.offsetLeft,
offsetY = canvas.offsetTop;
var mousedown = false;
function layer(ctx) {
ctx.fillStyle = 'gray';
ctx.fillRect(0, 0, w, h);
}
function eventDown(e){
e.preventDefault();
mousedown=true;
}
function eventUp(e){
e.preventDefault();
mousedown=false;
}
function eventMove(e){
e.preventDefault();
if(mousedown) {
if(e.changedTouches){
e=e.changedTouches[e.changedTouches.length-1];
}
var x = (e.clientX + document.body.scrollLeft || e.pageX) - offsetX || 0,
y = (e.clientY + document.body.scrollTop || e.pageY) - offsetY || 0;
with(ctx) {
beginPath()
arc(x, y, 80, 0, Math.PI * 2);
fill();
}
}
}
canvas.width=w;
canvas.height=h;
//canvas.style.backgroundImage='url('+img.src+')';
canvas.style.background='url('+img.src+')';
canvas.style.backgroundSize =''+ w+ 'px '+ h +'px';
ctx=canvas.getContext('2d');
ctx.fillStyle='transparent';
ctx.fillRect(0, 0, w, h);
layer(ctx);
ctx.globalCompositeOperation = 'destination-out';
canvas.addEventListener('touchstart', eventDown);
canvas.addEventListener('touchend', eventUp);
canvas.addEventListener('touchmove', eventMove);
canvas.addEventListener('mousedown', eventDown);
canvas.addEventListener('mouseup', eventUp);
canvas.addEventListener('mousemove', eventMove);
});
img.src = 'Chrysanthemum.jpg';
})(document.body.style);
</script>
</body>
</html>
-------demo 2-----------
<!DOCTYPE html>
<html>
<head>
<style>
*{margin:0;padding:0}
</style>
</head>
<body>
<div>
<img id="bgImg" src="bar.jpg" style="position:absolute; left:0px; top:0px;" />
<canvas id="CanvasDoodle" imgsrc='Chrysanthemum.jpg' style='position:absolute;left:0px; top:0px;background: transparent;'></canvas>
</div>
<script>
function CanvasDoodle(canvas){
this.canvas=canvas;
this.ctx=canvas.getContext("2d");
this.imgSrc=canvas.getAttribute("imgsrc");
document.getElementById('CanvasDoodle').width=window.innerWidth;
document.getElementById('CanvasDoodle').height=window.innerHeight;//2000;
document.getElementById('bgImg').width=canvas.width;
document.getElementById('bgImg').height=canvas.height;
//this.width=canvas.width;
//this.height=canvas.height;
this.left=parseInt(canvas.style.left);
this.top=parseInt(canvas.style.top);
this.touchX=0;
this.touchY=0;
this.needDraw=false;
this.init();
}
CanvasDoodle.prototype={
init:function(){
var _self=this;
var img=new Image();
img.onload=function(){
var pat=_self.ctx.createPattern(img,"no-repeat");
_self.ctx.strokeStyle=pat;
_self.ctx.lineCap="round";
_self.ctx.lineJoin="round";
_self.ctx.lineWidth="100";
}
img.src=this.imgSrc;
this.canvas.addEventListener('mousedown',function(e){
e.preventDefault();
_self.needDraw=true;
_self.ctx.beginPath();
_self.ctx.moveTo(e.clientX-_self.left,e.clientY-_self.top);
},false);
this.canvas.addEventListener('mousemove',function(e){
e.preventDefault();
if(_self.needDraw){
_self.ctx.lineTo(e.clientX-_self.left,e.clientY-_self.top);
_self.ctx.stroke();
}
},false);
this.canvas.addEventListener('mouseup',function(e){
e.preventDefault();
_self.needDraw=false;
});
}
};
new CanvasDoodle(document.getElementById('CanvasDoodle'));
</script>
</body>
</html>